CARBOHYDRATE METABOLISM

By

Prof. Dr SOUAD M. ABOAZMA

BIOCHEMISTRY DEP.

DIGESTION OF CARBOHYDRATE

•Salivary amylase partially digests starch and glycogen to

dextrin and few maltoses. It acts on cooked starch.

•Pancreatic amylase completely digests starch, glycogen,

and dextrin with help of 1: 6 splitting enzyme into maltose

and few glucose. It acts on cooked and uncooked starch.

Amylase enzyme is hydrolytic enzyme responsible for

splitting α 1: 4 glycosidic link.

•Maltase, lactase and sucrase are enzymes secreted from

intestinal mucosa, which hydrolyses the corresponding

disaccharides to produce glucose, fructose, and

galactose.

•HCl secreted from the stomach can hydrolyse the

disaccharides and polysaccharides.

ABSORPTION OF MONOSACCHARIDES

•Simple absorption (passive diffusion): The absorption depends upon the

concentration gradient of sugar between intestinal lumen and intestinal

mucosa. This is true for all monosaccharides especially fructose &

pentoses.

•Facilitative diffusion by Na+-independent glucose transporter system (GLUT5). There are mobile carrier proteins responsible for transport of

fructose, glucose, and galactose with their conc. gradient.

•Active transport by sodium-dependent glucose transporter system (SGLUT1). In the intestinal cell membrane there is a mobile carrier protein

coupled with Na+- K+ pump. The carrier protein has 2 separate sites one

for Na+ ,the other for glucose. It transports Na+ ions (with conc. Gradient)

and glucose (against its conc. Gradient) to the cytoplasm of the cell. Na+

ions is expelled outside the cell by Na+- K+ pump which needs ATP and

expel 3 Na+ against 2 K+.

Exit all sugars from mucosal cell to the blood occur by facilitative

transport through GLUT2.

It is proved that glucose and galactose are absorbed

very fast; fructose and mannose intermediate rate and

pentoses are absorbed slowly. Galactose is absorbed

more rapidly than glucose.

There are 2 pathways for transport of material absorbed by intestine:• The hepatic portal system, which leads directly to the liver

and transporting water-soluble nutrients.

• Lymphatic vessels: which lead to the blood by way of thoracic

duct and transport lipid soluble nutrients.

Carrier protein and transport of glucose

GLUCOSE UPTAKE BY TISSUESGlucose is transported through cell membrane of different tissues by

different protein carriers or transporters. Extracellular glucose binds to

the transporter, which then alters its conformations, then transport glucose

across the membrane.

•GLUT1: present mainly in red cells, and retina.

•GLUT2: present in liver, kidneys, pancreatic B cells, and lateral border of

small intestine, for rapid uptake and release of glucose.

•GLUT3: present mainly in brain.

•GLUT4: present in heart, skeletal muscles, and adipose tissues. It is for

insulin-stimulated uptake of glucose.

•GLUT5: present in small intestine and testes for glucose and fructose

transport.

•SGLUT1: present in small intestine and kidneys, sodium-dependent, for

active transport of glucose and galactose from lumen of small intestine and

reabsorption of glucose from glomerular filtrate in proximal renal tubules.

Role of insulin in transport of glucose in adipose tissue, skeletal muscles and heart through GLUT4:

1.Insulin produces transfer of GLUT-4 from their

intracellular pool to the outer membrane surface of these

tissues. So, increase GLUT-4 in the cell surface of these

tissues leads to increase glucose transport and uptake by

these tissues.

2-Transport through the previous tissues is insulin-

independent.

G G G G G

Insulin hormone

Insulin receptor

(nG)Intracellular signal

for insulin

Intracellular location of GLUT-4

lGucose

FATE OF ABSORBED SUGARS

The absorbed monosaccharides are either hexoses or

pentoses.

1.The absorbed pentoses are excreted in urine because

the body does not deal with them.

2.The absorbed hexoses are glucose, fructose, or

galactose. Fructose and galactose are converted into

glucose in the liver.

FATE OF ABSORBED GLUCOSE

Blood glucose comes from 3 main sources:

1- Absorbed glucose from diet.

2- Glcogenolysis of liver glycogen.

3- Synthesis of glucose from other substances by gluconeogenesis

The absorbed glucose has the following pathways:

1- Oxidation:

a- For provision of energy: glycolysis, and Kreb’s cycle.

b- Not for energy production:

- HMP for synthesis of phospho-pentoses & NADPH + + H+.

- Uronic acid pathway for synthesis of glucuronic acid.

2- Synthesis of other CHO substances as:

A- Mannose, fucose, neuraminic acid for glycoprotein

formation.

B- Galactose and lactose in mammary gland.

C- Fructose in seminal vesicles.

