Nature and biological irrelevance of mixed-type enzyme inhibition

Abstract

Mixed-type enzyme inhibitors were originally envisaged to decrease the enzyme affinity for the substrates and the maximum turnover rate by simultaneously targeting two distinct protein sites, i.e., the active site and an allosteric site. After a century from the first formulation of this hypothesis, the consensus on its validity is still unanimous, although several of its implications are in open conflict with the current knowledge on molecular recognition mechanisms. In particular, there is no plausible explanation for the experimental evidence that mixed-type inhibitors bind the enzyme active sites always more effectively than the allotopic sites. In an attempt to solve this controversy, it was found that the preference of mixed inhibitors for active sites emerges as an inevitable numerical artifact that is implicit in the equations used to model the apparent mixed inhibition caused under certain circumstances by active site-bound competitive inhibitors. Hence, proving that the consolidated model of mixed inhibition is incorrect and, more generally, strongly pointing to the biological irrelevance of mixed-type inhibition.