CBSE Class 10 Answered

• Acetyl CoA (2C) reacts with oxaloacetic acid (4C) to form citric acid (6C).
• The reaction takes place in the presence of citrate synthetase.
• A molecule of CoA is released during this reaction.

 2.   Isomerisation

• Citric acid is isomerised into isocitric acid in the presence of enzyme aconitase. 

• Isocitric acid undergoes oxidative decarboxylation in the presence of enzyme isocitrate dehydrogenase and Mn²⁺ ions to form an intermediate called oxalosuccinic acid. 
• Oxalosuccinic acid is further converted into the 5-carbon α-ketoglutaric acid. 

• α-Ketoglutaric acid (5C) undergoes simultaneous dehydrogenation and decarboxylation to form the 4-carbon compound Succinyl CoA. 
• The reaction takes place in the presence of α-ketoglutarate dehydrogenase complex. 

• Succinyl CoA (4C) is converted into succinic acid (4C) in the presence of succinyl CoA synthetase thiokinase.
• GDP is phosphorylated to form GTP.

• Succinic acid (4C) undergoes dehydrogenation and releases two hydrogen atoms to form fumaric acid (4C) in the presence of succinic dehydrogenase.
• The hydrogen atoms are accepted by FAD and it is reduced to FADH₂.

• A molecule of water is added to fumaric acid (4C) to form malic acid (4C).
• The reaction takes place in the presence of fumarase. 

• Malic acid (4C) releases two hydrogen atoms to form oxaloacetic acid (4C) in the presence of malic dehydrogenase.
• The released hydrogen atoms are accepted by NAD⁺ and it is reduced to NADH + H⁺.

 Summary Equation for the Krebs Cycle

• During the Krebs cycle, the following molecules are synthesised:
  • 8 molecules of NADH₂
  • 2 molecules of FAH₂
  • 2 molecules of ATP (GTP)