The Structures of the Active Center in Dark-Adapted Bacteriorhodopsin by Solution-State NMR Spectroscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS 2002-07, Vol.99 (15), p.9765-9770
• Main Authors: Patzelt, Heiko, Simon, Bernd, terLaak, Antonius, Kessler, Brigitte, Kühne, Ronald, Schmieder, Peter, Oesterhelt, Dieter, Oschkinat, Hartmut
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Atoms; Bacteriorhodopsins; Bacteriorhodopsins - chemistry; Binding Sites; Biological Sciences; Biophysics; Chemical equilibrium; Chromophores; Darkness; Family structure; Functional groups; Isomerization; Magnetic Resonance Spectroscopy - methods; Models, Molecular; Molecules; NMR; Nuclear magnetic resonance; Protein Conformation; Proteins; Protons; Schiff bases
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.132253899

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 Å 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.