E- Amino-sugar (glucosamine) for mucopolysaccharides, and

glycoprotein formation.

3- Synthesis of non essential amino acids.

4- Excess glucose is stored as glycogen in liver and muscles (glycogenesis).

5- More excess glucose is stored as lipid in adipose tissue (lipogenesis).

IMPORTANT ENZYMES IN CHO METABOLISM

1- KinaseThese are activating enzymes, which convert various metabolites into phosphorylated

form in presence of ATP, Mg++. They are irreversible enzymes with few exception.

1.Hexokinase (present in all tissues except liver) acts on any hexoses (glucose,

fructose, galactose) giving 6-phophorlyated hexose (glucose 6-P, fructose 6-P,

galactose 6-P).

2.Glucokinase (present only in liver) acts only on glucose converting it into glucose

6-P.

3.Fructokinase (present only in liver) acts on fructose to form fructose 1-P.

4.Galactokinase (present only in liver) acts only on galactose to give galactose 1-P.

2- DehydrogenasesThese are oxidizing enzymes that act by removal of H2 from a substrate → the

removed H2 will be carried by special coenzymes, which are hydrogen carriers as NAD,

FAD .

The name of the dehydrogenase is derived from the name of substrate upon, which it

acts as: Lactate dehydrogenase enzyme removes H2 from lactic acids.

The dehydrogenases are reversible enzymes.

Isomerases-31.These are enzymes that interconvert aldo-keto isomers. They are

reversible enzymes

Glucose 6-PFructose 6-Pisomerase

.

Mutases-4

Glucose 1-PGlucose 6-PMutase

These are enzymes that transfer a group from carbon to another carbon in

the same molecule. They are reversible enzymes.

e.g.

•Epimerases-5

UDP-glucoseUDP-glactoseEpimerase

They are enzymes that transfer a group from a side to the opposite

side of one carbon atom in the molecule. They are reversible enzymes e.g.

Phosphatases-6

Glucose 6-P Glucose + Phosphate

These are hydrolytic enzymes that remove a phosphate group from a

phosphorylated compound by addition of H2O. They are irreversible

enzymes.H2O

GLYCOGEN METABOLISM

Glycogen is the main storage form of carbohydrates in

animals. It is present mainly in liver and in muscles.

Glycogen is highly branched polymer of α, D-glucose. The

glucose residues are united by α 1: 4 glucosidic linkages

within the branches. At the branching point, the linkages

are α 1: 6. The branches contain about 8-12 glucose

residues.

Glycogen metabolism includes glycogen synthesis (glycogenesis) and glycogen breakdown (glycogenolysis).

GLYCOGENESIS

Def: it is the formation of glycogen from glucose in

muscles and from CHO and non CHO substances in

liver.

Site of location: In the cytoplasm of every cells

mainly liver and muscles.

Steps :- as the following :-

1- GlucoseGlucokinase,hexokinase

ATP

G-6-PPhosphoglucomutase

G-1-PMg++

ADP

2- G-1-PDUP-glucose pyrophosphorylase

UDP-glucose

UTP PPi

H2O

pyrophosphatase

2Pi

N.B.: G-6-P is converted to glucose-1-phosphate by

phosphoglucomutase, glucose-1, 6 diphosphate is an obligatory

intermediate in this reaction.

-Glycogen synthase enzyme in presence of pre-existing

glycogen primer or glycogenin (glycogenin is a small

protein that forms glycogen primer after glycosylation by

UDP-glucose) adds glucose molecule from UDP-glucose

through creation of α 1: 4 glucosidic link.

-When the chain has been lengthened, the branching

enzyme transfers a part of the chain forming α 1: 6

glucosidic link. Thus establishing the branching points in

the molecule. The branches grow by further addition of 1: 4

glucosyl units.

-The key regulatory enzyme of glycogenesis is glycogen synthase, which present in 2 forms:

1.Active form, which is dephosphorylated enzyme (GSa).

2.Inactive form, which is phosphorylated enzyme.(GSb).

GLYCOGENOLYSIS

Def.: It is the breakdown of glycogen into glucose in

liver and lactic acid in muscles.

Site of location: cytoplasm of many tissues mainly

liver, kidney, and muscles.

Steps:•Phosphorylase is the first acting enzyme which

is the rate-limiting and key enzyme in

glycogenolysis. With proper activation and in

presence of inorganic phosphate (Pi), the enzyme

breaks the glucosyl α-1:4 linkage and removes by

phosphorolytic cleavage the 1:4 glucosyl residues

from outermost chains of the glycogen molecule

until approximately four (4) glucose residues remain

on either side of α-1 :6 branch (“limit dextrin”).

By phosphorlyase activity glucose is liberated as glucose-1-P and NOT as free glucose.

