Question

presence of malonate inhibits the activity of succinate dehydrogenase enzyme

Answer 1

Succinate Dehydrogenase or SDH is an enzyme of The Citric Acid cycle(TCA) and Electron Transport Chain(ETC).
Succinate is the ligand(substrate) for enzyme SDH, malonate competes with succinate to bind with the active site of SDH.
malonate is an inhibitor of SDH. Hence SDH becomes devoid of its usual substrate and the further process is inhibited because malonate does not produce same reaction as succinate.