What defines a prosthetic group in biochemistry?

A prosthetic group in biochemistry is defined as a subset of cofactors that are tightly bound to enzymes. These groups are non-polypeptide units that are essential for the biological activity of certain enzymes. Unlike other cofactors that may bind and dissociate easily from the enzyme, prosthetic groups remain firmly attached, often covalently, throughout the enzyme's functional life. This stable association is crucial because it allows for the proper functioning of the enzyme, whether that involves facilitating chemical reactions or stabilizing the enzyme's structure.

Prosthetic groups can be composed of various types of molecules, including metal ions and organic compounds, and they play significant roles in enzyme catalysis and metabolic processes. For instance, heme is a classic example of a prosthetic group found in hemoglobin and myoglobin, which aids in oxygen transport and storage in the body.

The other options do not accurately capture the essence of what a prosthetic group is. While there are many types of molecules involved in enzyme function, only prosthetic groups are identified specifically by their tight binding and essential role in enzyme activity.