Gluconeogenesis is the formation of glucose from non-carbohydrate sources and occurs mainly in the liver and to a limited extent in the kidney and small intestine. It utilizes many of the same enzymes as glycolysis but must bypass three essentially irreversible steps. This is accomplished through hydrolysis reactions catalyzed by glucose-6-phosphatase and fructose-1,6-bisphosphatase. The bypass of pyruvate kinase involves two enzymes, pyruvate carboxylase and PEP carboxykinase, which convert pyruvate to oxaloacetate and then to PEP using ATP and GTP as energy sources.