Categorizing reactivity of bacteriorhodopsin cysteine mutants crosslinkingto 4-bromoretinal

The structure of bacteriorhodopsin (bR) has been probed by a large number o f experimental methods. In earlier work distance constraints measured from the 1BRD Brookhaven structure (1, 2) were used to guide site-directed mutag enesis/affinity labeling experiments (3-5). In the present study we report on the use of limited molecular dynamics (MD) investigations of the same bR /affinity label system. We show here that the chiral center introduced when 4-bromo-all-trans retinal is synthesized produces variable impact on poten tial crosslinking. Our MD analysis suggests the following ranking of bindin g site mutants in order of reactivity: R118C> S118C>> S121C> R141C>> S141C> >> R121C, R138C, S138C. Chirality appears to have limited effect for the M1 18C mutants but shows more dramatic impact for the T121C and S141C mutants. These results are in excellent agreement with the experimental observation s and offer encouragement that MD can be a useful component of experimental design with considerable predictive power.