Amino acid degradation

Most of absorbed dietary amino acids are catabolized by 2 subsequent

steps:

I- Removal of α-amino group: α-amino group is removed in the form of

ammonia (NH3). This occurs in most amino acids by transamination

followed by oxidative deamination of the resulting glutamate.

II- Breakdown of carbon skeleton:

Carbon skeleton (R side chain) is degraded to give either energy or some

compounds in the body

I- Removal of α-amino group:

The removal of amino group takes place in two steps which are Transamination (that produce glutamate) followed by oxidative deamination of the produced glutamate to give ammonia.

Transamination: is the transfer of α-amino group from α-amino acid to α-keto acid to yield α-keto acid of the original amino acid and a new amino acid. The enzymes that catalyze transamination are called transaminases or Aminotransferases. which need coenzyme pyridoxal phosphate (PLP)

General reaction is:

In most transamination reactions, the α-keto acid is α-ketoglutarate.

The most common examples on transaminases are:

1- Alanine Aminotransferase (ALT) or called Glutamate-Pyruvate Transaminase (GPT):

ALT

2- Aspartate Aminotransferase (AST) or called: Glutamate Oxaloacetate Transaminase (GOT).

Clinical significance of aminotransferases:

Aminotransferases are normally intracellular enzymes, and found only in low levels

in plasma. The presence of elevated plasma levels of aminotransferases indicates

damage of cells rich in these enzymes. e.g. ALT and AST are present in liver, so their

elevation in blood indicate liver cell damage such as in hepatitis, toxic injury,

cirrhosiss,……

AST

Oxidative deamination of the glutamate:

Glutamate produced from transamination is oxidatively deaminated by the enzyme glutamate dehydrogenase (GDH) yielding ammonia and regenerating α-ketoglutarate that is used in additional transamination reactions.

GDH is oxidoreductase enzyme that catalyze the oxidative deamination of glutamate (using NAD or NADP) and catalyze the reversal reduction reaction using NADH (or NADPH)

The produced ammonia is toxic to CNS (neurotoxic) and it must be

removed.

Ammonia (if increased) will react with α-ketoglutarate in the

presence of glutamate dehydrogenase enzyme (GDH) and

therefore interfere with Kreb’s cycle by consuming α-

ketoglutarate .

Disposal routes of NH3 OR Removal of ammonia:

1- excreted with urine.

2- react with glutamate yielding glutamine by the enzyme

glutamine synthetase

this reaction occurs primarily in brain, muscles and liver. This

reaction is the main disposal route of ammonia in brain

3- Used in formation of urea in liver. It is the most important

disposal route of ammonia.

Urea cycle or urea synthesis

Site: Urea is synthesized in the liver by 5 enzymes.

Steps of urea cycle

1)Ammonia reacts with CO2 and 2ATP in mitochondria to form

carbamoyl phosphate in the presence of carbamoyl phosphate

synthestae I.

2) Carbamoyl phosphate combines with ornithine to produce

citrulline which is then diffuse to cytoplasm

3) Aspartat e, carrying the second nitrogen atom of urea, enters the

cycle by condensing with citrulline to form argininosuccinate.

4) Argininosuccinate is cleaved to fumarate and arginine.

5) Arginine is further hydrolyzed to yield urea and regenerate the

ornithine

Urea cycle involves 5 steps: The first two reactions occur in

Mitochondria while the steps 3,4 and 5 occur in Cytoplasm).

The rate limiting step in the cycle is the first reaction which is the

formation of carbamoyl phosphate from CO2 and NH3 in the presence

of carbamoyl phosphate synthetase I (CPSI) which is the rate limiting

enzyme in the synthesis.

Regulation of urea cycle:

Carbamoyl phosphate synthetase-I is absolutely activated by N-

acetylglutamate.

Energy used for urea cycle: 3ATP

Overall reaction:

CO2 + NH3 + aspartate + 3ATP → Urea +fumarate

Sources of nitrogen atoms in urea formation:

One nitrogen is derived from ammonia, the second is derived from aspartate

The carbon of urea is derived from CO2

Fate of urea:

1- diffuse from liver, transported in blood to kidney and excreted with urine.

Urea is used as kidney function test, increased blood urea

indicate renal disease.

2- a portion diffuse from blood to intestine and cleaved by bacterial urease into

ammonia and CO2. Ammonia passes with stool.

Hyperammonemia

High ammonia decrease α-ketoglutarate and interfere with Kreb’s cycle so decrease ATP production

α-ketoglutarate + NH3 + NADH (or NADPH) → glutamate + NAD(P)

Ammonia toxicity in brain: low ATP COMA (a symptom of high ammonia)

Hyperammonemia:

1) Genetic (hereditary hyperammonemia):

Due to deficiency of one of the 5 enzymes of urea cycle leading to increased level of ammonia during the first week of birth leading to mental retardation.

Examples:a. Type I: Carbamoyl phosphate synthetase deficiencyb. Type II: Ornithine transcarbamoylase deficiency

Precaution: limiting protein in diet

2) Acquired hyperammonemia

-Is most commonly due to liver disease, especially in last stage where the capacity of live to synthesize urea is decreased

Symptoms of hyperammonemia:

Neurotoxic effects on CNS leading to:

-Tremors , slurring of speech, blurring of vision, behavioral changes

and cerebral edema

-At very high levels: coma and death

-A hyperammonemic coma occurs when serum ammonia

concentrations are greater than 300 μmol/L and is a medical

emergency requiring immediate treatment designed to prevent

irreversible brain damage.