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IntroductionVitamins are an organic chemical compound which the body requires in small amounts for the metabolism and

to protect your health .

Vitamins assist the body in Vitamins assist the body in functioning properly by helping in functioning properly by helping in the formation of hormones, blood the formation of hormones, blood cells, nervous-system chemicals and cells, nervous-system chemicals and genetic growth. An over dose can be genetic growth. An over dose can be harmful to your health.harmful to your health.

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The Body & Vitamins

The body can only produce one vitamin naturally by itself. This is vitamin D. All other vitamins that the body requires to function properly have to be derived from the diet. Lack of vitamins can have a serious affect on your health and may end in metabolic and other

dysfunctions.

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Origin of the word VITAMIN• Casimir Funk, a Polish biochemist, isolated an anti-

berberi substance from rice polishing.– Named it vitamine

• An amine

• Vital for life

• Originally it was thought these necessary compounds were all amines. Since they were vital to our health they became known as “vital amines”, ie. vitamines.

• When it was discovered that some were not amines, i.e., not ' --ines', the name was changed to vitamins

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What are Vitamins?• Vitamins are micronutrients (nutritionally

important organic compounds required in very small amounts).

• Plants and animals synthesize vitamins. – Vitamins form through biochemical life processes of the

plants and animals we eat.

Examples:– Most mammals can synthesize vitamin C; not humans

and primates.– No mammal can synthesize B vitamins but rumen bacteria

do.

• Some function as vitamins after undergoing a chemical change– Provitamins (e.g., β-carotene to vitamin A).

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Vitamin Groups

Vitamins are divided up into two main groups which are fat-soluble vitamins and water-soluble vitamins. Fat-soluble vitamins are usually found in

foods that contain fat .The body stores the fat soluble The body stores the fat soluble vitamins and because of this, vitamins and because of this, people don’t usually need to people don’t usually need to make a special effort to include make a special effort to include them in their diet. them in their diet.

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Vitamin Groups

Water soluble vitamins can’t be stored in the body for a long time and have to be

replenished everyday .

In some cases when it’s not In some cases when it’s not possible to obtain these possible to obtain these vitamins in a regular diet, vitamins in a regular diet, they have to be acquired by they have to be acquired by other vitamin supplements.other vitamin supplements.

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Vitamin classificationName(Letter) RDI

Retinol (A) 5000 IU

Calciferol (D) 400 IU

Tocopherol (E) 30 IU

Phylloquinone (K) 70 g

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Classification, Requirements, AbsorptionWater-soluble

Name(Letter) RDA

)mg(

Thiamin (B1) 1.5

Riboflavin (B2) 1.7

Niacin (B3) 2

Pantothenic acid (B5) 10

Pyridoxine (B6) 2

Biotin (B7) 0.3

Folic acid (B9) 0.4

Cobalamin (B12) 6 g

Ascorbic acid (C) 60

• Absorbed at the small intestine.

• Absorption often highly regulated by either other vitamins or binding proteins in the small intestine.

• Transported away from small intestine in blood.

• Typically not stored; instead, kidney filters excess into urine– Thus, important to get these

vitamins daily.– Toxicities almost unheard of.

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Classification, Requirements, Absorption

Oil-soluble Name(Letter) RDI

Retinol (A) 5000 IU

Calciferol (D) 400 IU

Tocopherol (E) 30 IU

Phylloquinone (K) 70 g

• Absorbed with dietary fat in small intestine• 40-90% absorption efficiency• Absorption typically regulated by need

need absorption

• Transported away from small intestine in chylomicra via blood and lymph (depending on size)

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• Metabolically they have diverse functions as:

• Coenzymes (B vitamins)

• Hormones (retinoic acid, vitamin D)

• Modulators or regulators of growth and development (retinoic acid, folic acid)

• (apparently non-specific) antioxidants (Vitamins C and E)

What do vitamins do?

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Coenzymes, Cofactors, and Prosthetic groups

• Vitamins bind the enzyme either loosely or tightly:– Coenzymes are lost upon dialysis because they bind

the enzyme loosely.– When they bind enzymes tightly, they are considered

prosthetic group. – The term cofactor includes such compounds but also

includes other molecules such as metal ions that may be necessary for enzyme activity.

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Cofactors and coenzymes

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Vitamin ACompounds with 20-carbon structure.

Contain a methyl substituted cyclohexenyl ring (-ionone ring),

and an isoprenoid side chain with either a hydroxyl group, and aldehyde group, a carboxylic acid group, or an ester group (retinyl ester) at the terminal C15.

All-trans-retinal 11-cis-retinal

Retinol Retinoic AcidCan’t be reduced to retinol or retinal in the body.

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Vitamin A1. Active vitamin A- Preformed vitamin A can be obtained

either directly from foods that are substantial in vitamin A (beef liver, fish liver oils, egg yolks and butter)

• The active form of vitamin is retinol, an alcohol which can be converted to other forms (e.g. vitamin A esters) for storage in liver and tissues.

• much the body's vitamin A is stored in the liver as retinyl palmitate

2. Provitamin A- or from provitamins, substances that are transformed into vitamins in the body

• Beta-carotene is the most abundant and widespread provitamin A.• Beta-carotene comes from a group of compounds called the

"carotenoids ".• Dark-green leafy vegetables (spinach) and yellow-orange

fruits (apricots and mango) and vegetables (carrots, yellow squash and sweet potatoes) are high in beta-carotene and other carotenoids .

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Vitamin A: Biological functions

• Vitamin A (retinal) is an essential precursor for formation of the visual pigment, rhodopsin, in the retina of the eye. Retinal plays an important role in vision, especially night vision.

• Helps regulate cell development.

• Promotes the proper growth of bones and teeth. Bone cells (osteoblasts and osteoclasts) depend on vitamin A for their normal functioning.

