Question

BIOCHEMISTRY QUESTION

Examine each of the reactions of the citric acid cycle. What happens in each step? Compare this to the name of each enzyme. How does the description compare to the name?

Answer 1

reactions of citric acid cycle

Oxidative decarboxylation of pyruvate to acetyl Co A by pyruvate dehydrogenase complex.

1 Formation of citrate - krebs cycle proper starts with condensation of acetyl coA and oxaloacetate to synthesise citrate by enzyme citrate synthase.

2 and 3 citrate is isomerised to isocitrate.

Two stage reaction of dehydration followed by hydration through formation of intermediate cis aconitase. Aconitase is a lyase enzymes. Lyases are enzymes that break various bonds by means other than hydrolysis and oxidation to form new compound. In this step of TCA cycle dehydration occurs where hydrogen molecule is removes from citrate to form cis aconitate and also to form isocitrate from cis aconitase. It does not use hydrolysis or oxidation reactions. Hence the enzyme aconitase.

4 and 5. Formation of alpha ketoglutarate.

Enzyme isocitrate dehydrogenase catalyses conversion of isocitrate to oxalosuccinate and then to alpha keto glutamate. Co2 is released. In the chemical formula of isocitrate hydrogen atom is removed from it and double bond to oxygen is formed in oxalosuccinate ( dehyrogenation) hence the enzyme isocitrate dehydrogenase.

6. Alpha KG to succinyl Co. A. occurs by oxidative decarboxylation by the enzyme alpha keto glutamate dehydrogenase.

7 formation of succinate.

Succinate Coa converted to succinate by succinate thiokinase. Thiokinase enzymes are ligase enzymes which synthesise Co A thioesters... Ligase enzymes couple two large molecules to form . Succinate thiokinase helps in coupling of the conversion of succinyl Coa to succinate. First step involves displacement of CoA from succinyl CoA to form succinyl phosphate , the enzyme then removes phosphate from succinyl Co A to form succinate. Thus coupling two large molecules. Hence the name thiokinase.

8 Conversion of succinate to fumarate.

The enzyme eliminates 2 Hydrogen atoms grok succinate to form fumarate. Hence a dehydrogenase.

9 formation of malate

Fumarase enzyme forms malate from Fumarase by addition of H20. Reversible hydration or dehydration step.

10. Malate to oxaloacetatemalate dehydrogenase catalyses oxidation of malate to OAA. By removing hydrogen molecule (since oxidation means removal of hydrogen) . So dehydrogenase.