Metabolism of ornithine and arginine

See also Table 5.1, Genetic Code, Urea, Ornithine, Citrulline, Glycine, Creatine, Creatine Phosphate, Metabolism of Ornithine and Arginine, Essential Amino Acids... 

See also Metabolism of Ornithine and Arginine, S-Adenosylmethionine and Biological Methylations, AdoMet... 

The 5-carbon skeleton of glutamic acid gives rise directly to those of proline, ornithine, and arginine. The reactions are outlined in Fig. 24-9. Arginine, in turn, is involved in the urea cycle, which is shown in detail in Fig. 24-10. Arginine is also a biosynthetic precursor of the polyamines. Another important biosynthetic product of glutamate metabolism is 8-aminolevulinate, a precursor to porphyrins (Eq. 24-44) in some organisms.139... 

Citrulline takes its name from the watermelon genus (Citrullus) in which it was first found in 1930. It was also discovered the same year as a bacterial degradation product of arginine. Krebs, who elucidated the form of the urea cycle, demonstrated that citrulline was the intermediate between ornithine and arginine. The urea cycle was the first metabolic cycle to be discovered. In Krebs words, it revealed a new pattern of the organization of metabolic processes. ... 

Putrescine tetramethylenediamine, a ubiquitous Polyamine (see), formed by decarboxylation of ornithine and, in some organisms, by decarboxylation of arginine to agmatine, followed by cleavage to P. and urea. It is the precursor of spermine and spermidine in ordinary metabolism, and is essential for cell division. It accumulates during bacterial degradation of arginine. Increased protein decomposition (e. g. in cholera) leads to the appearance of P. in urine and feces. 

Cystinuria is a hereditary disease characterized by the excessive excretion of cystine, lysine, ornithine, and arginine in the urine, probably resulting from a deficiency in the renal tubular transport mechanism. Sir Archibald E. Garrod postulated that cystinuria resulted from a metabolic block involving the oxidation of cystine to sulfate. Later investigations of the pathogenesis of cystinuria demonstrated that the hereditary deficiency does not involve a metabolic block. If a metabolic impairment existed, cystine would be expected to accumulate in the plasma of cystinurics, but plasma levels of cystine are normal or low in cys-... 

A number of legume plants, especially seeds of tropical legumes, but also seeds of common vetch species, contain structural analogues of arginine, which may interfere with its metabolism (metabolism of ornithine) in the urea cycle. An example of these amino acids is a unique amino acid i-canavanine (see 2-44), which was first isolated from Jack beans Canavalia ensiformis), probably native to South America, which is cultivated for its edible pods and seeds, or as a reclamation plant and for green manure. The concentration in jack bean seeds is around 50g/kg of dry matter. Later, this amino acid was identified in a number of other legumes. 

Certain amino acids and their derivatives, although not found in proteins, nonetheless are biochemically important. A few of the more notable examples are shown in Figure 4.5. y-Aminobutyric acid, or GABA, is produced by the decarboxylation of glutamic acid and is a potent neurotransmitter. Histamine, which is synthesized by decarboxylation of histidine, and serotonin, which is derived from tryptophan, similarly function as neurotransmitters and regulators. /3-Alanine is found in nature in the peptides carnosine and anserine and is a component of pantothenic acid (a vitamin), which is a part of coenzyme A. Epinephrine (also known as adrenaline), derived from tyrosine, is an important hormone. Penicillamine is a constituent of the penicillin antibiotics. Ornithine, betaine, homocysteine, and homoserine are important metabolic intermediates. Citrulline is the immediate precursor of arginine. 

Figure 31-3. Arginine, ornithine, and proline metabolism. Reactions with solid arrows all occur in mammalian tissues. Putrescine and spermine synthesis occurs in both mammals and bacteria. Arginine phosphate of invertebrate muscle functions as a phosphagen analogous to creatine phosphate of mammalian muscle (see Figure 31-6).

D-Arginine and D-ornithine metabolism D-Alanine metabolism Glutathione metabolism Alkaloid biosynthesis I Alkaloid biosynthesis II Metabolism of Complex Carbohydrates Starch and sucrose metabolism Biosynthesis and degradation of glycoprotein... 

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