•When four glucose residues are left around the

branch point, another enzyme, α-1:4 Glucan

transferase transfers a “trisaccharide” unit from one side

to other thus exposing α-1: 6 branching point.

•The hydrolytic splitting of α-1:6 glucosidic linkage

requires the action of a specific debranching enzyme.

As α-1:6 linkage is hydrolytically split, one molecule of

free glucose is produced.

•Fate of glucose-1-P: The combined action of

phosphorlyase and other enzymes convert glycogen

mostly to glucose-1-P. By the action of

phosphoglucomutase enzyme glucose-1-P is easily

converted to glucose-6-P as the reaction is reversible.

•In liver and kidney, a specific enzyme glucose-6-phosphatase is

present that removes PO4 from glucose-6-P enabling “free glucose” to

form and diffuse from the cells to extracellular spaces including blood.

This is the final step in hepatic glycogenolysis, which is reflected by a rise in blood glucose.

•In muscles, enzyme glucose-6-phosphatase is absent. Hence

glucose-6-P enters into glycolytic cycle and forms lactate. Muscle

glycogenolysis does not contribute to blood glucose directly. But

indirectly, lactic acid can go to glucose formation in liver.

The key regulatory enzyme of glycogenolysis is glycogen

phosphorolase enzyme which is present in 2 forms:

1.Active form (phosphorylated form) = phosphorylase a . 2.Inactive form (dephosphorylated form) = phosphorylase b.

Steps in glycogenolysis

1-Phosphorylase enzyme is a phosphorolysis enzyme which

responsible for breaking α 1: 4 glucosidic link of glycogen in

presence of inorganic phosphorus giving G-1-P.

2-Debranching enzyme is hydrolytic enzyme acts on α1: 6

glucosidic link giving free glucose.

NB :- The main function of muscles glycogen is to supply

glucose within muscles during contraction. Liver

glycogen is concerned with the maintenance of blood

glucose especially between meals. After 12-18 hours

fasting, liver glycogen is depleted, whereas muscle

glycogen is only depleted after prolonged exercise

CA2+ SYNCHRONIZES THE ACTIVATION OF PHOSPHORYLASE

WITH MUSCLE CONTRACTION

Glycogenolysis in muscle increases several 100-folds at the

onset of contraction; the same signal (increased cytosolic Ca2+

ion concentration) is responsible for initiation of both contraction

and glycogenolysis. Muscle phosphorylase kinase, which

activates glycogen phsophorylase, is a tetramer of four different

subunits, α, β, γ, and δ. The δ subunit is identical to the Ca2+ -

binding protein calmodulin and binds 4 Ca2+. The binding of Ca2+

activates the catalytic site of the δ subunit allowing activation of

glycogen phosphorylase and stimulation of glycogenolysis

REGULATION OF GLYCOGEN METABOLISM

1. Regulation of glycogen metabolism is achieved by a balance in

activities between glycogen synthase and glycogen phosphorylase.

2. Not only "phosphorylase" enzyme is activated by a rise in

concentration of phosphorylase kinase via c-AMP, but "Glycogen

synthase" enzyme is at the sametime converted to "inactive" form,

both effects are mediated via "cyclic -AMP-dependant protein-kinase".

3. Thus glycogenolysis is stimulated while glycogenesis is inhibited. Both

processes cannot occur simultaneously together .

4. Dephosphorylation of "phosphorylase 'a', phsophorylase kinase 'a' and

glycogen synthase 'b' is accomplished by a single enzyme of wide specificity

"protein phsophatase-1", which in turn is inhibited by c-AMP dependant

protein kinase via the protein "Inhibitor-1". Thus, glycogenolysis can be

inhibited and glycogenesis can be stimulated synchronously, or vice versa,

because both processes are geared to the activity of c-AMP dependant

protein-kinase.

HORMONAL CONTROL OF GLYCOGEN METABOLISM

•Epinephrine stimulates α1 adrenergic receptors in liver → activation of

phospholipase-C which hydrolyses phosphatidyl inositol into 1,2 diacylglycerol

and inositol triphosphate → release Ca++ from its intracellular stores into the

cytoplasm raising the intracytoplasmic concentration of Ca++ which reacts with

calmodulin to give Ca++ - calmodulin complex → activation of Ca++ calmodulin

dependent protein kinase → conversion of glycogen synthase a (active) into

glycogen synthase b (inactive) and conversion of phosphorylase kinase b into

phosphorylase kinase a which converts phosphorylase b (inactive) into

phsophorylase a (active) → stimulation of glycogenolysis and inhibition of glycogenesis →so stimulation of glycogenolysis in liver can be cAMP

independent.

