Ornithine transamination

The major pathway for catabolism of arginine is arginase to form ornithine, followed by ornithine transamination and oxidation of the resulting glutamic semialdehyde to form glutamate. For the most part, this catabolism occurs... 

Transamination is not restricted to a-amino groups. The 5-amino group of ornithine—but not the e-amino group of lysine—readily undergoes transamination. Serum levels of aminotransferases are elevated in some disease states (see Figure 7-11). 

In the branched-chain amino acids (Val, Leu, He) and also tyrosine and ornithine, degradation starts with a transamination. For alanine and aspartate, this is actually the only degradation step. The mechanism of transamination is discussed in detail on p. 178. 

Muscle activity involves processes such as aerobic and anaerobic glycolysis and is therefore accompanied by an increased pyruvate production. Consequently, the pyruvate transamination product alanine will be increased after exercise. Heavy exercise may be associated with an increased need of creatine biosynthesis from arginine. Ornithine is a by-product of this pathway and may be increased under these conditions. 

In some cases, the function of the metal ion is more to deactivate alternative sites of reaction than to activate a particular atom towards attack by an electrophile. A good example of this is seen in the transamination reaction of ornithine (5.12) with urea. Co-ordination of the ornithine to copper(n) results in the formation of a five-membered chelate ring, leaving the amino group of the 3-aminopropyl substituent as the most nucleophilic site in the complex. Reaction of this complex with urea results in a transamination process and the formation of the copper(n) complex of the substituted urea, which is the amino acid citrulline (5.13) (Fig. 5-20). The complex may be demetallated to yield the free amino acid in respectable yields. 

Glutamic y-semialdehyde cyclizes with a loss of H2O in a nonenzymatic process to give A i-pyrroline-5-carboxylate, which is reduced by NADPH to proline. Alternatively, the semialdehyde can be transaminated to ornithine, which is converted in several steps into arginine (Section 23.4.1). 

Arginine, via three reactions of the urea cycle, can be derived from ornithine, which is produced by transamination of glutamate semialdehyde. 

Arginine can be cleaved by arginase in the liver to form urea and ornithine. Ornithine can be transaminated to glutamate semialdehyde, which can be oxidized to glutamate. 

B. Glutamate semialdehyde is transaminated to form ornithine. Glutamate provides the... 

All of the amino acids except lysine, threonine, proline, and hydroxyproline participate in transamination reactions. Transaminases exist for histidine, serine, phenylalanine, and methionine, but the major pathways of their metabolism do not involve transamination. Transamination of an amino group not at the a-position can also occur. Thus, transfer of 3-amino group of ornithine to a-ketoglutarate converts ornithine to glutamate-y-semialdehyde. 

In a recent optimization study, an alternative coupling system has been developed [29 c], The chemical yield in the synthesis of two non-proteinogenic a-amino acids was remarkably improved by applying a coupled transamination process using additionally an ornithine co-aminotransferase to couple L-ornithine co-trans-amination to L-glutamate a-transamination [29 c],... 

Canaline is the product of the hydrolytic cleavage of canavanine with the simultaneous formation of urea. Canaline is an ornithine analogue which also shows neurotoxicity in the adult sexta where it adversely affects central nervous system functions (jj ). It also is a potent inhibitor of vitamin B -containing enzymes (20-22). It forms a stable Schiff base with the pyridoxal phosphate moiety of the enzyme and drastically curtails enzymatic activity. Pyridoxal phosphate-containing enzymes are vital to insects because they function in many essential transamination and decarboxylation reactions. Ornithine is an important metabolic precursor for insect production of glutamic acid and proline (23). 

Other methods can also be used for driving the transamination reaction to produce amino adds in high yields. For example, if L-lysine or L-ornithine are used as the donor in the two-enzyme process shown in Fig. 12.7-9, the cyclization of the aldehyde is strongly favored, creating an essentially irreversible reaction that can lead to high yields of a desired amino add from the corresponding 2-keto-... 

Glutamate is a central amino acid in general amino-acid metabolism. It plays a major role in transamination, ammonia production, formation of ornithine, proline, glutamine, and g-amino butyric acid (GABA). 

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