Final answer:
Under fasting conditions, pyruvate kinase activity is inhibited to conserve energy. This inhibition occurs through various mechanisms, including allosteric effects of ATP and regulation by fructose-1,6-bisphosphate, as well as covalent modifications such as phosphorylation.
Step-by-step explanation:
Under fasting conditions, the conversion of PEP to pyruvate by pyruvate kinase is inhibited. This inhibition is due to multiple regulatory mechanisms based on the energy and metabolic needs of the cell. If energy is not needed or if alanine levels are sufficient, pyruvate kinase activity decreases. Moreover, the enzyme's activity is modulated by several factors: it is increased in the presence of fructose-1,6-bisphosphate, an intermediate of glycolysis, indicating a demand for energy via increased glycolytic flux.
Additionally, the enzyme is subject to regulation via covalent modifications; phosphorylation by pyruvate kinase kinase leads to a less active form, whereas dephosphorylation reactivates it. Allosteric regulation by molecules such as ATP, which acts as a negative allosteric effector, also plays a role in controlling the activity of pyruvate kinase.