•Epinephrine stimulate β adrenergic receptors in liver and in muscles &

glucagon stimulate its receptors in liver but not in muscles→ stimulation of

adenylate cyclase enzyme → stimulation of cyclic AMP formation → stimulation

of protein kinase A → conversion of glycogen synthase a (active) into glycogen

synthase b (inactive) and conversion of phosphorylase kinase b into

phosphorylase kinase a which converts phosphorylase b (inactive) into

phsophorylase a (active) → stimulation of glycogenolysis and inhibition of

glycogenesis.

•Insulin stimulates phosphatase enzyme so converts

inactive glycogen synthase into active one and converts

active phosphonylase enzyme into inactive one →

stimulation of glycogenesis and inhibition of

glycogenolysis. Also it stimulates phosphodiesterase

enzyme → destruction of cyclic AMP.

cAMP DEPENDANT PROTEIN KINASE

PHOSPHORYLASE KINASE b

GLYCOGEN SYNTHASE b

PHOSPHORYLASE KINASE a

GLYCOGEN SYNTHASE a

PROTEIN PHOSPHATASE-1

PROTEIN PHOSPHATASE-1

PROTEIN PHOSPHATASE-1

PHOSPHORYLASE b

PHOSPHORYLASE

a

cAMP5’AMP Inhibitor-1

Inhibitor-1 phosphate

Epinephrine (liver, muscle)

Glucagon (liver)

UDPGIc

Glucose 1-phosphate

Glycogen

Glucose(liver) Glucose Lactate(muscle)

-

-

-

++

PHOSPHODIESTERASE

Control of glycogen metabolism

Liver glycogen Muscle glycogen

Amount Liver has more conc. Muscle has more amounts.

Sources Blood glucose and

other radicals

Blood glucose only

Hydrolysis Give blood glucose Due to absence of phosphatase enzyme not give free

glucose but give lactic acid

Starvation Changes to blood

glucose

Not affected

Muscular ex. Depleted Depleted

Hormones Insulin → ↑

Adrenaline → ↓

Thyroxin → ↓

Glucagon → ↓

Insulin → ↑

Adrenaline → ↓

Thyroxin → ↓

Glucagon → no effect due to absence of its receptors

Differences between muscle and liver glycogen

GLYCOGEN STORAGE DISEASES

A group of diseases results from genetic defects of certain enzymes. The

absence of glucose-6-phosphatase enzyme results in the classical hepatorenal

this is characterized by : (type I), GierkeVon glycogen storage disease

1- It occurs in only 1 person per 200,000 and is transmitted as an

autosomal recessive trait.

:2- Symptoms include

Fasting hypoglycemia, because the liver cannot release enough glucose by

means of glycogenolysis; only the free glucose from debranching enzyme activity

is available.

3- Lactic academia, because the liver cannot form glucose from lactate .The

increased blood lactate reduces blood pH and the alkali reserve.

4- Hyperlipidemia, because the lack of hepatic gluconeogenesis (results in

increased mobilization of fat as a metabolic fuel).

5- Hyperuricemia (with gouty arthritis), due to hyperactivity of the hexose

monophosphate shunt

glycogenosesOther types of

A number of other genetic glycogen storage

defects (glycogenoses) have been described.

Pompe’s (lysosmal glucosidase deficiency),

Forb’s (Debranching enzyme deficiency),

Andersen’s (Branching enzyme system deficiency),

Macardle’s (Muscle phosphorylase deficiency),

Here’s (Liver phosphorylase deficiency) and Taui’s(Phosphofuctokinase deficiency).

OXIDATION OF GLUCOSE

The pathways for oxidation of glucose are classified into two main

groups:

a- The major pathways for complete oxidation of glucose

into CO2, H2O and energy are:

1- Glycolysis → convert one molecule of glucose into 2 mol of

pyruvic acid + 2 NADH.H+.

2- Oxidative decarboxylation of pyruvic to acetyl CoA +

NADH.H++CO2

3- Complete oxidation of acetyl CoA in Kerb’s cycle into CO2, H2O

and energy .

b- The minor pathways for oxidation, which are not for energy

production.

1- Hexose monophosphate pathway (HMP).

2- Uronic acid pathway.

GLYCOLYSIS

EMBDEN-MEYERHOF PATHWAY

acid in presence of pyruvicoxidation of glucose to give Def.:O2 and lactic acid in absence of mitochondria (RBCs) and in

absence of O2 .

Site: Cytoplasm of all cells especially muscles and RBCs.

Steps:H – C = O

H C – OH

OH – C – H

H – C – OH

H – C – OH

CH2OH

H – C = O

H – C – OH

OH – C – H

H – C – OH

H – C – OH

CH2O-P

Mg

ATP ADP

Hexokinase, glucokinase

G-6-PD-Glucose

Mechanism of oxidation of glyceraldehydes 3-phosphate. Enz: glyceraldehydes 3-P dehydrogenase which is inhibited by the –SH poison iodoacetate, thus able to inhibit glycolysis.

