Analysis of a Novel Diacylglycerol Kinase from Dictyostelium discoideum: DGKA

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Abstract
Diacylglycerol kinases (DGKs) catalyze the ATP-dependent phosphorylation of diacylglycerols to generate phosphatidic acid and have been investigated in prokaryotic and eukaryotic organisms. Recently, a protein that is significantly similar to human DGK-θ, DGKA, was identified in Dictyosteliumdiscoideum. It has been shown to possess DGK activity when assayed using a medium-chain diacylglycerol, 1,2-dioctanoyl-sn-glycerol (DiC8). A complete understanding of DGK catalytic and regulatory mechanisms, as well as physiological roles, requires an understanding of its biochemical and kinetic properties. This report presents an analysis of these properties for DGKA. The enzyme catalyzes the phosphorylation of DiC8, and another medium-chain DAG, DiC6 (1,2-dihexanoyl-sn-glycerol), in a Michaelis−Menten manner. Interestingly, the kinetics of DGKA using physiologically relevant long-chain DAGs was dependent on substrate surface concentration and the detergent that was used. DGKA displayed Michaelis−Menten kinetics with respect to bulk substrate concentration (1,2-dioleoyl-sn-glycerol) in octyl glucoside mixed micelles when the surface substrate concentration was at or below 3.5 mol %. At higher surface concentrations, however, there was a sigmoidal relationship between the initial velocity and bulk substrate concentration. In contrast, DGKA displayed sigmoidal kinetics with respect to bulk substrate concentrations at all surface concentrations in Triton X-100 mixed micelles. Finally, we show the catalytic activity of DGKA was significantly enhanced by phosphatidylserine (PS) and phosphatidic acid (PA).

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