3. Removal of amino groups of amino acids during degradation.

Amino acid degradation in mammals usually starts with the loss of the a-amino nitrogens group, and finishes with the return of the carbon skeletons to the central metabolic pathways.
2 major routes to remove the a-amino nitrogen group (deamination): 
The universal acceptor of the a-amino group is the a-ketoglutarate.
There is no net loss of a-amino group because another amino acid is formed.
Major function of the transamination reactions in amino acid catabolism is to funnel the amino nitrogen group to one or few amino acids (such asglutamate).
Glutamate, after collected all the a-amino nitrogen group, then plays a role in the net conversion of amino group to ammonia.
A mechanism of glutamate deamination is then needed to actual remove the a-amino group and to re-generate the a-ketoglutarate for further transamination reactions.
Glutamate + NAD+ + H2O ---> a-Ketoglutarate + NH4+ + NADH
Oxidative deamination is catalyzed by a NAD+-linked enzyme, called glutamate dehydrogenase (in mitochondria where NAD is readily available).
This reaction is reversed in the biosynthesis of amino acid glutamate (in cytosol where NADH is abundant).

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