Question

Citric Acid Cycle.

I am a little confused on all the Carbons #...

1) what carbons from glucose are in acetyl CoA ?

Glucose --> Pyruvate (4,5,6) SO... acetyl CoA is carbon 4,5,6 ?

2) What glucose carbons are removed as CO2 in the citric acid cycle ?

Carbon 1 ???

3) Are any carbons from acetyl-CoA removed as CO2 during the first TURN of cytric acid cycle ? explain ?

NO. because it is claisen condensation not oxidative decarboxylation rxn so CoA-SH was removed. and methyl group for acetyl-CA turn to methylene in citrate ???

Thank you for your HELP :)

Answer 1

The citric acid cycle is a key metabolic pathway that unifies carbohydrate, fat, and protein metabolism. The reactions of the cycle are carried out by 8 enzymes that completely oxidize acetyl-CoA into two molecules of carbon dioxide. Through catabolism of sugars, fats, and proteins, a two-carbon organic product acetate in the form of acetyl-CoA is produced which enters the citric acid cycle. The reactions of the cycle also converts three equivalents of nicotinamide adenine dinucleotide (NAD⁺) into three equivalents of reduced NAD⁺ (NADH), one equivalent of flavin adenine dinucleotide (FAD) into one equivalent of FADH₂, and one equivalent each of guanosine diphosphate (GDP) and inorganic phosphate (P_i) into one equivalent of guanosine triphosphate (GTP). The NADH and FADH₂ generated by the citric acid cycle are in turn used by the oxidative phosphorylation pathway to generate energy-rich adenosine triphosphate (ATP).

One of the primary sources of acetyl-CoA is from the breakdown of sugars by glycolysis which yield pyruvate that in turn is decarboxylated by the enzyme pyruvate dehydrogenase generating acetyl-CoA according to the following reaction scheme:

• CH₃C(=O)C(=O)O^? (pyruvate) + HSCoA + NAD⁺ ? CH₃C(=O)SCoA (acetyl-CoA) + NADH + CO₂

The product of this reaction, acetyl-CoA, is the starting point for the citric acid cycle. Acetyl-CoA may also be obtained from the oxidation of fatty acids. Below is a schematic outline of the cycle:

• The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).
• The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO₂. The carbons lost as CO₂ originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO₂ requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they might not be lost, since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules.^[9]
• Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD⁺, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.
• Electrons are also transferred to the electron acceptor Q, forming QH₂.
• At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.
• I hope this will help you out.