• Boosts the body's immune system helping to increase our resistance to infectious diseases.

• Is important in the formation and maintenance of healthy hair, skin and mucous membranes.

• Vitamin A holds an important place in sexual reproduction. Adequate

levels of vitamin A are needed for normal sperm production. The female reproductive cycle requires sufficient amounts of vitamin A.

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Role of Vitamin A in Vision1. When the rhodopsin is exposed to light it is bleached releasing the 11-cis-retinal from opsin. 2. Isomerization of the cis-isomer of retinal to all-trans-retinal, causes conformational changes in rhodopsin, hyperpolarization of the retinal rod cell, and extremely rapid transmission of electrical activity to the brain via the optic nerve3. Trans-retinal is isomerized to cis-retinal in the dark, which associates with opsin to regenerate rhodopsin.

Visual Pigment

All trans retinol = main circulating

form of Vit A

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Vitamin A: Deficiency symptoms

1. Night blindness" - lessened ability to see in dim light.

2. Increased susceptibility to infection and cancer and anemia equivalent to iron deficient.

3. Prolonged lack of vitamin A (keratinization of the cornea, a condition known as

xerophthalmia).

4. Abnormal bone development in fetal and neonatal life.5. Various congenital defects.

Retinol and its precursors are used as dietary supplements to prevent the above symptoms.

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Vitamin A: Toxicity • Skeletal malformations

• spontaneous fractures

• internal hemorrhages

• loss of appetite

• slow growth or weight loss.

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Vitamin D: Types and Sources• Vitamin D2 (ergocalciferol) is derived

from plants and irradiated yeast and fungi.

• Vitamin D3 (cholecalciferol) is synthesized in the body when skin is exposed to sunlight– Cholesterol + sunshine = Vitamin D3– “sunshine vitamin” – UV-B rays (5-10

minutes arms and legs, mid-day sun).

• We can obtain vitamin D3 from foods like milk, fortified cereals, tuna, salmon and fish oils.

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Diet

Sunlight

Ergosterol(in plants)

Vitamin D2(Ergocalciferol)

7-Dehydrocholesterol

Vitamin D3Cholecalciferol)

Sunlight

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Activation of Vitamin D• Vitamin D2 and vitamin D3 are

biologically inactive but can have equal biological activity:

• Both can be converted first to calcifediol in the liver and then to calcitriol, also known as 1,25-dihydroxycholecalciferol, in the kidneys.

• Calcitriol, which is the most active form of vitamin D3, is then transported via a carrier protein to the various sites in the body where it is needed.

Calcitriol is also called 1,25-dihydroxy vitamin D3, or (1,25-(OH)2D3.

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1,25-dihydroxyvitamin D3

25-hydroxyvitamin D3In the liver is hydroxylated at the 25 position cholecalciferol by a specific D3-25-hydroxylase generating 25-hydroxy-D3 [25-(OH)D3] which is the major circulating form of vitamin D.

Conversion of 25-(OH)D3 to its biologically active form, calcitriol, occurs through the activity of a specific D3-1-hydroxylase present in the proximal convoluted tubules of the kidneys, and in bone and

placenta. Cytochrome P450, O2 and NADPH are needed.

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Vitamin D Functions: Hormone for Calcium and Phosphate regulation• Nerves and muscles must function properly; calcium is vital

for nerve cell transmissions and muscle fiber contractions.• Calcitriol functions in concert with parathyroid hormone

(PTH) and calcitonin to regulate serum calcium and phosphorous levels by affecting:

– Dietary calcium absorption from the small intestine.– Urinary calcium excretion– Bone calcium metabolism

• There is evidence that vitamin D (specifically, vitamin D3) is involved in regulation of the body's immune system.

• Vitamin D is essential for normal insulin secretion by the pancreas and therefore control of blood sugar levels.

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Vitamin D: Deficiency symptoms– Rickets (bone deformities in children)

– Osteoporosis

– Osteomalacia (weak bones)

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Vitamin D: Toxicity

• Nausea, thirst, loss of appetite, stupor

• Hypercalcemia: calcium gets deposited in soft tissues, Arteries and kidneys.

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Vitamin E

-Tocopherolis the most potent of the tocopherols.

Four of the eight vitamin E molecules are called tocopherols (alpha, beta, gamma and delta)). alpha-tocopherol is the most biologically active in humans.

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Functions• Vitamin E in the form of alpha-tocopherol is

an important fat-soluble antioxidant, scavenging oxygen free radicals, lipid peroxy radicals and singlet oxygen molecules before these radicals can do further harm to cells. [Free radicals are very reactive atoms or molecules that typically possess a single unpaired electron.]

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Free radical = unpaired electron very reactive

Oxygen radicals: Hydroxy (HO•) / Peroxy (HOO•)

OH

OH

OH

OH

.

.

Free Radicals - the Metabolic Oxidizers

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An antioxidant is a chemical so easilyoxidized itself that it protects others from oxidation.

Double Bondeg. Vitamin A

Phenoleg. Vitamin E or C

OH

and / or

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• Deficiency: rare in adults usually due to impaired fat absorption or transport; seen usually in children (anemia, edema in infants)

• Excess: very safe below 800 IU/day• Source:

– Vitamin E is present in animal fats, meat, green vegetables, nuts/seeds.

– Alpha-tocopherol is found in a number of vegetable oils, including safflower and sunflower. It is also found in wheat germ. Soybean and corn oils contain mainly gamma-tocopherol.

• The major site of vitamin E storage is in adipose tissue.• Estimated requirements: 5mg/day = 0.6mg/day of

unsaturated fat. • Uses: • Hemolytic anemia in premature infants, unresponsive to B12, Fe

and folic acids.– Macrocytic megaloblastic anemia seen in children with severe protein-

calorie malnutrition.

