Question

What is the role of the citric acid cycle? In metabolism and in biosynthesis?

Answer 1

The importance of the citric acid cycle in the metabolism

The citric acid cycle is the second of three stages of cellular respiration, in which glucose, fatty acids and certain amino acids, the so-called fuel molecules, are oxidized .

The oxidation of these molecules is primarily used to transform the energy contained in these molecules into ATP.

ATP provides for example energy for muscle contractions and can therefore be referred to as "energy currency" of the cells.

The citric acid cycle is a key metabolic pathway that connects carbohydrate, fat, and protein metabolism.

The reactions of the cycle are carried out by eight enzymes that completely oxidize acetate, in the form of acetyl-CoA, into two molecules each of carbon dioxide and water.

Through catabolism of sugars, fats, and proteins, the two-carbon organic product acetyl-CoA (a form of acetate) is produced which enters the citric acid cycle.

The reactions of the cycle also convert 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 ATP.

The importance of the citric acid cycle in the biosynthesis:

Some of the citric acid cycle are intermediates for other important reactions like the biosynthesis of glucose, fatty acids and amino acids.

For example, most of the carbon atoms in porphyrins come from succinyl CoA. Many of the amino acids are derived from α-ketoglutarate and oxaloacetate.