Figure 2. Flexibility of the HC as a Function of Ca²⁺.

The D19A-RD (upper left) and Ca²⁺-RD structures are shown as a B-factor color ramp (high B-factors, red, and low B-factors, blue) in ribbon diagrams. Main-chain mean B-factors are plotted as a function of residue number for each chain (right and below). Only in the D19A-RD structure, HC B-factors peak near the ELC/RLC interface and at the hook (residues Trp824-Trp826) (red arrows). B-factors spike in the Ca²⁺ binding site only in the absence of Ca²⁺ (green arrows). In the RLC N-lobe, a rise in B-factors is more pronounced for D19A-RD than Ca²⁺-RD. ELC residue 95 interacts hydrophobically with the Ca²⁺ binding site of a symmetry molecule in the crystal lattice and experiences a B-factor spike only in the absence of Ca²⁺ (black arrow). These results are consistent with the possibility that Ca²⁺ dissociation leads to significantly greater flexibility by the ELC/RLC interface. We cannot currently account for the B-factor rise in the HC hook and RLC N-lobe of the Ca²⁺-free D19A-RD structure.