Vitamin E (deficiency)

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Vitamin K• The "K" in vitamin K comes from the German word

"koagulation," which refers to blood clotting (coagulation). • Vitamin K is essential for the functioning of several proteins

involved in normal blood clotting. Vitamin K is needed for the body to make four of the blood's coagulation factors, including prothrombin (also known as factor II), proconvertin (factor VII), Christmas factor (factor IX) and the Stuart-Power factor (factor X).

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__                                                                                               

Vitamin K1• Naturally occurring vitamin K is absorbed from the

intestines only in the presence of bile salts and other lipids through interaction with chylomicrons. Therefore, fat malabsorptive diseases can result in vitamin K deficiency.

• Present in green leafy vegetables like lettuce, parsley, spinach and various greens (beet and mustard). Broccoli and certain vegetable oils (soybean, cottonseed, and olive).are also a good source of vitamin K1.

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Vitamin K2• Vitamin K2 is a group of compounds called the "menaquinones." • Synthesized by intestinal bacteria "n" can be 6, 7 or 9 isoprenoid

groups.• • Vitamin K2, which is the most biologically active form of vitamin K, is

found in egg yolks, butter, liver, cheddar cheese and yogurt.

• It has been suggested that products like yogurt, may help to increase the functioning of these useful bacteria.

__                                                                 _______                              

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Vitamin K3

• The synthetic (man-made) vitamin K3 is water solubleand absorbed irrespective of the presence of intestinal lipids and bile.

Uses : essential cofactor in blood clotting. Deficiency: Rare, (bruising/bleeding in infants).Excess: Dangerous if taking anti-coagulants.

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Vitamin K cycle

NH

R

R

C H C H2 C H2

C = O

-C O 2

NH

R

R

C H C H2 C H

C = O

-C O 2

-C O 2

K (re d )

carboxylase

epoxidereductase

vitam in Kreductase

coum arin sK (e p o x )

K (o x )

C a

G L U res id u e

G L A res id u e

O + C O2 2 H O + H2+

D ie t

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Thrombin Activation

co llagen

WO UNDv W F

endo th eliu m

p la te le tPL surfa c e

C a

C a

Va

Xa

C ircu la tio n

N H2C O O H

G la G la

P ro -T h ro m b in

SS

p ro teo ly t ic cu t

N H2C O O H

G la G la

T h ro m b inP ro -

SS

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The common pathway

p ro th ro m bin * th ro m bin

fib rino ge n

F ib rin m on om er

F ib rin po ly m erC L O T

* X a

X III X IIIa

Va V

C o m m o np a thwa y

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Vitamin B Complex• Originally thought to be one vitamin, BUTVitamin Chemical name B1

B2

B3

BB44

B5

B6

B7

B8

B9

BB1010

BB1111

B12

Thiamine

Riboflavin

Nicotinamide (niacin)

Adenine (no longer considered a vitamin)Adenine (no longer considered a vitamin)

Pantothenic acid

Pyridoxine

Biotin

Folacin (folic acid)

Folacin (folic acid)

pp-aminobenzoic acid (PABA) / H1-aminobenzoic acid (PABA) / H1 L-carnitine / b-hydroxy-g-trimethylammonium butyrate L-carnitine / b-hydroxy-g-trimethylammonium butyrate

Cyanocobalamin

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Chemical structurepyrimidine + thiazole

Thiamine

TPP

Thiamine diphosphotransferase

MgATP2-

MgAMP-

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Thiamine pyrophosphate (TPP) is a derivative of thiamine (vitamin B1). Nutritional deficiency of thiamine leads to the disease beriberi. It affects especially the brain, because TPP is required for CHO metabolism, and the brain depends on glucose metabolism for energy.

th iam ine pyrophosphate (T P P )

N

NH 3C NH 2

CH 2S

C

N

H 3CCH 2 O P O P O

O O

CH 2

H

O O

+

acid ic H +

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Dry beriberi – peripheral neuropathy, atrophy

Wet beriberi – dilated cardiomyopathyDue to peripheral dilation of arterioles

Wernicke Korsakoff (see alcohol)

Thiamine Deficiency (B1) Beriberi

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Alcohol dilated cardiomyopathy

Opthalmoplegia – can’t follow light sourceNystagmus-involuntary jerking of the eye

Ataxia (inability to coordinate muscular movements due to nervous disorders) and confusion

Memory loss/confabulation (to fill in gaps in memory by fabrication)

Alcohol, Wernicke Korsakoff syndrome:

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Sources

• Widely distributed.

• Brewers' yeast is very rich source.

• Cereal grains are rich sources, especially in germ and seed coat.

• Fresh green, leafy plants

• Animal products (especially egg yolk, liver, kidney) are good sources.

• Synthetic vitamin is usually available as thiamin hydrochloride.

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Vitamin B2 or Riboflavin• Yellow, crystalline compound with yellow-green

fluorescence in aqueous solution. • Only sparingly soluble in water. • Stable in acid or neutral, but not alkaline solutions. • Unstable in light.• Riboflavin is phosphorylated in the intestine to

generate FMN (riboflavin 5’-phosphate) by the action of Flavokinase.

• FMN then reacts with ATP, yielding FAD:

FMN + ATP FAD + ppi

ppi = inorganic pyrophosphate.

FAD synthetase

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Chemical structure• Isoalloxazine ring system = dimethylbenzene + pteryn • Ribitol (Reduced ribose) attached to N10

Riboflavin

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Chemical structure and atom numbering of the flavin mononucleotide

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FMN FAD

1 1

5 5

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FAD (Flavin Adenine Dinucleotide is derived from the vitamin riboflavin. The dimethylisoalloxazine ring system undergoes oxidation/reduction.

FAD is a prosthetic group, permanently part of E3.

