PubMed · 6352363
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J L Stirling, H P Bennetto, G M Delaney, J R Mason, S D Roller, K Tanaka, C F Thurston. 1983. Microbial fuel cells.. https://doi.org/10.1042/bst0110451
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The known high-resolution structure and dynamics characterization of the lipid bilayer-bound polypeptide gramicidin A provides a unique opportunity to study structure-function and dynamics-function correlations in a model membrane protein. In particular, the indoles have a variety of very important functional roles in this cation channel that will undoubtedly be recognized in membrane proteins. That indoles and phenols are oriented at the hydrophobic-hydrophilic interface of lipid bilayers is already well-recognized in membrane proteins. The most buried indole of the gramicidin channel, Trp9, is shown by 15N solid state NMR to be exposed to the hydrophilic surface through hydrogen exchange. Here the importance of the indole dipole moments is described for cation conductance. Preparation of samples with high concentrations of Na+ is shown by high-resolution orientational constraints derived from 2H NMR to have no structural effect on the indole side chain conformations. These dipoles stabilize cations in the binding sites near the channel entrance and substantially reduce the potential energy barrier at the bilayer center. This latter finding conclusively documents that the rate-limiting step in cation conductance by this channel involves the barrier at the bilayer center. Furthermore, dynamics of the indole rings cause significant fluctuations in the energy of stabilization at the binding site that may result in a rapid mechanism for gating the channel.
The measurements described demonstrate the variety of protein or protein-mediated interactions that can be probed directly with the SFA. The distance, force and dynamic measurement capabilities provide tremendous potential for the identification and quantification of the molecular mechanisms and forces that both determine protein properties and influence protein function in diverse environments.
The vibrational frequency of CO bound to myoglobin can be varied by up to 60 cm-1 by making site-specific mutations in the distal pocket. These changes may result from specific chemical interactions between distal amino acids and the CO or from changes in the electrostatic field of the distal pocket. In this paper, we separate the relative contributions of these two effects by comparing the IR spectra of the carbonmonoxy complexes of human myoglobin mutants V68N, V68D, and V68E. The effect of replacing valine with these polar amino acids on the electrostatic environment of the distal heme pocket has been independently determined earlier by measurements of the heme reduction potential and electronic absorption spectral band shifts. While all three mutations result in a negative dipole pointing towards the CO ligand, the CO stretch frequency shifts differently in each case. These differences are attributed to specific chemical interactions between the amino acids and the CO ligand.