Processes for Biosynthesis of Urea | Protein Metabolism

The following points highlight the three main processes for biosynthesis of urea. The processes are: 1. Transamination 2. Oxidative Deamination 3. Ammonia Formation and Transport.

Process # 1. Transamination:

a. Transamination involves inter-conversion of a pair of α-amino acids and a pair of α- keto acids and is catalysed by transami­nases or aminotransferases.

b. Pyridoxal phosphate (B₆ PO₄) is the co­enzyme essential for the transaminase ac­tivity.

c. Alanine-pyruvate transaminase (alanine transaminase) and glutamate-α-ketoglutarate transaminase (glutamate transami­nase) present in most mammalian tissues, catalyze the transfer of amino groups from most amino acids to form alanine (from pyruvate) or glutamate (from α-ketoglutarate) shown in Fig. 20.2.

d. Transamination is a reversible process. This reversibility allows transaminases to function in amino acid catabolism and bio­synthesis.

e. Each transaminase is specific for the speci­fied pair of amino acid and keto acid as one pair of substrates.

f. Most amino acids are substrates for transamination except lysine, threonine, proline and hydroxyproline. Not only δ- amino acid, δ-amino group of ornithine undergo transamination.

Process # 2. Oxidative Deamination:

a. Oxidative conversion of many amino ac­ids to their corresponding α-keto acids oc­curs in mammalian liver and kidney tis­sues.

b. Both L- and D-amino acid oxidase activi­ties occur in mammalian liver and kidney tissue and are also widely distributed in other animals and micro-organisms.

c. The amino acid oxidases are auto-oxidizable flavoproteins i.e. the reduced FMN or FAD is re-oxidized directly by molecu­lar oxygen forming H₂O₂ without the help of cytochromes or other electron carriers shown in Fig. 20.3. The toxic product (H₂O₂) is then split to O₂ and H₂O by catalase which occurs widely in tissues, espe­cially liver. If catalase is absent, the α- keto acid is decarboxylated by H₂O₂ form­ing a carboxylic acid with one carbon atom less.

In the amino acid oxidase reactions shown in Fig. 20.3, the amino acid is first dehydrogenated by the flavoprotein of the oxi­dase forming an α-imino acid which spontaneously decomposes to the correspond­ing α-keto acids by the addition of water.

d. Mammalian L-amino acid oxidase, an FMN-flavoprotein, is restricted to kidney and liver tissue and its activity is quite slow. Mammalian D-amino acid oxidase, an FAD-flavoprotein, occurs in the liver and kidney tissue of most mammals.

Significant Remarks:

i. If the enzyme catalase is absent geneti­cally, α-keto acid produced by oxidative deamination is decarboxylated by H₂O₂ forming a carboxylic acid with one car­bon atom less.

ii. The activity of mammalian L-amino acid oxidase is restricted to liver and kidney only and the activity of this enzyme is quite slow in these tissues.

iii. The enzyme L-amino acid oxidase has no effect on glycine.

iv. This enzyme does not have a major role in the catabolism of mammalian amino acid and formation of ammonia.

Process # 3. Ammonia Formation and Transport:

a. The amino groups of most amino acids are ultimately transferred to α-ketoglutarate by transamination. Release of this nitrogen as ammonia is catalyzed by L-glutamate dehydrogenase which is widely distributed in mammalian tissues. Certain hormones also influence gluta­mate dehydrogenase activity.

b. Glutamate dehydrogenase uses NAD⁺ or NADP⁺ as cosubstrate. The reaction is re­versible.

c. Intestinal bacteria produce ammonia from dietary protein as well as from the urea present in fluids secreted into the gas­trointestinal tract. This ammonia is ab­sorbed from the intestine into the portal venous blood. Under normal conditions, the liver promptly removes the ammonia from the portal blood. Minute quantities of ammonia are toxic to the central nerv­ous system.

The symptoms of ammonia intoxication include slurring of speech, blurring of vi­sion, a peculiar flapping tremor and in se­vere cases coma and death. These symp­toms occur when brain ammonia levels are increased. Ammonia is produced in the kidney from intracellular amino acid, glutamine, catalyzed by renal glutaminase.

Ammo­nia production by the kidney is highly increased in metabolic acidosis and de­pressed in alkalosis.

d. Ammonia is excreted as ammonium salts during metabolic acidosis but the major­ity is excreted as urea. Ammonia is present only in traces in blood (10-20 µg/100 ml) because it is rapidly removed from the cir­culation by the liver and converted to glutamine or urea.

e. Formation of glutamine is catalyzed by glutamine synthetase, a mitochondrial enzyme present in renal tissues in highest concentration, synthesis of glutamine is accompanied by the hydrolysis of ATP to ADP and Pi. The reaction is irreversible. Asparaginase and glutaminase are em­ployed as antitumor agents because cer­tain tumors require glutamine and asparagine.

f. In brain, the major mechanism for removal of ammonia is glutamine formation and in the liver, the most important pathway is urea formation. Brain tissue can form urea but this does not play a significant role in ammonia removal.