Chapter 2

Citric Acid Cycle (Krebs)

Marco Brito-Arias


This is an essential process considered a second step on the respiratory chain, consisting of a series of eight reactions that will produce 8 electrons transported by 3 NADH/H+ and 1 FADH2 molecules, aside from an ATP molecule. <p> The cycle begins with the condensation of acetyl-CoA with oxaloacetate catalysed by citrate synthase to form citrate, a six-carbon molecule and CoA. Next, a dehydration reaction by aconitase occurs to produce cis-aconitate, which by the addition of a water molecule is converted to D-isocitrate. This citrate isomer is oxidized by NAD+ and transformed to α-ketoglutarate by isocitrate dehydrogenase, along with the formation of a second molecule of NADH/H+ and CO2. A condensing reaction between α-ketoglutarate and CoA catalysed by α- ketoglutarate dehydrogenase resulted in the formation of succinyl-CoA, NADH/H+ and CO2. The succinyl-CoA is hydrolysed by succinyl-CoA synthase to give succinate, CoA, and one ATP or GTP molecule. The succinate is oxidized to fumarate by succinate dehydrogenase in the presence of FAD as an electron acceptor to produce FADH2. Then, fumarate is converted to malate by the enzyme fumarase, and finally, oxidation of the hydroxyl group to the ketone by malate dehydrogenase to provide oxaloacetate and the third molecule of NADH/H+, completing the cycle (Fig. 51).

Total Pages: 34-56 (23)

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