Question

What happens in the citric acid cycle?

Answer 1

Tricarboxylic Acid Cycle, also known as citric acid cycle. The TCA cycle is a metabolic pathway that connects carbohydrate, fat and protein metabolism. There are eight enzymes that carry out the reactions in the cycle. These enzymes completely oxidize acetate (a two carbon molecule), in the form of acetyl-CoA, into two molecules of carbon dioxide and water each.

Complete answer:
At the starting of this cycle the acetyl group is condensed with oxaloacetic acid (OAA) and water to yield citric acid. The enzyme citrate synthase catalyzes this reaction and a molecule of CoA is released. Then the molecule of citrate isomerized to isocitrate and this reaction is followed by two successive steps of decarboxylation, leading to the formation of alpha-ketoglutaric acid and then succinyl-CoA. In the remaining steps, succinyl CoA is oxidized to oxaloacetic acid. The cycle continues and during the formation of succinic acid from succinyl CoA a molecule of GTP is synthesized and this step of formation of GTP is a substrate level phosphorylation. After that, in a coupled reaction GTP is converted to GDP. With this there is simultaneous synthesis of ATP from ADP. There are three step in the cycle where $NA{{D}^{+}}$ is reduced to $NADH + {{H}^{+}}$ and one point where $FA{{D}^{+}}$ is reduced to $FAD{{H}_{2}}$. There is a requirement of the continued replenishment of oxalic acid, the first member of the cycle for the continuous oxidation of acetyl CoA via the TCA cycle. It also requires the generation of $NA{{D}^{+}}$ and $FA{{D}^{+}}$ form $NADH$ and $FAD{{H}_{2}}$ respectively. The equation for the phase of respiration in this process is as follows:$Pyruvic\text{ }acid\text{ }+\text{ }4NA{{D}^{+}}\text{ }+\text{ }FA{{D}^{+}}\text{ }+\text{ }2{{H}_{2}}O\text{ }+\text{ }ADP\text{ }+\text{ }Pi\text{ }\to \text{ }3C{{O}_{2}}+\text{ }4NADH\text{ }+\text{ }4{{H}^{+}}\text{ }+\text{ }FAD{{H}_{2}}\text{ }+\text{ }ATP.$This whole reaction is taking place inside mitochondrial matrix.

Note:
The citric acid cycle is also known as the Krebs cycle. It is at the center of the cellular metabolism. It plays a starring role in the process of energy production and biosynthesis. It ends up the sugar-breaking job started in the process of glycolysis. It fuels the production of ATP in the whole process.