Are alpha-ketoglutarate and glutamate involved in all transamination reactions?
Is it true that for all transamination reactions, that alpha-ketoglutarate and glutamate serve as the amino group acceptor and donor, respectively?
If this is true, then is it safe to assume that alpha-ketoglutamate is necessary for every transamination reaction?
In liver, the amino groups from many of the α-amino acids are transferred (by transamination reactions) to the α-carbon atom of α-ketoglutarate and in this way collected in the form of the amino group of L-glutamate molecules. The glutamate then functions as an amino group donor for most other amino acids or for excretion pathways that lead to the elimination of nitrogenous waste products. Thus, it is true that alpha-ketoglutarate and glutamate serve as the amino group acceptor and donor, respectively! HOWEVER, during glutamate catabolism, aspartate aminotransferase transfers amino groups from glutamate to oxaloacetate, forming aspartate and α-ketoglutarate. In this reaction, oxaloacetate is an acceptor of amino groups. Moreover, this reaction (and all transamination reactions) is reversible. It means that aspartate can be a donor of amino groups for α-ketoglutarate. It should be noted that in different biochemistry books, authors write that α-ketoglutarate and glutamate act as a “main”, “predominant” or “major” amino group acceptor and donor, respectively. However, they never use words “absolute”, “universal” or “obligatory” etc. Thus, I conclude that such strong expressions as “alpha-ketoglutarate and glutamate are involved in all transamination reactions” or “α-ketoglutarate is necessary for every transamination reaction” should be avoided. Finally, in in vitro enzyme reactions, other acceptors of amino groups (in addition to α-ketoglutarate) were shown to be possible.