Krebs Cycle or Citric Acid Cycle MCQ - Practice Questions with Answers

• Krebs’ cycle is called so after the scientist Hans Krebs who first elucidated it in the 1930s in pigeon flight muscles.
• It is called the citric acid cycle because the first intermediate formed is citric acid or citrate.
• It is called tricarboxylic acid cycle because citric acid or citrate and isocitrate are tricarboxylic acids.
• It takes place in the matrix of mitochondria.
• The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
• Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion
• For each glucose molecule, the link reaction and Kreb’s cycle occur twice. 
   

Step 1: Formation of Citrate
This is a condensation step, combining the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.

Step 2: Formation of Isocitrate
The citrate is rearranged to form an isomeric form, isocitrate by an enzyme aconitase. This reaction is inhibited by fluoroacetate

Step 3: Oxidation of Isocitrate to α-Ketoglutarate
In this step, isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to form α-ketoglutarate. It generates NADH from NAD

Step 4: Oxidation of α-Ketoglutarate to Succinyl-CoA
α-Ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl-CoA. During this oxidation, NAD+ is reduced to NADH + H+. The enzyme that catalyzes this reaction is α-Ketoglutarate dehydrogenase. This reaction is inhibited by arsenite dehydrogenase

Step 5: Conversion of Succinyl-CoA to Succinate
CoA is removed from succinyl-CoA to produce succinate. The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by substrate-level phosphorylation. GTP can then be used to make ATP. The enzyme succinyl-CoA synthase catalyzes this reaction of the citric acid cycle.