Reaction: FAD + 2 e- + 2 H+ FADH2

C

CCH

C

C

HC

NC

CN

NC

NHC

H3C

H3C

O

O

CH2

HC

HC

HC

H2C

OH

O P O P O

O

O-

O

O-

Ribose

OH

OH

Adenine

C

CCH

C

C

HC

NC

C

HN

NH

C

NHC

H3C

H3C

O

O

CH2

HC

HC

HC

H2C

OH

O P O P O

O

O-

O

O-

Ribose

OH

OH

AdenineFAD FADH2

2 e + 2 H+

dimethylisoalloxazine

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Riboflavin Functions

• Essential constituent of the– Flavoproteins– Flavin mononucleotide (FMN) – Flavin adenine dinucleotide (FAD)

• These play key roles in hydrogen transfer reactions associated with– Glycolysis– TCA cycle

• Oxidative phosphorylation.

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Deficiency symptoms1. Inappetence, poor growth,

vomiting, skin eruptions and eye abnormalities in pigs.

• Cheilosis/Angular stomatitis (fissure at the angle of the mouth)

• Localized seborrheic dermatitis of the face

• Vascular changes in the cornea• Purple smooth tongue due to

loss of tongue papillae (Glossitis).

2. Poor growth and "curled toe paralysis" in chicks.

• The toes frequently curl inward and they may be unable to stand.

Cheilosis/Angular stomatitis

Glossitis

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Dietary Sources

• Dairy products

• organ meats (liver and heart) but not muscle meat.

• Green leafy plants (especially alfalfa)

• Yeast and animal products

• Cereals are poor sources so poultry fed cereal-based rations should receive supplemental riboflavin.

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Niacin = Vitamin B3

• Beta pyridine carboxylic acid• Two forms: Nicotinic acid and Nicotinamide.

Nicotinamide is the amide derivative of nicotinic acid. • In most animal species (including humans) niacin can be

synthesized from the essential amino acid, tryptophan.• Both forms contain a pyridine ring.

Nicotinic acid Nicotinamide

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NAD+

NADP+

Functions:Active coenzymes: nicotinamide-adenine dinucleotide (NAD+) nicotinamide-adenine phosphate (NADP+).

Both are extremely important in hydrogen transfer reactions catalyzed by dehydrogenase enzymes.

ATP synthesis, from oxidation of primary fuels (glucose, fatty acids and to a lesser extent, amino acids) (NAD+)

Also important in reductive biosynthesis (NADP+)

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Deficiency symptoms1. Pellagra in farm animals and humans (fiery inflammation of tongue, mouth and upper

esophagus).2. Poor growth, enteritis and dermatitis.3. Occurs in people who subsist mainly on corn which is low in both niacin and

tryptophan4. The signs of pellagra include dermatitis, diarrhea, dementia (the three Ds) and loss of

tongue papillae.Sources of B3

Most non-corn-based diets contain adequate amounts of nicotinamide or its precursor, tryptophan.

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vitamin B5 / pantothenic acid • Chemical nature• Dipeptide derivative of the amino acid B-

alanine and a butyric acid derivative.

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Coenzyme A and Acetyl coenzyme A• Essential constituent of coenzyme A, Pantothenic acid combines with

ATP and cysteine in the liver to generate CoA-SH.• CoA-SH transfers activated acyl groups, R-(C=O)-, such as acetyl

group by binding them as a thioester. Acyl transfer is important in the TCA cycle and de novo fatty acid synthesis.

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• Deficiency symptoms• 1. Poor growth, diarrhea, loss of hair,

characteristic "goose-stepping" in pigs.• 2. Poor growth and feather development,

dermatitis in chickens.• Sources• Widely distributed in plants (especially legumes

and cereal) and animal products. • Deficiency has been observed in pigs fed a low

protein (14%) corn-soybean ration fortified with minerals and vitamins except pantothenic acid.

Vitamin B5 deficiency

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Lipoic Acid & DiHydroLipoic Acid (DHLA)

lipoic acid = Internal disulfur of 6,8-dithiooctanoic acid.Lipoic Acid (LA) is part of a redox pair.

oxidized form

reduced form

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Lipoamide includes a

dithiol that undergoes oxidation/ reduction .

S CH2

CH2

CHS

CH2 CH2 CH2 CH2 C NH (CH2)4 CH

NH

C O

O

HS CH2

CH2

CHHS

CH2 CH2 CH2 CH2 C NH (CH2)4 CH

NH

C O

O

2e + 2H+

lipoamide

dihydrolipoamide

lysine lipoic acid

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The carboxyl at the end of lipoic acid's hydrocarbon chain forms an amide bond to the -amino group of a

lysine residue of E2, yielding lipoamide.

A long flexible arm, including hydrocarbon chains of lipoate and the lysine R-group, links each dithiol of

lipoamide to one of two lipoate-binding domains of E2 .

S CH2

CH2

CHS

CH2CH2CH2CH2CNH(CH2)4 CH

NH

C O

O

HSCH2

CH2

CHHS

CH2CH2CH2CH2CNH(CH2)4 CH

NH

C O

O

2e + 2H+

lipoamide

lysine lipoic acid

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Structure

PDH = Pyruvate dehydrogenase complex

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Lipoic acid

• Alpha Lipoic acid is a natural substance found in certain foods and also produced in the human body.

• Alpha Lipoic acid is a disulfide compound found naturally in mitochondria as the coenzyme for pyruvate dehydrogenase and -ketoglutarate dehydrogenase.

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The coenzyme function for pyruvate dehydrogenase and -ketoglutarate dehydrogenase

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Pyruvate dehydrogenase complex (PDH)

E1 = Pyruvate dehydrogenase

E2 = Dihydrolipoamide acyltransferase

E3 = Dihydrolipoamide dehydrogenase

The reaction is:

Pyruvate + NAD+ +CoASH Acetyl CoA + NADH + H+ + CO2PDH

5 non-protein molecules (coenzymes) required for this enzyme catalyzed reaction are:

NAD+ and CoASH (coenzyme A); (these are present in the equilibrated reaction formula, as can be seen above)

TPP (thiamine pyrophosphate), Lipoic acid and FAD (flavin adenein dinucleotide) participate in the reaction but do not show up in the equilibrated reaction formula.

