Molecular Regulation of Enzymes


This plot shows rate of reaction versus substrate concentration for an enzyme in the absence of inhibitor, and for enzyme in the presence of competitive and noncompetitive inhibitors. Both competitive and noncompetitive inhibitors slow the rate of reaction, but competitive inhibitors can be overcome by high concentrations of substrate, whereas noncompetitive inhibitors cannot.
Competitive and noncompetitive inhibition affect the reaction’s rate differently. Competitive inhibitors affect the initial rate but do not affect the maximal rate; whereas, noncompetitive inhibitors affect the maximal rate.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Enzymes can be regulated in ways that either promote or reduce their activity. There are many different kinds of molecules that inhibit or promote enzyme function, and various mechanisms exist for doing so. For example, in some cases of enzyme inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to the active site and simply block the substrate from binding. When this happens, the enzyme is inhibited through competitive inhibition, because an inhibitor molecule competes with the substrate for active site binding. Alternatively, in noncompetitive inhibition, an inhibitor molecule binds to the enzyme at an allosteric site, a binding site away from the active site, and still manages to block substrate binding to the active site.

– What do you call the enzymatic regulation where a molecule binds to an enzyme somewhere other than the active site and makes the enzyme no longer in the optimal arrangement to stabilize the transition state and catalyze the reaction?

Some inhibitor molecules bind to enzymes in a location where their binding induces a conformational change that reduces the enzyme’s affinity for its substrate. This type of inhibition is an allosteric inhibition. More than one polypeptide comprise most allosterically regulated enzymes, meaning that they have more than one protein subunit. When an allosteric inhibitor binds to an enzyme, all active sites on the protein subunits change slightly such that they bind their substrates with less efficiency. There are allosteric activators as well as inhibitors. Allosteric activators bind to locations on an enzyme away from the active site, inducing a conformational change that increases the affinity of the enzyme’s active site(s) for its substrate(s).

The left part of this diagram shows allosteric inhibition. The allosteric inhibitor binds to the enzyme at a site other than the active site. The shape of the active site is altered so that the enzyme can no longer bind to its substrate. The right part of this diagram shows allosteric activation. The allosteric activator binds to the enzyme at a site other than the active site. The shape of the active site is changed, allowing substrate to bind at a higher affinity.
Allosteric inhibitors modify the enzyme’s active site so that substrate binding is reduced or prevented. In contrast, allosteric activators modify the enzyme’s active site so that the affinity for the substrate increases.

Source: OpenStax Biology 2e

Many enzymes don’t work optimally, or even at all, unless bound to other specific non-protein helper molecules, either temporarily through ionic or hydrogen bonds or permanently through stronger covalent bonds. Two types of helper molecules are cofactors and coenzymes. Binding to these molecules promotes optimal conformation and function for their respective enzymes. Cofactors are inorganic ions such as iron (Fe++) and magnesium (Mg++). One example of an enzyme that requires a metal ion as a cofactor is the enzyme that builds DNA molecules, DNA polymerase, which requires a bound zinc ion (Zn++) to function. Coenzymes are organic helper molecules, with a basic atomic structure comprised of carbon and hydrogen, which are required for enzyme action. The most common sources of coenzymes are dietary vitamins. Some vitamins are precursors to coenzymes and others act directly as coenzymes. Vitamin C is a coenzyme for multiple enzymes that take part in building the important connective tissue component, collagen. An important step in breaking down glucose to yield energy is catalysis by a multi-enzyme complex scientists call pyruvate dehydrogenase. Pyruvate dehydrogenase is a complex of several enzymes that actually requires one cofactor (a magnesium ion) and five different organic coenzymes to catalyze its specific chemical reaction. Therefore, enzyme function is, in part, regulated by an abundance of various cofactors and coenzymes, which the diets of most organisms supply.

– What do you call the protein portion of an enzyme requiring a cofactor for its reaction?

Shown are the molecular structures for Vitamin A, folic acid, Vitamin B1, Vitamin C, Vitamin B2, Vitamin D2, Vitamin B6, and Vitamin E.
Vitamins are important coenzymes or precursors of coenzymes, and are required for enzymes to function properly. Multivitamin capsules usually contain mixtures of all the vitamins at different percentages.

Source: OpenStax Biology 2e

Enzymes, such as lysozyme, can be used to digest the cell wall of most bacteria. Once the cell wall is digested, the cells burst owing to a weakened cell wall that cannot counterbalance the normal osmotic pressure of the cell. Lysozymes are found in saliva, milk, tears and egg white.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:


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