Functional waters in intraprotein proton transfer monitored by FTIR difference spectroscopy

Abstract

Much progress has been made in our understanding of water molecule reactions on surfaces, proton solvation in gas-phase water clusters and proton transfer through liquids. Compared with our advanced understanding of these physico-chemical systems, much less is known about individual water molecules and their cooperative behaviour in heterogeneous proteins during enzymatic reactions. Here we use time-resolved Fourier transform infrared spectroscopy (trFTIR) and in situ H2(18)O/H2(16)O exchange FTIR to determine how the membrane protein bacteriorhodopsin uses the interplay among strongly hydrogen-bonded water molecules, a water molecule with a dangling hydroxyl group and a protonated water cluster to transfer protons. The precise arrangement of water molecules in the protein matrix results in a controlled Grotthuss proton transfer, in contrast to the random proton migration that occurs in liquid water. Our findings support the emerging paradigm that intraprotein water molecules are as essential for biological functions as amino acids.

Mechanism of a proton pump analyzed with computer simulations

“…Earlier experiments indicated the involvement of E204 either as the proton release group [80] or as the group that delivers the released proton to E194 [81,82]. Based on more recent data, it was proposed that the proton is stored in a cluster of water molecules that interacts with protein amino acids [83,84]. This proposal was apparently supported by the observation of a continuum band in spectra calculated from QM/MM Carr-Parinelo dynamics in which the proton release group was modeled as a dimer of water molecules sharing a proton (Zudel ion, H 5 O 2 ? )…”

Experimental evidence of a 3-centre, 2-electron covalent bond character of the central O–H–O fragment on the Zundel cation in crystals of Zundel nitranilate tetrahydrate

“…[1][2][3][4] The basic step in a variety of processes, ranging from enzyme catalysis 3 to electric conductivity via the Grotthuss mechanism 1,5 , is migration of a hydrogen cation (i.e. a proton) between two water molecules and it involves two well-known solvated forms: Eigen cation [H 3 2 7 .…”

Controlled ionic condensation at the surface of a native extremophilemembrane

“…Water-filled cavities inside bR are fundamental in this process. [29][30][31][32] Many important questions remain unanswered, in particular neither 30 the proton release nor the proton uptake is understood 33 and plausible explanation of the functional role of the crystalline arrangement of bR trimers has not yet been found. These surface processes are controlled by very complex solvent mediated local interactions in which the role of ions is 35 expected to be fundamental, especially since bR adaptation to its extreme and highly asymmetric native environment (extracellular NaCl > 3 M, cytoplasmic KCl > 3 M) has produced specificity of surface interactions between proteins, lipids and ions.…”