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The final electron acceptor

is NAD+.

Coenzym e A -SH + HO C

O

CH 3

Coenzym e A -S C

O

CH 3 + H2O

acetic acid

acetyl-CoA

N

R

H

CN H 2

O

N

R

CN H 2

OH H

+

2 e + H

+

N A D + N A D H

In the overall reaction catalyzed by the

Pyruvate Dehydrogenase

complex, the acetic acid generated is transferred

to coenzyme A.

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Sequence of reactions catalyzed by Pyruvate Dehydrogenase complex:

1. The keto C of pyruvate reacts with the carbanion of TPP on E1 to yield an addition compound.

The electron-pulling (+) charged N of the thiazole ring promotes CO2 loss. Hydroxyethyl-TPP remains.

2. The hydroxyethyl carbanion on TPP of E1 reacts with the disulfide of lipoamide on E2. What was the keto C of pyruvate is oxidized to a carboxylic acid, as the lipoamide disulfide is reduced to a dithiol.

The acetate formed by oxidation of the hydroxyethyl is linked to one of the thiols of the reduced lipoamide as a thioester (~).

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Sequence of reactions (continued)

3.Acetate is transferred from the thiol of lipoamide to the thiol of coenzyme A, yielding acetyl CoA.

4.The reduced lipoamide, swings over to the E3 active site. Dihydrolipoamide is reoxidized to the disulfide, as 2 e- + 2 H+ are transferred to a disulfide on E3 (disulfide interchange).

5.The dithiol on E3 is reoxidized as 2 e- + 2 H+ are transferred to FAD.

The resulting FADH2 is reoxidized by electron transfer to NAD+, to yield NADH + H+.

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Acetyl CoA, a product of the Pyruvate Dehydrogenase reaction, is a central compound in

metabolism .

The "high energy" thioester linkage makes it an excellent donor of the acetate moiety.

a c e t y l - c o e n z y m e A

H 3 C C

O

S C o A

View an animation of the Pyruvate Dehydrogenase reaction sequence.

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Acetyl CoA functions as: input to Krebs Cycle, where the acetate moiety

is further degraded to CO2. donor of acetate for synthesis of fatty acids,

ketone bodies, & cholesterol.

glucose-6-P

Glycolysis

pyruvate fatty acids

acetyl CoA ketone bodies cholesterol

oxaloacetate citrate

Krebs Cycle

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Mechanism of the reaction catalyzed by PDH complex

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E1 uses TPP to release CO2 and produce HydroxyethylTPP (HETPP)

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E2 uses lipoic acid to transfer the hydroxyethyl group from TPP to CoASH in

order to produce AcetylCoA

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80

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6 .PYRIDOXINE (vitamin B6)

• Three interconvertible forms (Vitamers) exist in tissues:– Pyridoxine (alcohol) (PN)– pyridoxal (aldehyde) (PL)– pyridoxamine (amine) (PM)

• All can easily convert to each other and to the active form.

B6 is involved in: Amino acid metabolismBreakdown of glycogenSynthesis of epinephrine (adrenaline) and norepinephrine (noradrenaline)Synthesis of globular proteins Conversion of certain fatty acidsSynthesis of niacin (vitamin B3) from the amino acid tryptophan.

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Chemical nature

Pyridoxal (PL) Pyridoxamine (PM)Pyridoxine

Pyridoxol (PN)

Each of these forms can be phosphorylated at position 5 to form: PLP, PMP, and PNP.

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Active form

Pyridoxal phosphate (PLP)

• PLP and PL account for 90% of the total B6 in the blood.• In the blood B6 is transported both in the plasma and the RBCs.• In the blood PLP is hydrolyzed to PL because only free PL gets inside the cells.• In muscle and other tissues, PL is converted back to PLP by a reversible reaction with the help of alkaline phosphatase and pyridoxal kinase.

• Active functional form is pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP). • For absorption, the “phosphorylated” form must be hydrolyzed to “dephosphorylated” form by the enzyme alkaline phosphatase in the intestine.• In the portal vein Vit B6 is present as PL, PM, PN.• In the liver they are converted back to phosphorylated forms. This conversion is catalyzed by the ATP requiring enzyme, pyridoxal kinase.

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Functions

FUNCTIONS: A cofactor for enzymes involved in: •Transamination reactions required for the synthesis and catabolism of the amino acids.•Decarboxylation reactions. •Glycogenolysis as a cofactor for glycogen phosphorylase.

80-90% of body vit B6 is present in the muscles, most of it in PLP (coenzyme) form bound to glycogen phosphorylase. Only 1 mol or less is present in the blood,

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Vitamin-Coenzymes in Amino Acid Metabolism

• Vitamin B-6 : pyridoxal phosphate– Enzymes that bind amino

acids use PLP as coenzyme for binding

• Transaminases• Amino acid

decarboxylases• Amino acid deaminases

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Covalent bonds of -amino acids made labile by

their binding to PLP-containing enzyme In the reactions of amino acid metabolism, the formyl (CHO) group of PLP condenses with -NH2 group of an amino acid and forms a Schiffs base. This linkage weakens or labilizes all the bounds around the -carbon of the amino acid.

The specific bond of an amino acid that is broken depends on the particular enzyme to which PLP is attached.

