VCAC: Cellular Processes: The Citric Acid Cycle - The Reactions: Advanced Look: Reactions 1-4

The Citric Acid Cycle involves eight chemical reactions, here we will look at the first four of these reactions. Clicking on each of the thumbnail images will bring up a larger, labeled version of the described scene.

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	In eukaryotic cells, the citric acid cycle takes place in the matrix, or inner fluid, of the mitochondrion.
	The citric acid cycle is also called the tricarboxylic acid (TCA) cycle and the Krebs cycle.
	The first stage of the citric acid cycle is catalyzed by an ezyme called citrate synthase and requires acetyl CoA and oxaloacetate as substrates.
	During stage one, the two-carbon acetyl group from acetyl CoA is transferred to the four-carbon oxaloacetate to form a six-carbon citrate and CoA.
	Stage two of the cycle is catalyzed by an aconitase enzyme and begins with citrate as the substrate.
	Stage two involves two structural changes to the substrate. First a water molecule is removed from the citrate compound to form cis-aconitate.
	Finally the cis-aconitate is rehydrated to form isocitrate.
	Stage three of the citric acid cycle is catalyzed by an enzyme called isocitrate dehydrogenase, and begins with isocitrate and NAD+ as substrates.
	The oxidation and decarboxylation reactions of stage three result in the production of one carbon dioxide, one H+ ion, one NADH, and a five-carbon α-ketoglutarate.
	Stage four of the cycle is catalyzed by an α-ketoglutarate dehydrogenase enzyme and begins with CoA, NAD+, and α-ketoglutarate as substrates.
	During stage four, the CoA becomes succinyl-CoA, the NAD+ becomes NADH, and another carbon dioxide is produced, along with one H+ ion.

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