AN ENERGY-BASED APPROACH TO PACKING THE 7-HELIX BUNDLE OF BACTERIORHODOPSIN

Based on the heavy-atom coordinates determined by the electron microscopy for the seven main helical regions of bacteriorhodopsin with the all-trans retinal isomer, energy optimizations were carried out for helix bundles containing the all-trans retinal and 13-cis retinal chromophores, respectively. A combination of simulated annealing and energy minimization was utilized during the process of energy optimization. It was found that the 7-helix bundle containing the all-trans isomer is about 10 kcal/mol lower in conformational energy than that containing the 13-cis isomer. An energetic analysis indicates that such a difference in energy is consistent with the observation that absorption of a 570-nm proton is required for the conversion of a bacteriorhodopsin from its all-trans to 13-cis form. It was also found that the above conversion process is accompanied by a significant conformational perturbation around the chromophore, as reflected by the fact that the beta-ionone ring of retinal moves about 5.6 angstrom along the direction perpendicular to the membrane plane. This is consistent with the observation by Fodor et al. (Fodor, S.P.A., Ames, J.B., Gebhard, R., van der Berg, E.M.M., Stoeckenius, W., Lugtenburg, J., & Mathies, R.A., 1988, Biochemistry 27, 7097-7101). Furthermore, it is interesting to observe that although the retinal chromophore undergoes a significant change in its spatial position, the orientation of its transition dipole changes only slightly, in accord with experimental observations. In other words, even though orientation of the retinal transition dipole is very restricted, there is sufficient room, and degrees of freedom, for the retinal chromophore to readjust its position considerably. This finding provides new insight into the subtle change of the retinal microenvironment, which may be important for revealing the proton-pumping mechanism of bacteriorhodopsin. The importance of electrostatic and nonbonded interactions in stabilizing the 7-helix bundle structure has also been analyzed. Electrostatic interactions favor an antiparallel arrangement among adjacent helices. Nonbonded interactions, however, drive most of the closely packed helices into an arrangement in which the packing angles lie around -160-degrees, a value very near the -154-degrees value computed earlier as the most favorable packing arrangement of two poly(Ala) alpha-helices (Chou, K.-C., Nemethy, G., & Scheraga, H.A., 1983, J. Phys. Chem. 87, 2869-2881). The structural features of the 7-helix bundle and their relationship to those found in typical 4-helix bundle proteins are also discussed. The knowledge thus obtained suggests that many 7-helix membrane proteins might probably be built in a stepwise fashion from 4-helix bundle groups.