Kinetic and thermodynamic analysis of the interaction of cations with dialkylglycine decarboxylase.
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abstract
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(+), NH(4)(+), and Tl(+)) and three divalent cations (Mg(2+), Ca(2+), and Ba(2+)) have been studied. The catalytic rate constants for cation-bound enzyme (ck(cat) and ck(cat)/bK(AIB)) are cation-size-dependent, K(+) being the monovalent cation with the optimal size for catalytic activity. The divalent alkaline earth cations (Mg(2+), Ca(2+), and Ba(2+)) all give approximately 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 (bK(AIB)) varies with ionic radius. The larger cations (K(+), Rb(+), Cs(+), NH(4)(+), and Tl(+)) give smaller bK(AIB) ( approximately 4 mM), while smaller cations (Li(+), Na(+)) give larger values (approximately 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 aK(PLP). The divalent alkaline earth cations give aK(PLP) values approximately 10-fold higher than alkali cations of similar ionic radius. The cation dissociation constant for DGD-PLP-AIB-cation complexes (betaK(M)z+) 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 Ba(2+)) 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 approximately 24 s, while the half-life for dissociation of Rb(+) from the DGD-PLP-AIB-Rb(+) complex is approximately 12 min.