:GLYCOLYSISENERGY PRODUCTION FROM

A. glycolysis in presence of O2 (Aerobic glycolysis):

Reaction catalyzed by ATP production

Stage I

1. Hexokinase/Glucokinase reaction (for

phosphorylation)

-1 ATP

2. Phosphofrutokinase-1 (for

phosphorylation)

-1 ATP

Stage III

3. Glyceraldehyde-3-P dehydrogenase

(oxidation of 2 NADH in electron

transport chain)

+ 6 or +4 ATP

4. Phosphoglycerate kinase (substrate level

phosphorylation)

+2 ATP

Stage IV

5. Pyruvate kinase (substrate level

phosphorlyation)

+2 ATP

Net gain = 10 or 8 - 2

= 8 or 6ATP

):glycolysis(Anaerobic 2in Absence of OGlycolysisB.

•In absence of O2 re-oxidation of NADH at glyceraldehyde-3-P-

dehydrogenase stage cannot take place in electron-transport

chain.

But the cells have limited coenzyme. Hence to continue the

glycolytic pathway NADH must be oxidized to NAD+. This is

achieved by reoxidation of NADH by conversion of pyruvate

to lactate (without producing ATP) by the enzyme lactate

dehydrogenase. Occurs in cells with no mitochondria as RBCs

(mature) ,or under low O2 supply as intensive muscular exercise.

In anaerobic glycolysis per molecule of glucose oxidation 4 - 2 = 2 ATP

will be produced.

REGULATION OF GLYCOLYSIS

•:glycolysisEnzymatic regulation of -A

There are 3 types of mechanism can be identified as

responsible for regulation of the enzyme activity of enzymes

concerned with CHO metabolism which are:

1- Changed in rate of enzyme synthesis:

* Induction →↑ rate of enzyme synthesis at gene expression

→↑ mRNA synthesis.

* Repression →↓ rate of enzyme synthesis at gene expression

→↓ mRNA synthesis.

2- Covalent modification by reversible phosphorylation

dephosphorylation.

3- Allosteric effect.

irreversible reaction in 3 regulatory enzymes which responsible for 4 There are

.glycolysis

Hexokinase1.It is found in most tissues to give G-6-P when blood glucose level is low.

2.Acts on glucose and other hexoses to give hexose-6-P.

3.It has low km and Vmax→ acts maximally at fasting bl. glucose level.

4.It is inhibited by its products, which is G-6-P → feedback inhibition.

Glucokinase1.It is found in liver and acts maximally after meal.

2.Acts only on glucose.

3.It has a high km and high Vmax → so it is active when bl. glucose level is high (after meal).

4.It is induced (↑its rate of synthesis) by insulin.

5.It is not inhibited by G-6-P.

Phosphofructokinase1.It is the major regulatory enzyme in most tissues.

2.It is allosterically activated by F-6-P, AMP and inhibited by ATP, citrate, and H+.

Pyruvate kinase1.It is allosterically inhibited by ATP, fatty acids, alanine, and acetyl CoA. And activated by

F-1-6 diphosphate.

2. It is phosphorylated by cAMP dependent protein kinase, which becomes inactive

and dephosphorylated by phosphatase enzyme, which becomes active.

•Hormonal regulation:-B

Insulin/glucagons ratio is the main hormonal regulation of glucose

utilization; it increases during glucose feeding and decreases during

fasting.

A.Glucagons: it is secreted in case of carbohydrates deficiency or in

response to low blood glucose level (hypoglycemia). It affects liver

cells mainly as follows:

1.It acts as repressor of glycolytic key enzymes.

2.Through cAMP-dependent protein kinase A, it produces

phosphorylation of specific protein enzymes that lead to

inactivation of glycolytic key enzymes.

B.Insulin: it is secreted after feeding of carbohydrate or in response to

high blood glucose level (hyperglycemia). It stimulates all pathways of

glucose utilization. Insulin binds to a specific cell membrane receptors

and produces certain signal cascade, which results in the following:

1.It acts as inducer for glycolytic key enzymes.

2.It activats phosphodiesterase enzyme(decreases cAMP that

leads to inhibition of protein kinase A).

3.It activats protein phosphatase-1 that produces

dephosphorylation of glycolytic key enzymes and their activation.

INHIBITORS OF GLYCOLYSIS:

1- Aresnate: which used in oxidative step

insted of Pi→ so glycolysis proceeds in

presence of arsenate but ATP, which formed

from 1-3 diphosphoglycerate is lost.

2- Iodoacetate produces inhibition of

glyceraldehydes-3-P dehydrogenase (inhibitor

of SH group).

3- Flouride inhibits enolase →↓↓ glycolysis in

bacteria →no production of lactic acid produced

by bacteria, which cause dental caries. It used

as anticoagulant in blood sample used for

estimation of blood glucose →↓↓ glycolysis in

RBCs .

