= Solution
The zinc ion is coordinated by protein donors and a <water> <oxygen> placed near the reacting C6 center. \b[Zinc acts as a <Lewis acid> that activates and positions <water>.] Its positive charge withdraws <electron density> from <oxygen>, lowers the effective p$K_a$ of coordinated <water> and facilitates formation of the attacking <hydroxide>, potentially with assistance from a nearby base. The same <metal>–<oxygen> interaction can stabilize developing negative charge as the addition intermediate forms. Zinc stays Zn(II); this is <metal ion catalysis>, not a zinc <redox> cycle.
The structure identifies a plausible reaction geometry, but a static X-ray model does not by itself establish the <water>'s <protonation> state, proton-transfer order or the rate-limiting step. Structural work with bound transition-state analogues independently supports a zinc-containing catalytic center.<Atomic structure of adenosine deaminase complexed with a transition-state analog: understanding catalysis and immunodeficiency mutations by David K. Wilson and others>[1991, pp. 1278–1284, catalytic zinc and the bound transition-state analogue]{ref} An appropriate experimental test would compare native <enzyme>, a <metal>-depleted preparation and stoichiometrically zinc-reconstituted <enzyme>, checking retained fold and active <enzyme> concentration. Loss and recovery of activity with those controls distinguishes a catalytic <metal> requirement from nonspecific denaturation.
Back to article page
