Monday, June 18, 2007
Golden Eagle Eyrie (Boise Centre on the Grove)
137

Heme-ligand dissociation kinetics as a probe of proximal and distal coordination in Alcaligenes xylosoxidans cytochrome c'

Yukiko Cho, Julia C. Deutsch, Alison L. McKay, Christine A. Petersen, Alesha A. Porter, Elsie Samakai, Amy E. Servid, Arianne E. Tiwari, and Colin R. Andrew. Eastern Oregon University, La Grande, OR

Alcaligenes xylosoxidans cytochrome c' (AXCP) is a bacterial heme protein that has the unique ability to bind gaseous ligands to either side of its heme center (distal or proximal). Ferrous AXCP binds CO on its distal heme face to form a stable six-coordinate heme-carbonyl (6c-CO) complex, whereas NO forms a transient six-coordinate heme-nitrosyl (6c-NO) complex that converts to a five-coordinate heme-nitrosyl (5c-NO) with NO bound at the proximal position. Rate constants (koff) for the release of NO and CO from AXCP have been measured to determine how the proximal and distal heme environments influence the kinetics of ligand dissociation. Interestingly, the 6c-NO and 6c-CO complexes of AXCP exhibit opposite trends in koff values when compared to analogous complexes in other proteins: koff for 6c-NO AXCP is one of the highest values, whereas that of the 6c-CO complex is relatively low. Off-rates for the 5c-NO complex of AXCP were found to be dependent on the concentration of dithionite NO-scavenger, suggesting that dithionite is able to access the proximal heme pocket. The kinetic data are discussed in terms of the structures of the proximal and distal environments, as well as the overall mechanism of heme-NO binding and release.