Computer graphics image of an ATP molecule bound to phosphoglycerate kinase, a glycolytic enzyme. In glycolysis, glucose is converted to pyruvate with the production of ATP: in other words, a catabolic reaction provides metabolic energy (as ATP) at the expense of the degradation of food molecules (glucose). Kinase enzymes catalyse the transfer of phosphoryl groups from one substrate (usually ATP) to another. In this example of horse phosphoglycerate kinase, the alpha carbon backbone appears as a green ribbon. The ATP molecule is indicated in red with a purple Van der Waal's (bonding) surface.