The second phase of glycolysis begins with the oxidation of two molecules of glyceraldehyde 3-phosphate. It is catalyzed by the enzyme glyceraldehyde phosphate dehydrogenase, which transfers a phosphate group to the sugar, forming 1, 3- bisphosphoglycerate. Two electrons and a proton released during the reaction are picked up by NAD plus, forming NADH. Phosphoglycerate kinase then transfers a phosphate group from each 1,3- bisphosphoglycerate to ADP, creating two molecules of ATP and 3-phosphoglycerate. Next, the enzyme phosphoglycerate mutase converts 3-phosphoglycerate into its isomer 2-phosphoglycerate. Further, the enzyme enolase removes a water molecule from each 2-phosphoglycerate to form two molecules of phosphoenolpyruvate or PEP. With the help of pyruvate kinase, the phosphate groups are removed from PEP and taken up by ADP, generating two more molecules of ATP, along with the end product, pyruvate. In this way, at the end of the energy releasing phase of glycolysis– four ATPs, two NADHs, and two pyruvate molecules are produced.