Final answer:
Principal types of enzyme modulation include competitive inhibition, where inhibitors block the active site, and allosteric regulation, involving changes in the enzyme's conformation affecting substrate affinity.
Step-by-step explanation:
The two principal types of modulation of existing enzyme molecules are competitive inhibition and allosteric regulation. Competitive inhibition occurs when an inhibitor molecule is similar enough to a substrate that it can bind to the enzyme's active site, blocking the actual substrate from binding. Conversely, allosteric regulation involves molecules binding to parts of the enzyme other than the active site, known as the allosteric site.
In allosteric inhibition, this binding induces a conformational change in the enzyme, reducing its affinity for the substrate. On the other hand, allosteric activators can bind and induce a conformational change that increases the enzyme's affinity for the substrate. Another form of enzyme modulation is reversible covalent modification, which involves the cyclic conversion of the enzyme between its modified and unmodified forms.