Figure 16.13 The regulation of glycolysis in muscle. At rest (left), glycolysis is not very active (thin arrows). The high concentration of ATP inhibits phosphofructokinase (PFK) and pyruvate kinase, while glucose 6-phosphate inhibits hexokinase. Glucose 6-phosphate is converted into glycogen (Chapter 25). During exercise (right), the decrease in the ATP/AMP ratio resulting from muscle contraction activates phosphofructokinase and hence glycolysis. The flux down the pathway is increased, as represented by the thick arrows.