JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II Year BIOCHEMISTRY Previous Question Paper (March-2021 - R17)

The Citric Acid Cycle (also known as the Krebs cycle or TCA cycle) is a series of eight enzymatic reactions occurring in the mitochondrial matrix. It serves as the final common pathway for the oxidation of carbohydrates, lipids, and proteins.

The cycle begins with the condensation of Acetyl-CoA (2C) with Oxaloacetate (4C) to form Citrate (6C), catalyzed by citrate synthase. Through a series of steps involving isomerization, oxidative decarboxylation, and substrate-level phosphorylation, the cycle regenerates oxaloacetate, releasing two molecules of CO2 per turn.

The steps are as follows:

1. Citrate is converted to Isocitrate via Aconitase.
2. Isocitrate undergoes oxidative decarboxylation to α-Ketoglutarate (releasing NADH and CO2) via Isocitrate dehydrogenase.
3. α-Ketoglutarate undergoes oxidative decarboxylation to Succinyl-CoA (releasing NADH and CO2) via α-Ketoglutarate dehydrogenase complex.
4. Succinyl-CoA is converted to Succinate (producing GTP/ATP) via Succinyl-CoA synthetase.
5. Succinate is oxidized to Fumarate (producing FADH2) via Succinate dehydrogenase.
6. Fumarate is hydrated to Malate via Fumarase.
7. Malate is oxidized back to Oxaloacetate (producing NADH) via Malate dehydrogenase.

The energetics of one turn of the cycle (oxidation of one Acetyl-CoA) are as follows:

Total yield per Acetyl-CoA is 10 ATP. When considering the complete oxidation of one molecule of glucose (which yields two Acetyl-CoA), the cycle contributes 20 ATP to the total energy yield of cellular respiration.

Oxidative phosphorylation is the metabolic process in mitochondria where the oxidation of nutrient molecules releases energy, which is used to synthesize ATP. It involves the transfer of electrons from NADH and FADH2 to oxygen through the electron transport chain (ETC), coupled with the phosphorylation of ADP to ATP.

The mechanism involves two coupled processes:

1. Electron Transport Chain: Electrons pass through four protein complexes (I-IV) embedded in the inner mitochondrial membrane. As electrons move, protons are pumped from the matrix into the intermembrane space, creating an electrochemical gradient (proton-motive force).
2. Chemiosmosis: The accumulated protons flow back into the matrix through the enzyme ATP synthase. This flow drives the rotation of the enzyme, catalyzing the phosphorylation of ADP to ATP.

Inhibitors of oxidative phosphorylation block specific stages of the process:

• ETC Inhibitors: Rotenone and Amytal block Complex I; Antimycin A blocks Complex III; Cyanide, Carbon monoxide, and Azide block Complex IV, preventing electron flow.
• ATP Synthase Inhibitors: Oligomycin binds to the F0 subunit, preventing proton flow and thus inhibiting ATP synthesis.
• Uncouplers: Compounds like 2,4-Dinitrophenol (DNP) dissipate the proton gradient by allowing protons to leak across the membrane, uncoupling electron transport from ATP synthesis.

Substrate-level phosphorylation is the direct formation of ATP or GTP by the transfer of a phosphoryl group from a high-energy metabolic intermediate to ADP or GDP. Unlike oxidative phosphorylation, it does not require oxygen or the electron transport chain. Examples include the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in glycolysis and the conversion of succinyl-CoA to succinate in the citric acid cycle.