Pyruvate kinase also serves as a regulatory enzyme for gluconeogenesis, a biochemical pathway in which the liver generates glucose from pyruvate and other substrates. Gluconeogenesis utilizes noncarbohydrate sources to provide glucose to the brain and red blood cells in times of starvation when direct glucose reserves are exhausted.[12] During a fasting state, pyruvate carboxylase phosphorylates pyruvate kinase, inactivating the enzyme and therefore, preventing phosphoenolpyruvate from being converted into pyruvate. Instead, phosphoenolpyruvate is converted into glucose via a cascade of gluconeogenesis reactions.