Purification and Characterisation of Malate Dehydrogenase From Synechocystis sp. PCC 6803: Biochemical Barrier of the Oxidative Tricarboxylic Acid Cycle

Abstract

Cyanobacteria possess an atypical tricarboxylic acid (TCA) cycle with various bypasses. Previous studies have suggested that a cyclic flow through the TCA cycle is not essential for cyanobacteria under normal growth conditions. The cyanobacterial TCA cycle is, thus, different from that in other bacteria, and the biochemical properties of enzymes in this TCA cycle are less understood. In this study, we reveal the biochemical characteristics of malate dehydrogenase (MDH) from <i>Synechocystis</i> sp. PCC 6803 MDH (<i>Sy</i>MDH). The optimal temperature of <i>Sy</i>MDH activity was 45-50°C and <i>Sy</i>MDH was more thermostable than MDHs from other mesophilic microorganisms. The optimal pH of <i>Sy</i>MDH varied with the direction of the reaction: pH 8.0 for the oxidative reaction and pH 6.5 for the reductive reaction. The reductive reaction catalysed by <i>Sy</i>MDH was activated by magnesium ions and fumarate, indicating that <i>Sy</i>MDH is regulated by a positive feedback mechanism. The <i>K</i>_m-value of <i>Sy</i>MDH for malate was approximately 210-fold higher than that for oxaloacetate and the <i>K</i>_m-value for NAD⁺ was approximately 19-fold higher than that for NADH. The catalytic efficiency of <i>Sy</i>MDH for the reductive reaction, deduced from <i>k</i>_cat-values, was also higher than that for the oxidative reaction. These results indicate that <i>Sy</i>MDH is more efficient in the reductive reaction in the TCA cycle, and it plays key roles in determining the direction of the TCA cycle in this cyanobacterium.