[Malate oxidation by mitochondrial succinate:ubiquinone-reductase]

Area of Science:

• Biochemistry
• Enzyme kinetics

Background:

• Succinate:ubiquinone reductase (SQR) is a key enzyme in the electron transport chain.
• SQR primarily catalyzes succinate oxidation.

Purpose of the Study:

• To investigate the potential malate dehydrogenase activity of SQR.
• To characterize the kinetics and products of malate oxidation by SQR.

Main Methods:

• Enzyme assays using L- and D-malate as substrates.
• Kinetic analysis (Vmax, Km) for malate oxidation.
• Inhibition studies using specific enzyme site inhibitors (N-ethylmaleimide, malonate, carboxin).
• Product identification (oxaloacetate).

Main Results:

• SQR catalyzes the oxidation of both L- and D-malate, albeit at rates two orders of magnitude lower than succinate.
• Kinetic constants were determined for both stereoisomers.
• Malate oxidation activity was inhibited by dicarboxylate-binding site inhibitors but not by a ubiquinone-binding site inhibitor.
• The enol form of oxaloacetate was identified as the oxidation product.
• Both ketone and enol forms of oxaloacetate are potent inhibitors of SQR.

Conclusions:

• SQR exhibits a low-level malate dehydrogenase activity.
• The active site for malate oxidation appears to involve the dicarboxylate-binding site.
• Oxaloacetate, the product of malate oxidation, effectively inhibits SQR activity.