Principles of Regulation
Cellular energy metabolism adjusts to current physiological needs. The concentrations of specific metabolites serve as signals for energy deficit or excess. Key roles are played by the ATP/ADP ratio, which determines the energy charge, and the NADH/NAD+ ratio, which reflects the redox potential.
The primary allosteric activator of energy metabolism is ADP. Its accumulation signals energy consumption and activates common catabolic pathway enzymes, such as the pyruvate dehydrogenase complex (PDH) and isocitrate dehydrogenase. This ensures metabolic coordination between catabolism and the electron transport chain: the more ATP is consumed, the faster new molecules are synthesized.
Key Enzymes
The rate of the common catabolic pathway is controlled at four major reaction steps involving the following enzymes and complexes:
- Pyruvate dehydrogenase complex (PDH).
- Citrate synthase.
- Isocitrate dehydrogenase (the rate-limiting enzyme determining the overall cycle speed).
- $\alpha$-Ketoglutarate dehydrogenase complex.
Regulation of the PDH Complex
The pyruvate dehydrogenase complex is regulated by covalent modification, specifically phosphorylation and dephosphorylation. It includes regulatory subunits: a kinase and a phosphatase.
PDH regulatory mechanisms vary among tissues:
- In adipocytes (adipose tissue): Insulin increases mitochondrial calcium levels, which activates PDH phosphatase. The complex shifts to its active state, promoting the conversion of pyruvate to acetyl-CoA for fat synthesis and storage.
- In skeletal muscle: During muscle contraction, action potentials trigger a sharp rise in calcium concentration. $Ca^{2+}$ ions simultaneously inhibit PDH kinase and activate PDH phosphatase, rapidly converting it to the active form. This ensures prompt oxidation of acetyl-CoA and ATP synthesis for muscle work.
Regulation of TCA Cycle Enzymes
Tricarboxylic acid cycle enzymes also feature specific control mechanisms:
- Citrate synthase: Not an allosteric enzyme. Its activity depends on substrate (oxaloacetate) and product (citrate, which acts as an inhibitor) concentrations. When NADH is low, oxaloacetate formation accelerates, increasing the rate of citrate synthesis.
- Isocitrate dehydrogenase: Allosterically activated by ADP and calcium ions. Its activity heavily depends on the NADH/NAD+ ratio.
- $\alpha$-Ketoglutarate dehydrogenase complex: Structurally similar to PDH, but not regulated by phosphorylation. It is activated by calcium, inhibited by succinyl-CoA, and depends on ATP, ADP, NAD+, and NADH levels.