:IN RBCSDIPHOSPHOGLYCERATE3 ,2FORMATION OF

2:3 diphosphoglycerate has an effect on O2 binding power of

haemoglobin→ It lowers O2 affinity by haemoglobin →↑ dissociation of

O2 to the peripheral tissues as in cases of high altitude.

CLINICAL SIGNIFICANCE OF 2,3 DIPHSOPHOGLYCERATE:

1- Persons who live at high altitude undergo state of low

O2 affinity for HB due to simultaneous increase of 2,3

diphosphoglycerate. This increase can be reversed on

returning to sea level.

2- Fetal HB has less 2,3 diphosphoglycerate than adult

HB, so fetal HB has high O2 affinity.

3- During storage of blood in blood banks, there is

decrease in 2,3 diphosphoglycerate so, stored blood has

high O2 affinity, which is not suitable for blood transfusion

especially to ill patients. If 2,3 diphosphoglycerate is

added to stored blood, it can’t penetrate RBCs wall. So, it

is advisable to add insoine, which is a substance that can

penetrate RBCs wall and change it into 2,3

diphosphoglycerate through HMP shunt.

GLYCOLYSISDIFFERENCES BETWEEN AEROBIC AND ANAEROBIC

Aerobic glycolysis Anaerobic glycolysis

- Site Cytoplasm of all

tissues

RBCs and skeletal muscle

during muscular ex.

- End products Pyruvic acid +

NADH.H+

Lactic acid + NAD+

- Energy production 6 OR, 8 ATP 2 ATP

- Lactate dehdyrogenase Not needed Needed

GLYCOLYSISDISEASES ASSOCIATED WITH IMPAIRED

:deficiency Hexokinase-1•In patients with inherited defects of hexokinase activity, the red blood cells contain low

concentrations of the glycolytic intermediates including the precursor of 2,3-DPG.

•In consequence, the hemoglobin of these patients has an abnormally high oxygen affinity.

•The oxygen saturation curves of red blood cells from a patient with hexokinase deficiency

are shifted to the left, which indicates that oxygen is less available for the tissues.

2- Pyruvate kinase deficiency (hemolytic anemia):•All red blood cells are completely dependent upon glycolytic activity for ATP production.

•Failure of the pyruvate kinase reaction, the production of ATP will decrease leading to

hemolysis of red cells.

•Inadequate production of ATP reduces the activity of the Na+ - and K+ -stimulated ATPase

ion pump.

3- Lactic acidosis:-•Blood levels of lactic acid are normally less than 1.2 mM. In lactic acidosis, the values for

blood lactate may be 5 mM or more.

•The high concentration of lactate results in lowered blood pH and bicarbonate levels.

•High blood lactate levels can result from increased formation or decreased utilization of

lactate.

•Common cause of hyperlacticidemia is anoxia.

•Tissue anoxia may occur in shock and other conditions that impair blood flow, in respiratory

disorders, and in severe anemia.

AEROBIC AND ANAEROBIC EXERCISE USE DIFFERENT FUELS

distance running, while -Aerobic exercise is exemplified by longanaerobic exercise by sprinting or weight lifting.

organ -there is really very little interanaerobic exercise During

cooperation. The vessels within the muscles are compressed during peak

contraction, thus their cells are isolated from the rest of the body. Muscle

largely relies on its own stored glycogen and phosphocreatine.

energy phosphate for ATP -serves as a source of highPhosphocreatineGlycolysis. are stimulatedglycolysisand glycogenolysissynthesis until

becomes the primary source of ATP for want of oxygen.

is metabolically more interesting. For moderate Aerobic exercise of muscle glycogen. glycolysisenergy is derived from thetexercise, much of

chain amino acid oxidation, ammonium -stimulation of branchedThere is also

-. However, a wellrelease from the exercising musclealanineproduction, and fed individual doesn't store enough glucose and glycogen to provide the

energy needed for running long distances. The respiratory quotient, the ratio

of carbon dioxide exhaled to oxygen consumed, falls during distance running.

progressive switch from glycogen to fatty acid oxidation This indicates the

gradually increases as glucose stores are exhausted, Lipolysisduring a race. and, as in the fast state, muscles oxidize fatty acids in preference to glucose

. as the former become available

MAJOR FEATURES OF SKELETAL MUSCLE S METABOLISM

1.Skeletal muscle functions under both aerobic (resting) and anaerobic (eg, sprinting)

conditions, so both aerobic and anaerobic glycolysis operate, depending on conditions.

2.Skeletal muscle contains myoglobin as a reservoir of oxygen.

3.Insulin acts on skeletal muscle to increase uptake of glucose.

