Figure 1. Overall Comparison of Structures for Ca²⁺-RD and D19A-RD.

(a) The two structures are shown side by side. The RD consists of a heavy chain (HC, red) and associated light chains: the ELC (green), and the RLC (blue). The light chain helices are labeled A through H, starting from the N-terminus. The four "EF-hand" motif loops of each of the light chains are labeled with Roman numerals. The Ca²⁺ binding site is located in an ELC loop near the ELC/RLC interface. The overall RD conformation is the same in both structures, probably as a result of stabilization from crystal contacts. Inset (right): Schematic diagram showing the entire myosin head with its subdomains. The RD comprises the lever arm, which is a continuation of the converter. (b) A close view of the Ca²⁺ binding site in the Ca²⁺-RD structure in wall-eyed stereo. As previously reported³; ⁴, the Ca²⁺ ion is coordinated by seven oxygen ligands, including the carbonyl of Gly23. The electron density (light blue) is from a 2F_o−F_c difference map contoured at 1.0 sigma. All residues are highly ordered. (c) The same site as in b for the D19A-RD structure. A 2F_o−F_c omit map contoured at 1.0 sigma (light blue) as in b. The Gly23 carbonyl and the side-chain of Asp22 have rotated away from the Ca²⁺ site. We have modeled Asp25 with the side chain facing the Ca²⁺ site, but there is some density for a second conformation pointing away from the site. Arg24 is partially disordered. A water molecule has been modeled at the Ca²⁺ position (see text).