Dialkylglycine decarboxylase (DGD) is a tetrameric pyridoxal phosphate (PLP)-dependent enzyme that catalyzes both decarboxylation and transamination in its normal catalytic cycle. Its activity is dependent on cations. Metal-free DGD and DGD complexes with seven monovalent cations (Li+, Na +, K+, Rb+, Cs+, NH4 +, and T1+) and three divalent cations (Mg2+, Ca2+, and Ba2+) have been studied. The catalytic rate constants for cation-bound enzyme (ckcat and ckcat/bK AIB) are cation-size-dependent, K+ being the monovalent cation with the optimal size for catalytic activity. The divalent alkaline earth cations (Mg2+, Ca2+, and Ba2+) all give ∼10-fold lower activity compared to monovalent alkali cations of similar ionic radius. The Michaelis constant for aminoisobutyrate (AIB) binding to DGD-PLP complexes with cations (bKAIB) varies with ionic radius. The larger cations (K+, Rb+, Cs+, NH 4 +, and T1+) give smaller bKAIB (∼4 mM), while smaller cations (Li+, Na+) give larger values (∼10 mM). Cation size and charge dependence is also found with the dissociation constant for PLP binding to DGD-cation complexes (aK PLP). K+ and Rb+ possess the optimal ionic radius, giving the lowest values of aKPLP. The divalent alkaline earth cations give aKPLP values ∼10-fold higher than alkali cations of similar ionic radius. The cation dissociation constant for DGD-PLP-AIB-cation complexes (βKMz+) was determined and also shown to be cation-size-dependent, K+ and Rb+ yielding the lowest values. The kinetics of PLP association and dissociation from metal-free DGD and its complexes with cations (Na+, K+, and Ba2+) were analyzed. All three cations tested increase PLP association and decrease PLP dissociation rate constants. Kinetic studies of cation binding show saturation kinetics for the association reaction. The half-life for association with saturating Rb+ is ∼24 s, while the half-life for dissociation of Rb+ from the DGD-PLP-AIB-Rb+ complex is ∼12 min.
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