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Hexokinase is a transferase protein. It is the first enzyme in the glycolytic pathway that converts glucose into glucose-6-phosphate. This enzyme uses ATP to phosphorylate the 6-hydroxyl group of glucose, which is inhibited by its product, glucose-6-phosphate. It is also allosterically relieved of product inhibition by the addition of phosphate. Hexokinase controls the flow of glucose into energy metabolism in many mammalian tissues, such as blood cells, and brain tissue. Glucose-6-phosphate and glucose bind synergistically to hexokinase, as well as glucose and phosphate. This phosphate however plays a small role in the regulation of hexokinase during respiration. This is because gylcolysis is limited by the supply of glucose. During times of oxygen deprivation more ATP must come from glycolysis. This is because pyruvate forms lactic acid instead of entering the Kerb's cycle. As ATP phosphorylates glucose to glucose-6-phosphate, it increases its Gibbs free energy from -31 KJ to 14.3 KJ. This makes the reaction thermodynamically favorable. <ref> Worthingtin K. Hexokinase. Worthington Biochemical Corporation.2011; I.U.B.:2.7.1.1. [http://www.worthington-biochem.com/HK/default.html]</ref>