Which of the following is an example of an irreversible modification of an enzyme?

Proteolytic cleavage represents an irreversible modification of an enzyme because it involves the permanent alteration of the enzyme's structure. In this process, one or more peptide bonds in the enzyme are cleaved, resulting in a change that cannot be reversed. This type of modification is common in the activation of certain enzymes that are synthesized as inactive precursors (zymogens). Once cleaved, the enzyme often gains its functional conformation and cannot revert back to the inactive form.

In contrast, phosphorylation is a reversible modification that can be added or removed, allowing for dynamic regulation of enzyme activity. Allosteric modulation also involves reversible binding of molecules that can enhance or inhibit enzyme activity without permanently altering its structure. Competitive inhibition involves the temporary binding of inhibitors that can be displaced from the active site, allowing the enzyme to regain its function. These characteristics of the other options highlight why proteolytic cleavage stands out as an irreversible modification.