Question

6. (10 pts) For the following step in the citric acid cycle, COO H COO CH Succinate dehydrogenase + FAD + FADH CH, OOC H COO Succinate Fumarate a. What class of enzyme catalyzes this reaction? b. How does this step ultimately contribute to the formation of ATP? Be specific, c. Knowing that the next step is a hydration of the alkene, briefly explain why this step needed to occur? That is, why not hydrate the succinate?

Answer 1

a) Succinate dehydrogenase is a subclass of the class of enzymes called oxidoreductases. So, this reaction is catalysed by an oxidoreductase class enzyme.

b) In this step FAD gets converted into FADH₂ and that FADH₂ produces two ATP during ETC. It gives up its electron to Complex II, bypassing Complex I. By bypassing Complex I, we missed a chance to pump protons across the membrane, so less protons have been pumped by the time we get to Complex IV. Protons still have been pumped enough to fuel 2 ATP created by ATP synthase. If it didn't bypass Complex I then it would produce 3 ATP just as NADH.

c) There are two reasons for which this step should occur

I) For the formation of FADH₂ because it would lead to the formation of two ATP molecules

II) Succinate does not contain any double bond so by hydrating succinate malate production is not possible