<P> From measuring the physiological concentrations of metabolites in an erythrocyte it seems that about seven of the steps in glycolysis are in equilibrium for that cell type . Three of the steps--the ones with large negative free energy changes--are not in equilibrium and are referred to as irreversible; such steps are often subject to regulation . </P> <P> Step 5 in the figure is shown behind the other steps, because that step is a side - reaction that can decrease or increase the concentration of the intermediate glyceraldehyde - 3 - phosphate . That compound is converted to dihydroxyacetone phosphate by the enzyme triose phosphate isomerase, which is a catalytically perfect enzyme; its rate is so fast that the reaction can be assumed to be in equilibrium . The fact that ΔG is not zero indicates that the actual concentrations in the erythrocyte are not accurately known . </P> <P> The four regulatory enzymes are hexokinase, glucokinase, phosphofructokinase, and pyruvate kinase . The flux through the glycolytic pathway is adjusted in response to conditions both inside and outside the cell . The internal factors that regulate glycolysis do so primarily to provide ATP in adequate quantities for the cell's needs . The external factors act primarily on the liver, fat tissue, and muscles, which can remove large quantities of glucose from the blood after meals (thus preventing hyperglycemia by storing the excess glucose as fat or glycogen, depending on the tissue type). The liver is also capable of releasing glucose into the blood between meals, during fasting, and exercise thus preventing hypoglycemia by means of glycogenolysis and gluconeogenesis . These latter reactions coincide with the halting of glycolysis in the liver . </P> <P> In animals, regulation of blood glucose levels by the pancreas in conjunction with the liver is a vital part of homeostasis . The beta cells in the pancreatic islets are sensitive to the blood glucose concentration . A rise in the blood glucose concentration causes them to release insulin into the blood, which has an effect particularly on the liver, but also on fat and muscle cells, causing these tissues to remove glucose from the blood . When the blood sugar falls the pancreatic beta cells cease insulin production, but, instead, stimulate the neighboring pancreatic alpha cells to release glucagon into the blood . This, in turn, causes the liver to release glucose into the blood by breaking down stored glycogen, and by means of gluconeogenesis . If the fall in the blood glucose level is particularly rapid or severe, other glucose sensors cause the release of epinephrine from the adrenal glands into the blood . This has the same action as glucagon on glucose metabolism, but its effect is more pronounced . In the liver glucagon and epinephrine cause the phosphorylation of the key, rate limiting enzymes of glycolysis, fatty acid synthesis, cholesterol synthesis, gluconeogenesis, and glycogenolysis . Insulin has the opposite effect on these enzymes . The phosphorylation and dephosphorylation of these enzymes (ultimately in response to the glucose level in the blood) is the dominant manner by which these pathways are controlled in the liver, fat, and muscle cells . Thus the phosphorylation of phosphofructokinase inhibits glycolysis, whereas its dephosphorylation through the action of insulin stimulates glycolysis . </P>

What is the key regulatory enzyme in glycolysis
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