title: Photoacoustic calorimetry studies of CO photo-dissociation from chloramine-T modified horse heart cytochrome-c.
abstract: Treatment of horse heart Cytochrome-c (Cc) with N-chloro-4-toluosulfonamide (Chloramine-t, CT) results in the oxidation of methionine (Met) residues to the corresponding sulfoxide including the distal heme ligand, Met80. The resulting Fe-sulfoxide coordination is sufficiently labile in the ferrous form to be displaced by gaseous ligands, including CO. Photolysis of the CO-CT-Cc complex provides an opportunity to examine ligand binding dynamics that are associated with a relatively rigid distal heme pocket. In this work, photoacoustic calorimetry (PAC) was utilized to obtain the kinetics as well as enthalpy and molar volume changes subsequent to CO photo-dissociation from CO-CT-Cc. Previous photolysis studies of CO-CT-Cc have led to a proposed model for ligand recombination in which the Met80-sulfoxide and CO recombine with the heme on relatively slow timescales (50 μs and ∼500 μs, respectively). The PAC data presented here reveals two additional kinetic phases with lifetimes of <20 ns and 534 ± 75 ns. The fast phase (<20 ns) is associated with an ΔH of 44 ± 5 kcal mol-1and ΔV of -0.5 ± 0.5 mL mol-1, whereas the slower phase (534 ns) is associated with a small ΔH of 2 ± 3 kcal mol-1and ΔV of 1 ± 0.5 mL mol-1.
authors.affiliation: Department of Chemistry, University of South Florida, 4202 East Fowler Ave. CHE 424, Tampa, FL 33620, USA.
authors.forename: Tarah A
authors.initials: TA
authors.lastname: Word
authors.affiliation: Department of Chemistry, University of South Florida, 4202 East Fowler Ave. CHE 424, Tampa, FL 33620, USA. Electronic address: rwlarsen@usf.edu.
authors.forename: Randy W
authors.initials: RW
authors.lastname: Larsen
mesh_terms: D000162:Acoustics; D000818:Animals; D001665:Binding Sites; D002151:Calorimetry / Q000379:methods*; D002248:Carbon Monoxide / Q000737:chemistry*; D002700:Chloramines / Q000737:chemistry*; D045304:Cytochromes c / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D006736:Horses; D007501:Iron / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008027:Light; D009206:Myocardium / Q000201:enzymology*; D010782:Photolysis; D011487:Protein Conformation; D013056:Spectrophotometry, Ultraviolet; D013454:Sulfoxides / Q000737:chemistry; D013816:Thermodynamics; D014105:Tosyl Compounds / Q000737:chemistry*
chemical_list: D002700:Chloramines; D008024:Ligands; D013454:Sulfoxides; D014105:Tosyl Compounds; C016300:chloramine-T; D006418:Heme; D002248:Carbon Monoxide; D045304:Cytochromes c; D007501:Iron
keywords: CO photo-dissociation; Chemical oxidation; Chloramine-T cytochrome-c; Heme protein; Methionine-Sulfoxide; Modified cytochrome-c; Photoacoustic calorimetry