Amino acid degradation glutamate dehydrogenase

Figure 8.6 The three dehydrogenase (oxidase) reactions in amino acid degradation. The enzymes are D-amino acid oxidase, glutamate dehydrogenase and proline oxidase (dehydrogenase). Biochemical details are given in Appendix 8.4.

In general, amino acid degradation begins with deamination. Most deamination is accomplished by transamination reactions, which are followed by oxidative deaminations that produce ammonia. Although most deaminations are catalyzed by glutamate dehydrogenase, other enzymes also contribute to ammonia formation. Ammonia is prepared for excretion by the enzymes of the urea cycle. Aspartate and CO, also contribute atoms to urea. 

Glutamate is synthesized from NH4 + and a-ketoglutarate, a citric acid cycle intermediate, by the action of glutamate dehydrogenase. We have already encountered this enzyme in the degradation of amino acids (Section 23.3.1). Recall that NAD+ is the oxidant in catabolism, whereas NADPH is the reductant in biosyntheses. Glutamate dehydrogenase is unusual in that it does not discriminate between NADH and NADPH, at least in some species. 

F. 20.19. Major anaplerotic pathways of the TCA cycle. 1 and 3 (blue arrows) are the two major anabohc pathways. (1) Pyruvate carboxylase (2) Glutamate is reversibly converted to a-ketoglutarate by transaminases (TA) and glutamate dehydrogenase (GDH) in many tissues. (3) The carbon skeletons of valine and isoleucine, a 3-carbon unit from odd chain fatty acid oxidation, and a number of other comprounds enter the TCA cycle at the level of succinyl CoA. Other amino acids are also degraded to fumarate (4) and oxaloacetate (5), principally in the liver. 

The NHg-group of amines and other amino acids may be transferred by transamination (C 5) to x-ketoglutaric acid. Hence most of the ammonia liberated by the degradation of amino acids and amines is formed by glutamic acid dehydrogenase. 

The reactions of amino compounds, considered so far, have been largely concerned with the production of essential cell constituents firom ingested material. The remaining section is concerned with those reactions by which excess amino compounds are converted first to ammonia and then to common excretion products. The division between biosynthetic and degradative reactions is not clear cut as can be seen from the fact that the reaction catalysed by the enzyme glutamic dehydrogenase produces ammonia and has already been discussed in sections II and IV.B. Oxidative deamination of the D-amino acids has also been discussed (section V.A) because the reverse of this reaction appears to be the main route by which o-amino acids are formed in microorganisms but it is also believed to be the... 

---