Which of the following steps in the citric acid cycle does NOT produce reduced electron carriers?

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11ecba2d_0ebd_8aa2_a3bf_b7e7b6020efb_MD0008_00 The citric acid cycle is an oxidative process. The oxidation state of carbon can be estimated by the number of oxygen or sulfur bonds and the number of C=C double bonds. Increasing the number of these bonds increases the overall oxidation state. When one molecule is oxidized, another molecule must be reduced, and vice versa. Therefore, each of the four oxidative steps in the cycle yield reduced electron carriers, either in the form of NADH or FADH₂. The oxidative steps are: Conversion of isocitrate to α-ketoglutarate (Choice A). In this reaction, a single bond to oxygen (hydroxyl group) is oxidized to a ketone with two bonds to oxygen, and a carboxyl group leaves as CO₂. One NADH molecule is produced in this step. Conversion of α-ketoglutarate to succinyl-CoA (Choice B). In this step, the ketone that was formed in the first step is further oxidized by formation of a bond to sulfur, and another carboxyl group leaves as CO₂. Another NADH molecule is formed during this step. Conversion of succinate to fumarate (Choice D). In this step, a carbon is oxidized by forming a double bond with another carbon. Oxidation steps of this nature typically use FAD as a cofactor, and this step produces one FADH₂ molecule as a reduced electron carrier. Conversion of malate to oxaloacetate. In this step, a hydroxyl group is oxidized to a ketone, and a molecule of NADH is formed in the process. Conversion of succinyl-CoA to succinate does not involve oxidation because the C-S bond that breaks and the C-O bond that forms both have the same oxidation state. With no oxidation reaction, no reduction of electron carriers can occur. However, this conversion is still a high-energy step as it produces guanosine triphosphate (GTP), which is equivalent in energy to ATP. Educational objective: The citric acid cycle includes four oxidative steps. Each oxidative step is coupled to the reduction of an electron carrier, either NAD⁺ to NADH or FAD to FADH₂.

Which of the Following Steps in the Citric Acid Cycle