The structures of the active center in dark-adapted bacteriorhodopsin by solution-state NMR spectroscopy
H. Patzelt, B. Simon, A. terLaak, B. Kessler, R. Kühne, P. Schmieder, D. Oesterhelt, H. Oschkinat
Proc. Natl. Acad. Sci. USA (2002) 99, 9765-9770
The two forms of bacteriorhodopsin present in the dark-adapted state, containing either all-trans or 13-cis,15-syn retinal, were examined by using solution state NMR, and their structures were determined. Comparison of the all-trans and the 13-cis,15-syn forms shows a shift in position of about 0.25 A within the pocket of the protein. Comparing this to the 13-cis,15-anti chromophore of the catalytic cycle M-intermediate structure, the 13-cis,15-syn form demonstrates a less pronounced up-tilt of the retinal C12-C14 region, while leaving W182 and T178 essentially unchanged. The N-H dipole of the Schiff base orients toward the extracellular side in both forms, however, it reorients toward the intracellular side in the 13-cis,15-anti configuration to form the catalytic M-intermediate. Thus, the change of the N-H dipole is considered primarily responsible for energy storage, conformation changes of the protein, and the deprotonation of the Schiff base. The structural similarity of the all-trans and 13-cis,15-syn forms is taken as strong evidence for the ion dipole dragging model by which proton (hydroxide ion) translocation follows the change of the dipole.