87

Mechanism of catalyzed reaction

88

Biosynthesis of Amino Acids: Transaminations

Amino Acid1 +-Keto Acid2 Amino Acid2 +-Keto Acid1

NH3+

-O2CCH 2CH2CHCO 2-

Glutamate

OR-CCO 2

-+

O-O2CCH 2CH2CCO 2

-

-Ketoglutarate

NH2

R-CHCO 2-

+

Pyridoxal phosphate (PLP)-Dependent Aminotransferase

89

Transaminations: Role of PLP

N

C

CH2OPO3-2HO

H3C

N CHCH2CH2CO2-

N

HNH3+

CO2-

CHO

CH2OPO3-2HO

H3C

H

N

H

CH2

CH2OPO3-2HO

H3C

N CCH2CH2CO2-

H

CO2-

N

CH2NH2

CH2OPO3-2HO

H3C

O

H

H2O

-O2CCH 2CH2CCO 2-

+ +

++

-O2CCH 2CH2CHCO 2-

H2O

Tautomerization

90

Decarboxylation reactions

•Formation of neurotransmitters in the nervous system: norepinephrine, dopamine, histamine .

•Formation of -aminobutyric acid (GABA) from glutamate and formation of Serotonin.

91

Deficiency• Food sources:

– In animal foods major forms are PL and and PM along with their phosphorylated forms.

– In plants PN.– Bananas, beans, lentils, walnuts, salmon, chicken, beef, whole grain breads

and cereals, soybeans, liver, eggs, dairy products are excellent sources. • Requirements:

– The requirement for vitamin B6 in the diet is proportional to the level of protein consumption ranging from 1.4 - 2.0 mg/day for a normal adult.

– During pregnancy and lactation the requirement for vitamin B6 increases approximately 0.6 mg/day.

• TOXICITIES: – Megadoses of B6 (daily doses of >500mg) are used to treat pms symptoms.

They can cause neurotoxocity and photosensitivity in some individuals.• Deficiencies: are rare and usually are related to an overall deficiency of

all the B-complex vitamins.• Certain drugs form complexes with PL and PLP

– Penicillamine (used to treat rheumatoid arthritis and cystinurias). – Isoniazid (the hydrazide derivative of isonicotinic acid) is the primary drug for

chemotherapy of tuberculosis.

92

7 .BIOTIN

• Biotin is relatively small, bicyclic (two-ring) compound formed from a tetrahydrothiophene (thiophene) ring ,

• and a second ring, which contains a ureido group. • The thiophene ring also has a valeric acid side chain. • Although eight different stereoisomers of biotin exist,

only one stereoisomer is found naturally and to have biologically activity as a coenzyme. It is called d-(+)-biotin, D-biotin or simply biotin.

It is sometimes called vitamin H and also coenzyme R.

93

Biotin CycleBiotin cycle: the chain of chemical reactions involved in the use and reuse of the vitamin biotin. One important role of biotinidase is

1. To separate or free biotin from proteins to which it is bound in foods. Biotin in its free form can then be used by the body.

2. Biotinidase lets the body recycle or reuse the biotin over and over again so that we do not need to consume large amounts of this vitamin in our diets. •Within cells, the carboxylases (pyruvate carboxylase, acetyl-CoA carboxylase, methycrotonyl-CoA carboxylase, propionyl-CoA carboxylase) are biotinylated via holocarboxylase synthetase. Biotin and apocarboxylases are the substrates. ATP and magnesium also participate in the reaction.

Biotinidase deficiency is a treatable, inherited metabolic disorder in which the body cannot process the vitamin biotin in a normal manner.

94

Holocarboxylase

In humans, the four holocarboxylases are : acetyl-CoA carboxylase, propionyl-CoA carboxylase, pyruvate carboxylase and beta-methylcrotonyl-CoA carboxylase. Biotin is chemically bonded in each of these enzymes via an amide linkage between the carboxyl group of the valeric acid side-chain in biotin and the epsilon-amino group of the lysine residue in the apocarboxylase.

The enzyme that catalyzes the formation of this covalent bond is called holocarboxylase synthetase.

95

Biotin (functions)•Coenzyme for several reactions involving CO2 fixation into various

compounds e.g.

Pyruvate to oxaloacetate (pyruvate carboxylase)

Propionyl CoA to methylmalonyl CoA (propionyl CoA carboxylase)

Acetyl CoA to malonyl CoA(acetyl CoA carboxylase) - initial step in de novo fatty acid synthesis.

96

Deficiency symptoms• Rare because of widespread distribution in feeds

and significant lower gut synthesis.• Can be induced by eating raw egg white

– The fact is that nature created the egg in such a way that its yolk is very rich in biotin. One of the highest concentration in nature. Eat the egg whole together with the egg white and you will be fine.

– Egg whites contain a glycoprotein called "avidin" which binds biotin - one of the B vitamins - very effectively. The cooking process deactivates the avidin in the egg, much the same way it deactivates every other protein in the egg white.

• Biotin deficiency is chief cause of fatty liver and kidney syndrome.

Sources• Yeast, rice, soybeans, peanuts, fish

(herring and mackerel), mushrooms and bananas, safflower meal, liver and milk are rich sources.

This baby developed severe biotin deficiency during intravenous feeding without biotin.

Aajonus Vonderplanitz, in his book “We Want to live” is a strong proponent of raw eggs.

97

How Biotin Works

1- Biotin carrier protein

2- Biotin carboxylase

3- Transcarboxylase

98

VITAMIN B12 (cobalamin)• Vitamin B12, is also called cobalamin,

cyanocobalamin and hydroxycobalamin.

• It is built from : A nucleotide and a complex tetrapyrrol ring structure (corrin ring) and a cobalt ion in the center.

• Vitamin B12 is synthesized exclusively by microorganisms (bacteria, fungi and algae) and not by animals and is found in the liver of animals bound to protein as methycobalamin or 5'-deoxyadenosylcobalamin.

• When R is cyanide (CN), vitamin B12 takes the form of cyanocobalamin.

