Question

All the following steps of TCA cycle result in the ${\mathrm{NADH}}_2$ formation except in the step involving conversion of:

• A. α-KGA to succinyl CoA
• B. Malic acid to OAA
• C. Succinic acid to fumaric acid
• D. Isocitric acid to α-KGA

Answer 1

The correct option is C Succinic acid to fumaric acid

The TCA cycle is a central pathway that provides a unifying point for many metabolites, which feed in at various points. It takes place over eight different steps:

Step 1: Acetyl CoA (two carbon molecule) joins with oxaloacetate (4 carbon molecule) to form citrate (6 carbon molecule).

Step 2: Citrate is converted to isocitrate (an isomer of citrate)

Step 3: Isocitrate is oxidised to alpha-ketoglutarate (a five carbon molecule) which results in the release of carbon dioxide. One NADH molecule is formed.

Step 4: Alpha-ketoglutarate is oxidised to form a 4 carbon molecule. This binds to coenzyme A forming succinyl CoA. A second molecule of NADH is produced, alongside a second molecule of carbon dioxide.

Step 5: Succinyl CoA is then converted to succinate (4 carbon molecule) and one GTP molecule is produced.

Step 6: Succinate is converted into fumarate (4 carbon molecule) and a molecule of FADH₂ is produced.

Step 7: Fumarate is converted to malate (another 4 carbon molecule).

Step 8: Malate is then converted into oxaloacetate. The third molecule of NADH is produced.

An overview of TCA cyle: