Molecular Dynamics Simulations of BcZBP, A Deacetylase from Bacillus cereus: Active Site Loops Determine Substrate Accessibility and Specificity.

ABSTRACT

BcZBP is an LmbE-like, homohexameric, zinc-dependent deacetylase from the opportunistic pathogen Bacillus cereus with three, thus far uncharacterized, homologues in B. anthracis. Although its specific substrate is still unknown, the enzyme has been shown to preferentially deacetylate N-acetylglucosamine and diacetylchitobiose via an active site based on a zinc-binding motif of the type HXDDXnH. In the crystal structure, the active site is located at a deep and partially blocked cleft formed at the interface between monomers related by the molecular 3-fold axis, although the major, in structural terms, building block of the enzyme is not the trimer, but the intertwined dimer. Here, we report results from a 50 ns molecular dynamics simulation of BcZBP in explicit solvent with full electrostatics and show that (i) the view of the intertwined dimer as the major structural and functional building block of this class of hexameric enzymes is possibly an oversimplification of the rather complex dynamics observed in the simulation, (ii) the most mobile (with respect to their atomic fluctuations) parts of the structure coincide with three surface loops surrounding the active site, and (iii) these mobile loops define the active site's accessibility, and may be implicated in the determination of the enzyme's specificity.