Hexokinase:

Hexokinase, that catalyzes the first irreversible step of glycolysis, is inhibited through glucose 6-phosphate.   Therefore  when  PFK  is  inhibited and  fructose  6-phosphate builds  up and so does glucose  6-phosphate  because these two metabolites  are in equilibrium  through phosphoglucoisomerase. Hexokinase inhibition then reinforces the inhibition at the PFK step. At first sight this seems commonly since it is usually the first irreversible step of a pathway (the committed step) that is the main control step. On this basis, it may appear in which hexokinase should be the major control enzyme not PFK.  Moreover,   glucose 6-phosphate the product of the hexokinase reaction can also feed into glycogen synthesis or the pentose phosphate pathway. Therefore the first irreversible step which is exclusive to glycolysis is which catalyzed through PFK and therefore this is the major control step.