FIGURE 2.

Overall structure of MCD subunits. Shown are ribbon (A) and secondary element (B) representations of a human MCD subunit, which is composed of an all-helix N-terminal domain (yellow) and a catalytic C-terminal domain (brown). The positions of Cys-206 and Cys-243 forming intersubunit disulfide bonds are indicated. C, MCD subunits adopt two markedly different conformations (shown in yellow/brown and green, respectively). The main differences, around the active center binding pocket, are likely related to changes in the relative orientation of the N- and C-terminal domains. A putative substrate/product fragment represented with sticks was found with partial occupancy only in subunits presenting the conformation defined as bound. D, stereo views of the 2F_o − F_c electron density maps at 1σ, corresponding to helices α7 and α8 at the end of the N-terminal domain.