Pyridoxal phosphate (PLP) dependent enzymes are ubiquitous in nitrogen metabolism and catalyze many medically important transformations. As a group, they catalyze an extraordinarily wide variety of reactions, and a fundamental, unresolved question that bears on the design of enzyme inhibitors is how a given apoenzyme determines a unique reaction specificity. Dialkylglycine decarboxylase (DGD) is an unusual PLP dependent enzyme that rapidly catalyzes both decarboxylation and transamination in its normal catalytic cycle, thereby providing a vehicle for incisive studies on the control of enzymatic reaction specificity. High resolution X-ray structures for DGD are available, and reveal two alkali metal ion-specific binding sites. One, near the active site, is responsible for the activating effects of large (K+, Rb+) and inhibitory effects of small (Na+, Li+) ions. The elucidation of the mechanisms by which DGD discriminates between and is catalytically controlled by alkali metals has very broad physiological significance. DGD is structurally representative of a medically important class of aminotransferases capable of acting alternately on primary amines and alpha-amino acids.
The specific aims of this project are to test the hypotheses that: 1) C-C (decarboxylation) and C-H (transamination) bond cleavage occur via a single substrate binding subsite in the DGD active site, which activates these bonds by large stereoelectronic effects; 2) DGD specifically catalyzes oxidative vs. non-oxidative decarboxylation by proceeding through a concerted transition state in which CO2 loss and proton transfer occur simultaneously; 3) Active site structural changes in Ser80 and Tyr301 caused by exchange of activating for inhibitory ions control catalytic activity; 4) The ability of several aminotransferases to act in alternative half-reactions on primary amines and alpha-amino acids is largely determined by two active site residues, Glu215 and Va1244.
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