The first irreversible reaction on a biochemical pathway is called the committed step. An example is the phosphorylation of fructose 6-phosphate to fructose 1-6-bisphosphate in glycolysis. It is usually the enzyme catalyzing the committed step which is the control element for the pathway. In glycolysis, this enzyme is phosphofructokinase, such an important enzyme that it definitely might find its way onto the MCAT. The original substrate is committed to the glycolytic pathway after phosporylation by phosphofructokinase. Other alternatives before phosphorylation included the pentose phosphate pathway (for generation of NADPH and five carbon sugars) or polymerization into glycogen.
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