A biosynthetic pathway is made up of a series of enzymes which take some molecule and convert it into another molecule through a sequence of catalyzed reactions. For example, shown below is a generic biosynthetic pathway composed of four enzymes, labelled 1 through 4, that convert the molecule A into the molecule E through a series of intermediates (B, C, and D):
1 2 3 4 A --------> B -------> C -------> D -------> E | | | | \/ \/ other other pathway pathway
Biosynthetic pathways are necessary to make types of all of the major molecules in cells: nucleotides, amino acids, sugars, and lipids. Catabolic pathways are organized in the same manner, but they break down molecules.
Feedback regulation is the mechanism by which biosynthetic and catabolic pathways regulate themselves. These pathways have evolved self-regulation such that, if too much of the end product is around (E in the pathway above), it, or a byproduct, acts as an inhibitor of an earlier reaction. Generally, the end product acts as a noncompetitive inhibitor of the first committed step in the pathway. For example, if both A and B are involved in other pathways as well as the one shown above, and there is an excess of E in the cell, E (or a byproduct of E) will inhibit enzyme 2, preventing it from converting any more B to C. In this way, the cell keeps from synthesizing excessive amounts of E, and keeps A and B available for use in other pathways.
The next page illustrates solving feedback problems