The mechanistic role of the coordinated tyrosine in astacin

Crayfish astacin belongs to the only Zn protein family containing a coordin ated Tyr ligand in the active site, and is a rare example of Zn enzymes who se activity can be significantly restored by Cu2+. The highly active Co2+ a nd Cu2+ derivatives of astacin can serve as good models of the native enzym e for structural and mechanistic studies by means of optical and magnetic r esonance techniques. Upon the introduction of the inhibitor Tyr-hydroxamate to these two metal derivatives of astacin, the coordinated Tyr in the acti ve site is detached based on our optical and NMR studies in solution. The s ignificance of the detachment of the coordinated Tyr in the action of astac in is fourfold: (1) to enhance the Lewis acidity of the active site metal, (2) to balance the negative charge of the transition state gem-diolate-enzy me complex, (3) to relieve the steric crowding upon substrate binding, and (4) to stabilize the enzyme-substrate complex by way of a H-bonding. (C) 19 98 Published by Elsevier Science Inc. All rights reserved.