4.In the fed state, most glucose is used to synthesize glycogen, which acts as a store of glucose for

use in exercise, 'preloading' with glucose is used by some long-distance athletes to build up stores

of glycogen.

5.Epinephrine stimulates glycogenolysis in skeletal muscle, whereas glucagon does not because of

absence of its receptors.

6.Skeletal muscle cannot contribute directly to blood glucose because it does not contain glucose-6-

phosphatase.

7.Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to

synthesize glucose, which can then return to muscle, (the cori cycle).

8.Skeletal muscle contains phosphocreatine, which acts as an energy store for short-term (seconds)

demands.

9.Free fatty acids in plasma are a major source of energy, particularly under marathon conditions

and in prolonged starvation.

10.Skeletal muscle can utilize ketone bodies during starvation.

11.Skeletal muscle is the principle site of metabolism of branched chain amino acids, which are

used as energy source.

12.Proteolysis of muscle during starvation supplies amino acids for gluconeogenesis.

13.Major amino acids emanating from muscle are alanine (destined mainly for gluconeogenesis in

liver and forming part of the glucose-alanine cycle) and glutamine (destined mainly for the gut and

kidneys).

ACIDPYRUVICOF DECARBOXYLATIONOXIDATIVE

keto-αacid and other pyruvicIt is conversion of :Def.acids into CoA derivatives.

In mitochondrial matrix of all tissues except : SiteRBCs.

CoAacid into acetyl pyruvicThe conversion of : Stepsis catalyzed by pyruvate dehydrogenase complex, which

composed of 3 enzymes act cooperative with each other

in presence of 5 co-enzymes: TPP, lipoic acid, FAD,

NAD+, and CoASH.

Pyruvate +TTP + Lipoic acid + CoA +FAD+ NAD+ --→ CO2 + Acetyl-CoA + NADH + H+

acid:pyruvicof decarboxylationSteps of oxidative •Pyruvate is decarboxylated to form a hydroxyethyl

derivative bound to the reactive carbon of thiamine

pyrophosphate, the coenzyme of pyruvate decarboxylase.

•The hydroxyethyl intermediate is oxidized by transfer to the

disulfide form of lipoic acid covalently bound to dithydrolipoyl

transactylase.

•The acetyl group, bound as a thioester to the side chain of

lipoic acid, is transferred to CoA.

•The sulfhydryl form of lipoic acid is oxidized by FAD-

dependent dihydrolipoyl dehydrogenase, leading to the

regeneration of oxidized lipoic acid.

•Reduced flavoprotein is reoxidized to FAD by dihydrolipoyl

dehydrogenase and NAD+.

OF DECARBOXYLATIONREGULATION OF OXIDATIVE

ACID :PYRUVIC

1- Product inhibition : The enzyme complex is inhibited by acetyl CoA, which

accumulates when it is produced faster than it can be oxidized by

citric acid cycle. The enzyme is also inhibited by elevated levels of

NADH+.H, which occure when the electron transport chain is

overloaded with substrate and oxygen is limited.

2- Covalent modification:The pyruvate dahydrogenase complex exists in two forms: an active

nonphosphorylated form and an inactive phosphorylated form.Phosphorylated

and nonphosphorylated pyruvate dehydrogenase can be interconverted by

two separate enzymes, a kinase and a phosphatase. The kinase is activated

by increase in the ratio of acetylCoA/ CoA or NADH/ NAD+. An increase in the

ratio of ADP/ATP, which signals increased demand for energy production ,

inhibits the kinase and allows the phosphatase to produce more of the active

,nonphosphorylated enzyme.

:METABOLISMPYRUVATECLINICAL ASPECTS OF

metabolism leads to lactic pyruvateInhibition of :acidosis, which may be due to

1- Arsenite or mercuric ions complex the –SH

group of lipoic acid.

2--Dietary deficiency of thiamin as in alcoholics.

These two factors lead to inhibition of pyruvate

dehydrogenase.

3- Inherited pyruvate dehydrogenase deficiency,

which may be due to defects in one or more of

the components of the enzyme complex.

CITRIC ACID CYCLE

CYCLE) KREB’SACID CYCLE (TRICARBOXYLIC

Def.:

It is the series of reactions in mitochondria, which

oxidized acetyl CoA to CO2, H2O & reduced H2 carriers

that oxidized through respiratory chains for ATP

synthesis.

Site:

Mitochondria of all tissue cells except RBCs, which

not contain mitochondria. The enzymes of the cycle

are present in mitochondrial matrix except succinate

dehydrogenase, which is tightly bound to inner

mitochondrial membrane.

Steps:

ENERGY PRODUCTION:

:ENERGY PRODUCTION FROM OXIDATION OF ONE MOLECULE OF GLUCOSE

REGULATION OF KREB’S CYCLE:

1- As the primary function of TCA cycle is to provide energy, respiratory

control via the E.T.C and oxidative phosphorylation exerts the main control.

