The presence of glucokinase means that the liver is able to sequester excess glucose from the blood and convert it to glycogen for storage. This step is the second ATP investment of glycolysis. Note that this enzyme is named in the direction of ATP utilization, even though, under standard conditions, the strongly favored direction is ATP synthesis. Under physiological conditions, the enzyme operates close to equilibrium. Formation of ATP in this glycolytic reaction is an example of substrate-level phosphorylation , the generation of ATP without participation of the electron transport chain .
Pyruvate kinase is another important regulated enzyme of glycolysis .
In liver and muscle pyruvate can be converted to the amino acid alanine . This will become more apparent as we consider these other pathways. However, regulation of PFK and PK are good illustrations of the principles of metabolic regulation. Regulation of PFK-1 is allosteric . PFK-1 is activated by AMP and ADP, which both signal a low energy charge.