Alternative catalytic mechanisms driven by structural plasticity is an emerging theme in HAS-GTPases, Era and FeoB

GTP hydrolysis is the underlying basis for functioning of biological switches or GTPases. Extensively studied GTPases, Ras and EF-Tu, use a conserved Gln/His that facilitates the activation of attacking water for nucleophilic attack. However, this is insufficient to explain catalysis in Hydrophobic Amino acid Substituted (HAS)-GTPases that naturally possess a hydrophobic residue in lieu of Gln/His. We had previously reported a bridging water-chain mediated catalytic mechanism for HAS-GTPase FeoB; which utilizes two distantly-located but conserved glutamates. Curiously, mutating these does not abolish GTP hydrolysis. Similarly, in this study we report our observations on another HAS-GTPase Era, wherein the mutants of catalytically important residues continue to hydrolyze GTP. We attempt to rationalize these inquisitive observations on GTP hydrolysis by FeoB and Era mutants. We propose a general theory that appears common to at least three classes of GTPases, where alternative mechanisms emerge when the primary mechanism is disrupted. Based on the analysis of crystal structures of FeoB and Era mutants, bound to the transition state analogue GDP.AlFx, this work suggests that in the absence of catalytically important residues, the active site waters in both FeoB and Era undergo re-arrangements, which in turn helps in sustaining GTP hydrolysis. Similar employment of alternative mechanisms was also suggested for the catalytic mutants of hGBP1. Importantly, such alternatives underscore the robustness of GTP hydrolysis mechanisms in these systems, and raise important questions regarding the need for persistent GTP hydrolysis and the physiological relevance of structural plasticity seen here.