• In hydroxycobalamin, R equals the hydroxyl group (-OH).

• In the coenzyme forms of vitamin B12, R equals an adenosyl group in adenosylcobalamin.

• R equals a methyl (-CH3) group in methylcobalamin.

99

• Known as the "red" vitamin because it exists as a dark red crystalline compound, Vitamin B12 is unique in that it is the only vitamin to contain cobalt (Co3+) metal ion, which by the way, gives it the red color.

• The vitamin must be hydrolyzed from protein in order to be active.

• Intrinsic factor, a protein secreted by parietal cells of the stomach, carries it to the ileum where it is absorbed.

• It is transported to the liver and other tissues in the blood bound to transcobalamin II.

• It is stored in the liver attached to transcobalamin I.– It is released into the cell as

HydroxocobalaminIn the cytosol it is converted to methylcobalamin.

– Or it can enter mitochondria and be converted to 5’-deoxyadenosyl cobalamin.

Dorothy Crowfoot Hodgkin(1910-1994)

Dr. Stadtman in her lab

100

Absorption of Vitamin B-12 (Fig. 10-10)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

101

Functions• Only two reactions in the body require vitamin B12 as a

cofactor: 1. During the catabolism of fatty acids with an odd number of

carbon atoms and the amino acids valine, isoleucine and threonine the resultant propionyl-CoA is converted to succinyl-CoA for oxidation in the TCA cycle. – methylmalonyl-CoA mutase, requires vitamin B12 as a cofactor in

the conversion of methylmalonyl-CoA to succinyl-CoA.– 5'-deoxyadenosine derivative of cobalamin is required for this

reaction

2. The second reaction catalyzed by methionine synthase converts homocysteine to methionine – This reaction results in the transfer of the methyl group from N5-

methyltetrahydrofolate to hydroxycobalamin generating tetrahydrofolate and methylcobalamin during the process of the conversion.

102

103

CH3-THF

Methionine SAM

SAH

methyltransferases

CH3-

Methylacceptor

Homocysteine

MSTHF

B12CH2-THF

CBS

cystathionine

cysteine

B6

B6

Folate cycleFolate cycle

TransulfurationTransulfurationpathwaypathway

Methionine cycleMethionine cycle

Methylacceptor

Methionine and Folate cycles are interrelated

104

Vitamin-Coenzymes in Amino Acid Metabolism

• Vitamin B-12– Catabolism of BCAA

• Methyl-malonyl CoA mutase (25-9 &10)

105

Deficiency symptoms

• Pernicious anemia in humans (inability to absorb B12 because of lack of gastric intrinsic factor).

• Neurological disorders due to progressive demyelination of nerve cells. – This results from increase in methylmalonyl-CoA.– Methylmalonyl-CoA is a competitive inhibitor of malonyl-CoA in

fatty acid biosynthesis.– Can substitute malonyl-CoA in any fatty acid bisynthesis and

create branched-chain fatty acid altering the architecture of normal membrane structure of nerve cells.

• Sources– Synthesized only by microorganisms, so traces only are present

in plants; liver is a rich source.– B12 is found in organ and muscle meats, fish, shellfish, dairy

products, eggs and in fortified foods like breakfast cereals.

106

107

9 .FOLIC ACID (folacin)• Folacin includes several derivatives of folic acid

(monopteroylglutamic acid).• Active functional form is tetrahydrafolic acid.• Folic acid is obtained primarily from yeasts and leafy vegetables as

well as animal liver. Animal cannot synthesize PABA nor attach glutamate residues to pteroic acid, thus, requiring folate intake in the diet.

108

Structure

Folic acid exists in a polyglutamate form. Intestinal mucosal cells remove some of the glutamate residues through the action of the lysosomal enzyme, conjugase.

109

Structure

Folic acid

PABA (vitamin Bx)

110

Folic acid is reduced within cells (principally the liver where it is stored) to tetrahydrofolate (THF or H4folate) through the action of folate reductase [or dihydrofolate reductase (DHFR) ] which is an NADPH-requiring enzyme.

Folate Dihydrofolate Tetrahydrofolate

111

Active center (N5 and N10)

112

 

                                                                                                                             

                                         • Active center of tetrahydrofolate (THF). The N5 position is the site of

attachment of methyl  and  formimino groups, the N10 the site for attachment of formyl  group and that both N5 and N10 bridge the methylene and methenyl groups.

113

folate conversions

114

Function

• Carrier of one-carbon (e.g. methyl) groups that are added to, or removed from, metabolites such as histidine, serine, methionine, and purines.– Role of N5,N10-methylene-THF in dTMP synthesis is

the most metabolically significant function for this vitamin.

– Vitamin B12 and N5-methyl-THF in the conversion of homocysteine to methionine is important in helping cells to regenerate needed THF.

115

Participation of H4folate in dTMP synthesis

______Deoxyuridine______________          ________Deoxythymidine

____Monophosphate (dUMP)_______________Monophosphate (dTMP)_______         

116

Vitamin-Coenzymes in Amino Acid Metabolism

• Folacin: Tetrahydrofolate (THF)– Carrier of single

carbons• Donor & receptor• Glycine and serine• Tryptophan degradation• Histidine degradation• Purine and pyrimidine

synthesis

117

Deficiency symptoms• Identical to those for vitamin B12 deficiency. • Effect of folate deficiency on cellular processes is upon DNA

synthesis.– Impairment in dTMP synthesis– Cell cycle arrest in S-phase of rapidly proliferating cells, especially

hematopoietic cells.• The result is megaloblastic leukemia as for vitamin B12 deficiency. • The inability to synthesize DNA during erythrocyte maturation leads

to abnormally large erythrocytes termed macrocytic anemia.

