Final answer:
Under fasting conditions, the formation of PEP from pyruvate is a critical process in gluconeogenesis, which predominantly occurs in the liver. Enzymes such as pyruvate carboxylase and PEPCK are involved in converting pyruvate to PEP, enabling glucose production during periods of low glucose availability.
Step-by-step explanation:
Under fasting conditions, the formation of phosphoenolpyruvate (PEP) from pyruvate is a key step in gluconeogenesis. During starvation or when fasting, glucose availability is low and the body needs to produce glucose to supply essential tissues such as the brain. Gluconeogenesis is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate substrates, including pyruvate. The gluconeogenic pathway predominantly occurs in the liver and, to a lesser extent, in the kidney.
The enzyme that catalyzes the conversion of pyruvate to PEP is pyruvate carboxylase for the initial step that yields oxaloacetate, which is then converted into PEP by phosphoenolpyruvate carboxykinase (PEPCK). This series of reactions is critical during fasting because it allows for the production of glucose, which can be released into the bloodstream and used by various organs for energy, particularly when glycogen stores are depleted.
It's important to note that while glycerol from the breakdown of triglycerides can also be used in gluconeogenesis, it cannot directly form PEP. Instead, it is initially converted to dihydroxyacetone phosphate and then to glyceraldehyde-3-phosphate, both intermediates of glycolysis, which can be utilized in the gluconeogenic pathway.