Which substrate is essential for the activity of phosphofructokinase-1 (PFK-1)?

The essential substrate for the activity of phosphofructokinase-1 (PFK-1) is fructose 6-phosphate. PFK-1 is a key regulatory enzyme in the glycolytic pathway, responsible for phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate. This reaction is a crucial point of control for the pathway, making fructose 6-phosphate integral to the enzyme's function. The activity of PFK-1 is finely tuned by various factors, including energy status in the cell; however, the direct substrate required for its catalytic action is fructose 6-phosphate, affirming its central role in glycolysis.

ADP is important in the context of energy regulation but does not serve as a substrate for PFK-1. Glucose is upstream of this reaction and is converted into fructose 6-phosphate before it can enter the pathway. Pyruvate is a product of glycolysis rather than a substrate for PFK-1, as it is formed after the action of this enzyme. Thus, fructose 6-phosphate is the only correct choice that directly pertains to PFK-1's enzymatic activity.