Ketimine from pyridoxal phosphate

Ketimine 121,744s. See also Schiff base from pyridoxal phosphate 742 as electron acceptor 746, 747 a-Ketoacid. See 2-Oxoacid Ketoamine 434s Ketodeoxyoctonate. See KDO Ketone(s), acidity of 46 Khorana, H. Gobind 84 Kidney cells, alkaline phosphatase in 645... 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

The /3 decarboxylation of aspartate (equation 7) proceeds by elimination of a /3-carbanionic intermediate like that in figure 10.4c/ from the ketimine, analogous to the intermediate produced by loss of the a proton from the aldimine of aspartate with pyridoxal-5 -phosphate. 

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