Question

What are the three primary control points in the citrate cycle with regard to energy charge in the cell?

Short answer

Answer: The three primary control points in the citrate cycle related to the cell's energy charge are citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. These enzymes regulate the cycle by being sensitive to the energy charge in the cell. When the energy levels are high in the cell, these enzymes are inhibited, slowing down the citrate cycle. Conversely, when the energy levels are low, the enzymes are activated, allowing the citrate cycle to proceed and generate more energy. This regulation helps maintain the energy balance of the cell.

Step-by-step solution

Understand the citrate cycle

The citrate cycle is a series of chemical reactions that generates energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into ATP and carbon dioxide. The cycle involves several reaction steps and intermediate products.

Identify the control points in the cycle

The control points are the enzymes that regulate the rate of the citrate cycle and are sensitive to the energy charge in the cell. The primary control points are: 1. Citrate synthase 2. Isocitrate dehydrogenase 3. α-ketoglutarate dehydrogenase

Explain the role of citrate synthase in controlling the cycle

Citrate synthase is the first enzyme in the citrate cycle and catalyzes the reaction between acetyl-CoA and oxaloacetate to form citrate. The enzyme's activity is inhibited by high levels of ATP, NADH, and citrate, indicating a high-energy charge in the cell. This feedback inhibition helps maintain the energy balance by slowing down the citrate cycle when the cell energy levels are adequate.

Explain the role of isocitrate dehydrogenase in controlling the cycle

Isocitrate dehydrogenase is an enzyme that catalyzes the oxidative decarboxylation of isocitrate, producing alpha-ketoglutarate, NADH, and CO2. This enzyme is allosterically regulated by energy charge molecules. High levels of ATP or NADH inhibit isocitrate dehydrogenase activity, while high levels of ADP or NAD+ activate it. This regulation ensures that the citrate cycle proceeds at a pace consistent with the cell's energy needs.

Explain the role of α-ketoglutarate dehydrogenase in controlling the cycle

α-ketoglutarate dehydrogenase is another enzyme involved in regulating the citrate cycle. It catalyzes the reaction between α-ketoglutarate and CoA, producing succinyl-CoA, NADH, and CO2. This enzyme is inhibited by high levels of ATP, NADH, and succinyl-CoA, indicating a high-energy charge in the cell. The inhibition of α-ketoglutarate dehydrogenase activity helps slow down the citrate cycle when the cell's energy levels are sufficient. In conclusion, the three primary control points in the citrate cycle with regard to energy charge in the cell are citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. These enzymes regulate the pace of the cycle in response to the cell's energy needs, ensuring efficient energy production.