FIGURE 3.

Comparison of the back door region of the MD_dc·Mg·ADP·BeF_x and S1_sc·Mg·ADP crystal structures. The MD_dc·Mg·ADP·BeF_x (Fisher et al., 1995) (purple) and S1_sc·Mg·ADP crystal structures (Houdusse et al., 1999) (green) were overlaid based on the C_α atoms of the upper 50-kDa and 25-kDa subdomains. Mg·ADP·BeF_x (orange) is included for reference. The difference in the degree of constriction in the back door route, as seen in the solvent accessible surfaces of Fig. 1, d and f, arises from the movement of Arg-236 (Arg-232 in MD_dc) to form a new salt bridge with Glu-675 (Asn-660 in MD_dc). Also, the side chain of Glu-465 beyond the C_β is not resolved in the S1_sc structure. This is likely due to the breaking of its weak salt bridge with Arg-242 that in turn is caused by the slight movement of switch 2 and the lower 50-kDa subdomain.