The mechanistic role of the coordinated tyrosine in astacin

Crayfish astacin belongs to the only Zn protein family containing a coordinated Tyr ligand in the active site, and is a rare example of Zn enzymes whose activity can be significantly restored by Cu2+. The highly active Co2+ and Cu2+ derivatives of astacin can serve as good models of the native enzyme for structural and mechanistic studies by means of optical and magnetic resonance techniques. Upon the introduction of the inhibitor Tyr-hydroxamate to these two metal derivatives of astacin, the coordinated Tyr in the active site is detached based on our optical and NMR studies in solution. The significance of the detachment of the coordinated Tyr in the action of astacin 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-enzyme 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) 1998 Published by Elsevier Science Inc. All rights reserved.