|
The crystallographic structure of cytochrome c' from the purple phototrophic
bacterium Chromatium vinosum (CVCP) has been determined at 1.8 A 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 P1BBH).
|
