Succinic dehydrogenase oxidised the succinate to fumarate. In eukaryotes succinate dehydrogenase is tightly bound to the inner mitochondrial membrane, in prokaryotes to the plasma membrane. Electrons pass from succinate through the FAD and iron-sulphur centres before entering the chain of electron carriers in the mitochondrial inner membrane Malonate, an analogue of succinate is a strong competitive inhibitor of succinate dehydrogenase and therefore, blocks the activity of citric acid cycle.
In Krebs cycle the reversible hydration of fumarate to malate is catalysed by fumarase enzyme.
In the last reaction of Krebs cycle NAD-linked L-malate dehydrogenase catalyses the oxidation of L-malate to oxaloacetate. The isocitrate dehydrogenase in Krebs cycle catalyses oxidative decarboxylation of isocitrate to form $\alpha$-ketoglutarate, which in next step through oxidative decarboxylation converted to succinyl Co-A.