2- In addition to this overall and coarse control, several enzymes of TCA

cycle are also important in the regulation.

Three key enzymes are:

(a)Citrate synthase. (b)Mitochondrial isocitrate dehydrogenase.

(c)α-ketoglutarate dehydrogenase.

These enzymes are responsive to the energy status as expressed by the [ATP]/[ADP] ratio and [NADH]/[NAD+] ratio.

(a)Citrate synthase enzymes is allosterically inhibited by ATP and long-chain

acyl CoA.

(b)NAD+-dependent mitochondrial iso-citrate dehydrogenase (ICD) is activated

allosterically by ADP and is inhibited by ATP and NADH.

(c)α-ketoglutarate dehyrogenase complex which allosterically inhibited by succinyl

CoA, NADH-H+ and ATP.

3- In addition to above succinate dehydrogenase enzyme is inhibited by oxaloacetate

(OAA) and the avability of OAA is controlled by malate dehydrogenase, which

depends on [NADH]/[NAD+] ratio.

CYCLEKREB’SFUNCTIONS OF •lipids CHO,It is the final pathway for complete oxidation of all foodstuffs -1

and protein, which are converted to acetyl CoA.

2- It is the major source of energy for cells except cells without mitochondria

as RBCs.

3- It is the major source of succinyl CoA, which used for:

1.Perphyrine and HB synthesis.

2.Ketone bodies activation.

3.Converted to OAA → glucose.

4. Detoxication by conjugation

•4- Synthetic functions of Kreb’s cycle:

a- Amphibolic reactions.

Some components of Kreb’s cycle are used in synthesis of other

substances as:

In fasting state, oxaloacetic acid is used for synthesis of glucose

by gluconeogenesis.

In fed state, citric acid is used for synthesis of fatty acids.

Reactions of Kreb’s cycle are used for synthesis of amino acid

(transamination into non essential amino acids) eg:

-OAA + glutamic acid aspartic acid + α-ketoglutarate.

-Pyruvic acid + glutamic acid alanine + α-ketoglutarate.

•b- Anaplerotic reactions.

Synthesis of one or more component of Kreb’s cycle from outside

the cycle:

O.A.A. can be synthesized from pyruvic acid by pyruvate

carboxylase, and from aspartic acid by transamination.

Fumarate can be synthesized from phenylalanine and tyrosine.

Succinyl CoA can be synthesized from valine, isoleucine,

methionine, and threonine.

α-ketogluterate can be synthesized from glutamic acid by

transamination.

Inhibitors of Citric Acid Cycle

1-Flouro-acetate reacts with oxalacetate forming

flourocitrate, which inhibits the aconitase enzyme.

2-Arsenite inhibits α-ketogluterate dehydrogenase.

3-Malonate acts as competitive inhibitor for succinate

dehydrogenase.

ROLES OF VITAMINS IN CITRIC ACID CYCLE

Four of the soluble vitamins of B complex have

important roles in cirtic acid cycle. They are:

-αin the form of FAD, a cofactor in ,riboflavin-1

ketogluterate dehydrogenase complex and in

in the form of ,niacin-2; dehydrogenasesuccinate

NAD, the coenzyme for three dehydrogenases in the

cycle, isocitrate dehydrogenase, α-ketogluterate

thiamin-3; dehydrogenasemalateand dehydrogenase

(vitamin B1), as TPP, the coenzyme for decarboxylation

in α-ketogluterate dehyrdogenase reaction; and

as part of coenzyme A, which , pantothenic acid-4

present in the form of acetyl-CoA and succinyl-CoA.

4-Pyruvic acidPyruvate carboxylase

2biotin CO2NADPH-HMalic acid

1-Pyruvic acidPyruvate carboxylase

ATP

CO2 ,biotin, Mn++

ADP

O.A.A

3-Acetyl CoA

Acetyl CoA

carboxylase

ATP CO2 ,Mn++ ,biotin ADP

Malonyl CoA

2- Propionyl CoA

Propionyl CoA carboxylase

ATP CO2 ,Mn++ ,biotinADP

D- methylmalonyl CoA → L-MMCoA

CAC ← Succinyl CoA

carboxylationfixation or 2COIt is an addition of CO2 to the molecule in presence of CO2, biotin, Mn++, ATP,

and specific carboxylase

- CO2 is produced by α – ketoglutarate dehydrogenase , isocitrate

dehydrogenase and pyruvate dehydrogenase complex examples for

carboxylation :-

5- Synthesis of carbomyl phosphate of urea cycle and pyrimidine.

6- Formation of C number 6 of purine.

7- Synthesis of H2CO3/NaHCO3 buffer system