Substrate-level phosphorylation encompasses certain chemical reactions that occur in human cells during glycolysis, the conversion of glucose leading to the production of two high energy molecules, referred to as adenosine triphosphate (ATP). This is accomplished through the chemical transfer of a phosphate group from each of two phosphoenolpyruvate (PEP) molecules, forming ADP, which is then transformed into ATP. In addition to ATP, glycolysis also yields two molecules of nicotinamide adenine dinucleotide (NADH) in reduced form and pyruvate, which is put into the next phase of cellular respiration.
ATP is the chief energy molecule used by cells, driving all processes occurring within it, of which substrate-level phosphorylation has a small, although important, role and is actually one of two ways ATP is produced from ADP in humans. Oxidative phosphorylation is the other mechanism necessary for making energy, most of which happens inside of the mitochondria of the cell. Often referred to as the powerhouse of the cell, the mitochondria is the organelle in which all stages of cellular respiration occur, except glycolysis. All steps in glycolysis, including substrate-level phosphorylation, happen in the cell's cytosol, fluid containing all cellular components such as the nucleus and ribosomes.
Cellular respiration in humans occurs aerobically and consists of four stages of reactions through which food is converted into ATP. Glycolysis is the beginning of the process of which substrate-level phosphorylation is the last step. Next, pyruvate from glycolysis is used to form acetyl coenzyme A, from which the waste product carbon dioxide is released. With the Krebs cycle, part of the coenzyme is used to make yet another chemical called citrate, more carbon dioxide is released as ATP, NADH, and another energy-yielding molecule referred to as flavin adenine dinucleotide (FADH2) are also end products. The last of these stages is the electron transport chain and chemiosmosis, by which energy taken from glucose, NADH, and FADH2 is used in the movement of hydrogen ions across the membrane of the mitochondria, as well as for the production of more ATP.
Pyruvate kinase is the enzyme responsible for catalyzing substrate-level phosphorylation. Other chemical reactions of glycolysis and subsequent cellular respiration phases involve the action of a certain enzyme, a protein necessary for controlling the speed of reactions, which is very important for meeting energy requirements of the human body, as it takes only a minute to consume all available ATP. Once the reaction is complete, the enzyme is recycled and used again.