What makes an enzyme promiscuous?

Kinetic analyses of promiscuous enzymes reveal rate accelerations, (k(cat)/K(M))/k(2), of up to 10(18) for their secondary activities. Such large values suggest that binding and catalysis can be highly efficient for more than one reaction, challenging the notion that proficient catalysis requires specificity. Growing numbers of reported promiscuous activities indicate that catalytic versatility is an inherent property of many enzymes. The examples discussed here illustrate promiscuous molecular recognition mechanisms that, together with knowledge from structural and computational analysis, might be used for the identification or development of catalysts for new reactions.