ATOMIC-STRUCTURE OF A CYTOCHROME-C' WITH AN UNUSUAL LIGAND-CONTROLLED DIMER DISSOCIATION AT 1-CENTER-DOT-8 ANGSTROM RESOLUTION

The crystallographic structure of cytochrome c′ from the purple phototrophic bacterium Chromatium vinosum (CVCP) has been determined at 1.8 Å resolution using multiple isomorphous replacement. The molecule is a dimer, with each 131-residue chain folding as a four-helical bundle incorporating a covalently bound heme group at the core. This structure is the third of the ubiquitous cytochromes c′ to be solved and is similar to the known structures of cytochrome c′ from R. molischianum (RMCP) and R. rubrum (RRCP). CVCP is unique in exhibiting ligand-controlled dimer dissociation while RMCP and RRCP do not. The Tyr16 side-chain, which replaced Met16 in RMCP and Leu14 in RRCP, is parallel to the heme plane and located directly above the sixth ligand site of the heme Fe. Any ligand binding to this site, such as CO or CN-, must move the Tyr16 side-chain, which would be expected to cause other conformational changes of helix A, which contributes to the dimer interface, and consequently disrupting the dimer. Thus, the crystallographic structure of CVCP suggests a mechanism for dimer dissociation upon ligand binding. The dimer interface specificity is due to a lock and key shape complementarity of hydrophobic residues and not to any charge complementarity or cross-interface hydrogen bonds as is common in other protein-protein interfaces. The co-ordinates have been deposited in the Brookhaven Data Bank (entry PIBBH).