Enzyme promiscuity – A light on the “darker” side of enzyme specificity

Enzyme promiscuity can be defined as the capability of enzymes to catalyse side reaction in addition to its main reaction. The side reaction of an enzyme is termed as promiscuous or sometimes as the "darker" side of enzyme cross-reactivity/specificity. This unique property of enzyme allows organisms to adapt under varying environmental conditions. Promiscuous enzymes can modify their catalytic activities with altered substrates and can adjust their catalytic and kinetic mechanisms according to substrate properties. This group of enzymes evolved from ancestral proteins found in primitive organisms like archaea that survive under extreme environmental conditions. Such ancestral proteins possessed the potential to catalyse a wide range of reactions at low levels, hence create families or superfamilies of highly specialized enzymes. Further, some enzymes were identified which have non-catalytic functions in addition to their major catalytic activities. These enzymes are referred to as moonlighting enzymes. The study of these enzymes will provide important information regarding enzyme evolution and will help in optimizing protein engineering applications.