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a-Ketoisocaproate © |

Isovaleryl CoA

P-Methylcrotonyl CoA

P-Methylcrotonyl CoA

Acetyl CoA Acetoacetate

Fig. 4.13. Degradation of leucine. Enzymes: 1, branched-chain aminotransferase; 2, decarboxyl 3s8. 3, dehydrogenase.

Acetyl CoA Acetoacetate

Fig. 4.13. Degradation of leucine. Enzymes: 1, branched-chain aminotransferase; 2, decarboxyl 3s8. 3, dehydrogenase.

Lysine -

a-Keto-e-aminocaproate

Pipecolate

-► Saccharopine

a-Aminoadipate-5-semialdehyde a-Aminoadipate a-Ketoadipate

Acetyl CoA

Fig. 4.14. Degradation of lysine. Enzymes: 1, L-amino acid oxidase; 2, specific aminotransferase.

Cysteine

3- Me rcaptopy ruvate

Cysteine sulphinate p-Sulphinylpyruvate

Pyruvate

Fig. 4.15. Catabolism of cysteine. Enzymes: 1, cysteine dioxygenase; 2, transaminase; 3, transaminase.

intermediate in the urea cycle, enabling mammals to dispose of waste nitrogen. However, interest in arginine has recently been revived following demonstration of its involvement in the production of polyamines and nitric oxide. Together with glycine, arginine also contributes to the formation of creatine. In poultry, waste nitrogen is disposed of as uric acid, a purine synthesized from glycine, glutamine and aspartate. The latter two amino acids are involved again in the biosynthesis of the pyrimidine ring, whereas glycine is the starting molecule in the production of haem. In addition, both glycine and aspartate act as inhibitory neurotransmitters. Tyrosine and tryptophan are precursors in

Ala,Gly,Cys,Ser,Thr

Ala,Gly,Cys,Ser,Thr

CITRIC ACID CYCLE

Fig. 4.16. Gluconeogenesis and ketogenesis from amino acids. Asn, asparagine; Asp, aspartate; Cys, cysteine; Gin, glutamine; Glu, glutamate; Gly, glycine; His, histidine; lie, isoleucine; Leu, leucine; Lys, lysine; Met, methionine; Phe, phenylalanine; Pro, proline; Ser, serine; Thr, threonine; Trp, tryptophan; Tyr, tyrosine; Val, valine. Enzymes: 1, malate dehydrogenase; 2, citrate synthase; 3, aconitase; 4, isocitrate dehydrogenase; 5, a-ketoglutarate dehydrogenase complex; 6, succinyl CoA synthetase; 7, succinate dehydrogenase; 8, fumarase.

the production of neuroactive biogenic amines. Tryptophan is also a source of the B-complex vitamin, nicotinamide. Finally, the involvement of methionine in polyamine syn thesis and in the activated methyl cycle imposes competing metabolic demands, particularly when the tissue supply of cysteine is critical.