Understanding the TCA Cycle

Is there "substrate level" phosphorylation in TCA cycle?

Answer explanation

Yes, there is substrate-level phosphorylation in the TCA cycle. Specifically, it occurs during the conversion of succinyl-CoA to succinate, where GTP (or ATP) is produced directly.

What happens to Pyruvate?

Answer explanation

Pyruvate can undergo different fates: it is converted to acetyl-CoA for entry into the Krebs cycle under aerobic conditions, or it is reduced to lactate in anaerobic conditions, allowing for continued ATP production.

Conversion of pyruvate to acetyl-CoA

Answer explanation

The conversion of pyruvate to acetyl-CoA occurs in the mitochondria, catalyzed by the pyruvate dehydrogenase complex. This process involves decarboxylation and the reduction of NAD+, producing CO2 and NADH.

The citric acid cycle (CAC) also known as the TCA cycle or the Krebs cycle

Answer explanation

The citric acid cycle, also known as the TCA cycle or Krebs cycle, is a key metabolic pathway that plays a crucial role in cellular respiration, converting carbohydrates, fats, and proteins into carbon dioxide and water, while generating energy.

Each step in the cycle (including the link reaction → conversion of pyruvate to acetyl-CoA) occurs TWICE for every glucose molecule.

Answer explanation

Each glucose molecule produces two pyruvate molecules during glycolysis. Therefore, the subsequent steps, including the link reaction, occur twice to process both pyruvate molecules into acetyl-CoA.

What is the product of glycolysis that enters the TCA cycle?

• (S)-citryl-CoA

• Citrate

• Isocitrate

• α-Ketoglutarate

Answer explanation

The product of glycolysis that enters the TCA cycle is (S)-citryl-CoA, formed when acetyl-CoA combines with oxaloacetate. Citrate is produced subsequently, but (S)-citryl-CoA is the direct entry product.

Which enzyme catalyzes the reaction of citrate to isocitrate?

• Citrate synthase

• Aconitase

• Isocitrate dehydrogenase

• Succinyl-CoA synthetase

Answer explanation

Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate in the citric acid cycle, facilitating the isomerization of citrate.

What are the products of the reaction catalyzed by α-Ketoglutarate dehydrogenase complex?

• CO2, NADH, Succinyl-CoA

• Citrate, NADH, GTP

• Isocitrate, FADH2, CO2

• Fumarate, GTP, NADH

Answer explanation

The α-Ketoglutarate dehydrogenase complex catalyzes the conversion of α-ketoglutarate to succinyl-CoA, producing CO2 and NADH as byproducts. Thus, the correct products are CO2, NADH, and Succinyl-CoA.

What is the role of ATP in the regulation of the TCA cycle?

• Inhibitor

• Activator

• Substrate

• Product

Answer explanation

ATP acts as an inhibitor in the TCA cycle by signaling that the cell has sufficient energy. High levels of ATP reduce the activity of key enzymes, thus slowing down the cycle when energy is abundant.

What is the final product of the TCA cycle?

• Oxaloacetate

• Citrate

• Succinyl-CoA

• Fumarate

Answer explanation

The final product of the TCA cycle is oxaloacetate, which is regenerated at the end of the cycle. It combines with acetyl-CoA to initiate the cycle again, making it crucial for the continuation of cellular respiration.