Question

How is oxaloacetate regenerated to complete the TCA cycle?

Answer 1

The TCA cycle or Tricarboxylic Acid cycle is a series of chemical reactions taking place in the mitochondria of all aerobic organisms to release energy that are stored in the form of ATP by oxidation of Acetyl CoA derived from carbohydrates, fats and proteins. The TCA cycle is also called the Citric Acid cycle or Krebs cycle. There are many soluble enzymes involved in this cycle to catalyse the reactions.

Complete answer:
The TCA cycle is an eight-step pathway that plays an important role in the breakdown of organic molecules. Macromolecules like glucose, fatty acids and amino acids cannot directly enter the TCA cycle and thus are broken down first into two carbon compounds called Acetyl CoA. The Acetyl CoA enters the TCA cycle and undergoes a series of chemical reactions to produce carbon dioxide and energy. Every step of the pathway is catalysed by an enzyme.
The first reaction is the oxidation of pyruvate. Pyruvate obtained from glucose undergoes oxidation to produce acetyl CoA. Acetyl CoA is a two-carbon compound that enters the TCA cycle. It combines with a four-carbon compound called oxaloacetate to produce a six-carbon molecule called citrate and releases the CoA group.
This oxaloacetate molecule will be regenerated at the end of the cycle from malate. Malate dehydrogenase will catalyse this reaction and also during this reaction one molecule of NAD is reduced to NADH.

Note:
The end products of the citric acid cycle are two molecules of carbon dioxide, three molecules of NADH, three hydrogen ions, one molecule of FADH and one molecule of GTP. This cycle gives precursors of certain amino acids as well as the reducing agent NADH, that are used in various other reactions. In prokaryotic organisms like bacteria, the citric acid cycle is performed in the cytosol of the cell.