Amino acid metabolism

The important reaction commonly employed in the breakdown

of an amino acid is always the removal of its -amino group.

The product ammonia is excreted after conversion to urea or

other products and the carbon skeleton is degraded to

CO2 releasing energy.

Metabolism of amino acids differs, but 3 common reactions:

– Transamination

– Deamination

– Decarboxylation

2

Transamination

In transamination

• Amino acids are degraded in the liver.

• An amino group is transferred from an amino acid

to an -keto acid, usually -ketoglutarate.

• The reaction is catalyzed by a transaminase or

aminotransferase.

•

• A new amino acid, usually glutamate, and a new

-keto acid are formed.

Transamination reactions

Enzymatic Transamination

• Typically, -ketoglutarate

accepts amino groups

• L-Glutamine acts as a

temporary storage of nitrogen

• L-Glutamine can donate the

amino group when needed for

amino acid biosynthesis

• All aminotransferases rely on

the pyridoxal phosphate

cofactor

Amino Group Transfer - Aminotransferase

Enzymatic removal of -amino groups (transaminase

/aminotransferases - named for amino donor

i.e. Ala aminotranferase removes amino group from Ala)

• Ping-pong

kinetics of

aspartate

transaminase (next slide)

(from previous slide)

Transamination

The 3-C -keto acid pyruvate is produced from alanine, cysteine, glycine, serine, & threonine.

Alanine deamination via Transaminase directly yields pyruvate.

alanine -ketoglutarate pyruvate glutamate

Aminotransferase (Transaminase)

COO

CH2

CH2

C

COO

O

CH3

HC

COO

NH3+

COO

CH2

CH2

HC

COO

NH3+

CH3

C

COO

O + +

The 4-C Krebs Cycle intermediate oxaloacetate is produced from aspartate & asparagine.

Aspartate transamination yields oxaloacetate.

Aspartate is also converted to fumarate in Urea Cycle. Fumarate is converted to oxaloacetate in Krebs cycle.

aspartate -ketoglutarate oxaloacetate glutamate

Aminotransferase (Transaminase)

COO

CH2

CH2

C

COO

O

COO

CH2

HC

COO

NH3+

COO

CH2

CH2

HC

COO

NH3+

COO

CH2

C

COO

O + +

The Amino

Group is

Removed

From All

Amino

Acids First

12

Oxidative Deamination

Oxidative deamination

• Removes the amino group as an

ammonium ion from glutamate.

• Provides -ketoglutarate for

transamination.

Oxidative Deamination • Glutamate formed by transamination reactions

is deaminated to -ketoglutarate

• Glutamate dehydrogenase - NAD+ or NADP+ is coenzyme

• Other AA oxidases - (liver, kidney) low activity

It is one of the few enzymes that can use NAD+ or NADP+ as e acceptor.

Oxidation at the -carbon is followed by hydrolysis, releasing NH4

+.

OOC

H2

CH2

C C COO

O

+ NH4+

NAD(P)+

NAD(P)H

OOC

H2

CH2

C C COO

NH3+

Hglutamate

-ketoglutarate

Glutamate Dehydrogenase

H2O

Glutamate Dehydrogenase catalyzes a major reaction that effects net removal of N from the amino acid pool.

Summarized above:

The role of transaminases in funneling amino N to glutamate, which is deaminated via Glutamate Dehydrogenase, producing NH4

+.

Amino acid -ketoglutarate NADH + NH4+

-keto acid glutamate NAD+ + H2O

Transaminase Glutamate Dehydrogenase

Non-oxidative deamination

• Amino acids such as serine and

histidine are deaminated non-oxidatively

• The other reactions involved in the

catabolism of amino acids are

decarboxylation,

• transulfuration, desulfuration, dehydration

etc.

DEAMIDATION

• The amino acid, which contains an amide

linkage with ammonia at the γ-carboxyl,

degraded by process of deamidation.

• E.g, conversion of asparagine to aspartate

by removal of alpha amino group.

Decarboxylation

The decarboxylation process is important since the products of

decarboxylation

• reactions give rise to physiologically active amines.

The enzymes, amino acid decarboxylases are pyridoxal phosphate

dependent enzymes.

• Pyridoxal phosphate forms a Schiff's base with the amino acid so

as to stabilise the -carbanion formed by the cleavage of bond

between carboxyl and -carbon

• atom.

•

• The physiologically active amines epinephrine, nor-epinephrine,

dopamine,

• serotonin, -amino butyrate and histamine are formed through

decarboxylation of the corresponding precursor amino acids.

Transmethylation

• Resynthesis of methionine:

• Homocysteine accepts a methyl group from N5-

methyltetrahydrofolate (N5-methyl-THF)

requiring methylcobalamin, a coenzyme derived

from vitamin B12.

•

• The methyl group is transferred from the B12

derivative to homocysteine, and cobalamin is

• recharged from N5-methyl-THF.

Excretory

Forms of

Nitrogen

Fate of Individual Amino Acids

• Seven to acetyl-CoA – Leu, Ile, Thr, Lys, Phe, Tyr, Trp

• Six to pyruvate – Ala, Cys, Gly, Ser, Thr, Trp

• Five to -ketoglutarate – Arg, Glu, Gln, His, Pro

• Four to succinyl-CoA – Ile, Met, Thr, Val

• Two to fumarate – Phe, Tyr

• Two to oxaloacetate – Asp, Asn

Summar

y of

Amino

Acid

Cataboli

sm

NITROGEN BALANCE

Nitrogen balance = nitrogen ingested - nitrogen excreted

(primarily as protein) (primarily as urea)

Nitrogen balance = 0 (nitrogen equilibrium)

protein synthesis = protein degradation

Positive nitrogen balance

protein synthesis > protein degradation

Negative nitrogen balance

protein synthesis < protein degradation

UREA CYCLE

Detoxification

Of ammonia ()

REGULATION OF UREA CYCLE:

N-Acetylglutamate is an essential activator for carbamoyl phosphate

synthetase I—the rate-limiting step in the urea cycle

N-Acetylglutamate is synthesized from acetyl coenzyme A and glutamate by

N-acetylglutamate synthase in a reaction for which arginine is an activator.

UREMIA

urea and other waste products, are retained in the blood.

Early symptoms include anorexia , lethargy ,fatigue, nausea, vomiting, cold,

bone pain, itch, shortness of breath,, and late symptoms can include decreased

mental acuity and coma. when the glomerular filtration rate, a measure of

kidney function, is below 50% of normal.[2]

Uremia can also result in uremic pericarditis. There are many dysfunctions

caused by uremia affecting many systems of the body, such as blood (lower

levels of erythropoietin), sex (lower levels of testosterone/estrogen), and bones

(osteoporosis and metastatic calcifications).

Uremia can also cause decreased peripheral conversion of T4 to T3, producing

a functionally hypothyroid state.

Azotemia:

Refers to high levels of urea, but is used primarily when the abnormality

can be measured chemically but is not yet so severe as to produce

symptoms.

Uremia is the pathological manifestations of severe azotemia.

Hyperammonemia

The capacity of the hepatic urea cycle exceeds the normal rates of

ammonia generation .

The levels of serum ammonia are normally low (5–35 μmol/L).

CAUSES:

liver function is compromised, due to genetic defects of the urea cycle or

liver disease.

blood levels can rise above 1,000 μmol/L.

Such hy per ammon emia is a medical emergency, because ammonia

has a direct neurotoxi effect on the CNS.