English: Control of glycolysis in muscle cells (simplified scheme). In muscles, primary control of glycolytic flux to pyruvate is mediated by phosphofructokinase (PFK), with secondary control at pyruvate kinase (PK). Activation of PFK by a low ATP/AMP ratio (indicating a low energy status of the cell) enhances the level fructose-1,6-bisophosphate (Fru-1,6-P2), which is a feed-forward allosteric activator of PK. PK is allosterically inhibited by ATP to slow glycolysis when the energy status is high. Intermediates of the tricarboxylic acid (TCA) cycle and the regulatory compound fructose-2,6-bisphosphate (Fru-2,6-P2) also control glycolytic enzymes (not shown). Control of glycolysis in other organisms and tissues can be very different. For example, glycolysis in plant leaves is controlled in a ‘bottom up’ manner. Abbreviations: DHAP, dihydroxyacetone phosphate; G3P, glyceraldehyde-3-phosphate; PEP, phosphoenolpyruvate.