(color online) Structure of kinesin motor domain and the backbone hydrogen bonds of the β10 zipper. (a) Two motor heads and the neck coiled coil on microtubule. The neck linker (red) of the leading head is in the undocked state and that of the trailing head is in the docked state. (b) Definition of the four backbone HBs in the docked β10 region. The HBs are indicated with broken lines. The oxygen atoms are shown in red balls, the hydrogen atoms in white, the nitrogen atoms in blue and α carbon atoms in bigger black balls.