RESUMEN
The substrate specificity of enzymes is bound to be imperfect, because of unavoidable physicochemical limits. In extant metabolic enzymes, furthermore, such limits are seldom approached, suggesting that the degree of specificity of these enzymes, on average, is much lower than could be attained. During biological evolution, the activity of a single enzyme with available alternative substrates may be preserved to a significant or even substantial level for different reasons - for example when the alternative reaction contributes to fitness, or when its undesirable products are nevertheless dispatched by metabolite repair enzymes. In turn, the widespread occurrence of promiscuous reactions is a consistent source of metabolic 'messiness', from which both liabilities and opportunities ensue in the evolution of metabolic systems.