Lecture 25 Summary

The catabolism of amino acids is separable into a common pathway for amino groups and the pathways for the various carbon skeletons of the amino acids. The fates of these carbon skeletons generally converge to major metabolic intermediates: acetyl CoA (or acetoacetyl CoA), pyruvate, and intermediates of the citric acid cycle.

The citric acid cycle intermediate a-ketoglutarate is the acceptor of amino groups from most amino acids, forming glutamate and the a-keto acid corresponding to the amino acid donor. This aminotransferase reaction is dependent upon enzymes using pyridoxal phosphate (PLP) as a cofactor. The resulting a-keto acids that yield acetyl CoA or acetoacetyl CoA as a result of further degradation are termed ketogenic. Those that yield pyruvate or citric acid cycle intermediates are termed glucogenic. Glutamate is then oxidatively deaminated by glutamate dehydrogenase, yielding a-ketoglutarate and ammonium ion. The latter is assimilated into urea and excreted as such by most terrestrial vertebrates. A metabolic cycle called the urea cycle is linked to the citric acid cycle, and is responsible for the synthesis of urea.

Study questions

• Describe the fates of amino groups from excess amino acids, and the fates of the remaining carbon skeletons.

Page updated 12-27-06

References

1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) Ch.22, pp.649-657, 660-661, 666ff.