Deficiency is rare due to the adequate presence of folate in food. •Poor dietary habits as those of chronic alcoholics •Impaired absorption or metabolism or an increased demand for the vitamin. •Pregnancy

•folate will nearly double by the third trimester of pregnancy Certain drugs such as anticonvulsants and oral contraceptives can impair the absorption of folate.

118

Ascorbic Acid Structure

(AscH2

)

O

OH

OHO

HO

OH

119

1’

2’3’

4’ 5’

6’

1’

2’3’

4’

5’

6’

• It is derived from glucose via uronic acid pathway. Enzyme L-gluconolactone oxidase is reponsible for conversion of gluconolactone to ascorbic acid.

• This enzyme is absent in primates, including humans, some bats…. • The active form is ascorbic acid itself.

Vitamin C (Chemical nature)

120

AscH2 is a Di-acid

At pH 7.4, 99.95% of vitamin C will be present as AscH-; 0.05% as AscH2 and 0.004% as Asc2-. Thus, the antioxidant chemistry of vitamin C is the chemistry of AscH- .

O

OH

OHO

HO

OH

O

OH

OHO

O

OH

O

O

OHO

O

OH

pK1 = 4.1 pK2 = 11.8

AscH2 AscH- Asc2-

121

Forms of Ascorbate

pK = 4.1

pK = 11.8

pK = -0.86

O O

OHO

OHOH

HOO O

HOHO

OHOH

OH

HO

+H+ -H+

+H+ -H+

O

OH

OHO

HO

OH

AscH2

O

OH

OHO

O

OH

AscH-

+H+ -H+

O

O

OHO

O

OH

Asc2

-e

-e -e

Asc

O

O

OHO

O

OH

O

O

OHO

O

OH

DHA

+H2O-H2O

+H2O

-H2O

DHAA (2) DHAA (1) (>99%)

(pK ~ 8-9)

O

OH

OHO

O

OH

AscH

122

Ascorbate Falling Apart

+H2O

+2H+

-2H+

AscH2

HO

OH

O

OH

O

HO

-e

Asc

HO

OH

O

O

O

ODHA

HO

OH

O

O

O

O

+e

-e

+e

DHA

HO

OH

O

O

O

O

2,3-diketo-L-gulonic acid

L-xylonicacid

L-lyxonic acid

C

C

C

C

C

CH2OH

OHO

OH

HO

O

O

H

H

C

C

C

C

CH2OH

OHO

OH

OH

HO H

H

H

C

C

C

C

CH2OH

OHO

OH

HO

HO

H

H

H

+

L-xylose

CH2OH

C

C

C

CH2OH

OH

HO

O

H

L-threonicacid

oxalic acid

+

C

C

C

CH2OH

OH

HO

O OH

HC

C

O OH

OHO

123

AscH- is a Donor Antioxidant

AscH- donates a hydrogen atom (H or H+ + e-) to an oxidizing radical to produce the resonance-stabilized tricarbonyl ascorbate free radical. AscH has a pKa of -0.86; thus, it is not protonated in biology and will be present as Asc-.

AscAscH

O

OH

OHO

O

OH

R+ RH+

O

O

HO

O

OH

O

124

Ascorbate, Summary

Ascorbate is a versatile, water soluble, donor, antioxidant.

Thermodynamically, it can be considered to be the terminal, small-molecule antioxidant.

AscAscH

O

OH

OHO

O

OH

R+ RH+

O

O

HO

O

OH

O

125

126

VITAMIN C• Vitamin C is L-ascorbic acid, which is a colorless,

crystalline acid with strong reducing properties.• Functions• Vitamin C has antioxidant properties similar to those of

vitamin E, – Protects cells from free radicals.– Protects iron from oxidative damage, thus enhancing iron

absorption in the gut.

• The main function is as a reducing agent.– It has the potential to reduce cytochrome a and c of the

respiratory chain and molecular oxygen and nitrates.

• It is required for various hydroxylation reactions e.g. proline to hydroxypoline for collagen synthesis (see next slide).

127

Hydroxylation of proline and lysine residues in collagen

• Vitamin C is required for the maintenance of normal connective tissue as well as for wound healing since synthesis of connective tissue is the first event in wound tissue remodeling.

128

Other activities• Several other metabolic reactions require

vitamin C as a cofactor: • The catabolism of tyrosine and the synthesis of

epinephrine from tyrosine and the synthesis of the bile acids.

• It is also believed that vitamin C is involved in the process of steroidogenesis.

• The adrenal cortex contains high levels of vitamin C which are depleted upon adrenocorticotropic hormone (ACTH) stimulation of the gland.

129

Roles in the bodySources• Citrus fruits and green leafy vegetables • Vitamin C is readily absorbed and so the primary cause of vitamin C

deficiency is poor diet and/or an increased requirement.

Deficiency symptoms1. Scurvy

– Bleeding gums– Small red spots on skin– Rough skin– Wounds fail to heal– Weak bones and teeth– Anemia and infections

2. Stress (e.g., infections, smoking)– Mechanism unknown, but vitamin C requirements increase during stress

3. Common cold?4. Disease prevention?

– Cancer, heart disease

130

Some activated carriers in metabolism Carrier molecule in

activated formGroup carried Vitamin precursor

ATP Phosphoryl

NADH and NADPH Electrons Nicotinate (niacin)

FADH2 Electrons Riboflavin (vitamin B2)

FMNH2 Electrons Riboflavin (vitamin B2)

Coenzyme A Acyl Pantothenate

Lipoamide Acyl

Thiamine pyrophosphate Aldehyde Thiamine (vitamin B1)

Biotin CO2 Biotin

Tetrahydrofolate One-carbon units Folate

S-Adenosylmethionine Methyl

Uridine diphosphate glucose

Glucose

Cytidine diphosphate diacylglycerol

Phosphatidate

Nucleoside triphosphates Nucleotides