Water is required for proton transfer from aspartate-96 to the bacteriorhodopsin Schiff base

Glucose alone does not completely hydrate bacteriorhodopsin in glucose-embedded purple membrane

Glucose embedding is a simple and highly effective method for preparing biological macromolecules for high-resolution electron microscopy. The investigation of conditions that can trap the M-state intermediate in the bacteriorhodopsin (bR) photocycle has revealed, however, that when glucose-embedded bR is prepared at ambient humidity, it does not fully retain the capability to execute a proper photocycle. However, 'native' photocycle properties are returned after glucose-embedded samples are equilibrated at 81% relative humidity. Equilibration at relative humidities significantly higher than 81% causes glucose to dissolve in its own water of hydration, resulting in samples that may be too thick to be suitable for electron microscopy. The results obtained with bR indicate that caution should be taken with other biological specimens, and it cannot be assumed that glucose-embedded biological macromolecules retain completely their native, hydrated structure, even when high-resolution electron diffraction patterns are obtained. Equilibration of such samples at high humidity may generally be a worthwhile precaution when using the glucose-embedding technique.

The two consecutive M substates in the photocycle of bacteriorhodopsin are affected specifically by the D85N and D96N residue replacements

The photocycle of the proton pump bacteriorhodopsin contains two consecutive intermediates in which the retinal Schiff base is unprotonated; the reaction between these states, termed M1 and M2, was suggested to be the switch in the proton transport which reorients the Schiff base from D85 on the extracellular side to D96 on the cytoplasmic side (Váró and Lanyi, Biochemistry 30, 5016-5022, 1991). At pH 10 the absorption maxima of both M1 and M2 could be determined in the recombinant D96N protein. We find that M1 absorbs at 411 nm as do M1 and M2 in wild-type bacteriorhodopsin, but M2 absorbs at 404 nm. Thus, in M2 but not M1 the unprotonated Schiff base is affected by the D96N residue replacement. The connectivity of the Schiff base to D96 in the detected M2 state, but not in M1, is thereby established. On the other hand, the photostationary state which develops during illumination of D85N bacteriorhodopsin contains an M state corresponding to M1 with an absorption maximum shifted to 400 nm, suggesting that this species in turn is affected by D85. These results are consistent with the suggestion that M1 and M2 are pre-switch and post-switch states, respectively.

Characterization of the conformational change in the M1 and M2 substates of bacteriorhodopsin by the combined use of visible and infrared spectroscopy

A combination of visible and Fourier transform infrared (FTIR) spectroscopies is used to characterize the formation of the M1 and M2 substates of the bacteriorhodopsin photocycle in glucose-embedded, hydrated thin films. Difference FTIR bands in the amide I region verify the previously reported existence of a significant peptide backbone conformational change in the transition from M1 to M2. The visible absorption spectra demonstrate that contamination of the M-intermediate samples by L, N, or other non-M species should contribute negligibly to the observed changes in the amide I region, and this conclusion is supported by comparison of specific carboxyl group peaks with corresponding bands in published L and N FTIR difference spectra. Based upon spectroscopic results, an extension of the C-T Model (Fodor, S., Ames, J., Gebhard, R., van den Berg, E., Stoeckenius, W., Lugtenberg, J., and Mathies, R. (1988) Biochemistry 27, 7097-7101) is presented. The results of this work suggest that protein structural changes should be clearly visible in M-bR, difference Fourier density maps and that these structural changes may in turn elucidate how bacteriorhodopsin actively pumps ions across the purple membrane of Halobacterium halobium.

Proton transfer and energy coupling in the bacteriorhodopsin photocycle

A description of the rate constants and the energetics of the elementary reaction steps of the photocycle of bacteriorhodopsin has been helpful in understanding the mechanism of proton transport in this light-driven pump. The evidence suggests a single unbranched reaction sequence, BR-hv----K in equilibrium with L in equilibrium with M1----M2 in equilibrium with N in equilibrium with O----BR, where coupling to the proton-motive force is at the energetically and mechanistically important M1----M2 step. The consequences of site-specific mutations expressed homologously in Halobacterium halobium have revealed characteristics of the Schiff base deprotonation in the L----M1 reaction, the reorientation of the Schiff base from the extracellular to the cytoplasmic side in the M1----M2 reaction, and the reprotonation of the Schiff base in the M2----N reaction.