The substrates for the next reaction in a pathway are typically the products of the preceding enzyme-catalyzed reaction. Enzymes like hexokinase and phosphofructokinase regulate glycolysis, which is a biochemical pathway, and the cell's ATP levels indicate whether the pathway should proceed.
In biochemical pathways, substrates and enzymes interact to produce a sequence of reactions leading to a final product. According to the information given, substrate A is converted to product D with intermediate products B and C. In this kind of pathway, after each enzyme-catalyzed step, the product of one reaction becomes the substrate for the next reaction. For example, if we consider the glycolysis pathway, phosphoenolpyruvate is a substrate that, through the action of pyruvate kinase, gets converted into pyruvic acid and generates ATP via substrate-level phosphorylation.
Enzymes like hexokinase and phosphofructokinase are key regulatory points in the glycolysis pathway, functioning according to cell energy needs represented by ATP, ADP and AMP levels. The activity of these enzymes signals whether the cell should continue energy production via additional reactions in the pathway.
Additionally, the investigation of enzyme kinetics using analogs like acetylthiocholine (ASCh) can be a method to study enzyme catalysis, as certain substrates provide observable changes such as color transition, aiding in the understanding of enzyme function.