title: Effect of axial coordination on the kinetics of assembly and folding of the two halves of horse heart cytochrome C.
abstract: The kinetics of the assembly of two complementary fragments of oxidized horse heart cytochrome c (cyt c), namely the heme-containing fragment-(1-56) and the fragment-(57-104), have been characterized at different pH values. At neutral pH the fragment-(1-56) is hexacoordinated and has two histidines axially ligated to the heme-Fe(III) (Santucci, R., Fiorucci, L., Sinibaldi, F., Polizio, F., Desideri, A., and Ascoli, F. (2000) Arch. Biochem. Biophys. 379, 331-336), thus mimicking what occurs in the folding intermediate of cyt c. The kinetics of the formation of the complex between the two fragments are characterized at pH 7.0 by a slow rate constant that is independent of the concentration of the reactants; conversely, at a low pH the kinetics are much faster and depend on the concentration of the fragments. This behavior suggests that the rate-limiting step observed in the recombination process of the fragments at neutral pH (that leads to the final coordination of Met-80) has to be ascribed to the detachment of the "misligated" histidine. Thus, the faster recombination rate at a low pH can be related to the fact that histidine is protonated and not able to coordinate to the metal. Furthermore, the independence of the rate constant on the concentration of the reactants observed at pH 7.0 can be accounted for by the occurrence of a conformational transition, which takes place immediately after the two fragments collapse together, likely simulating what induces the detachment of the misligated histidine during cytochrome folding.
authors.affiliation: Department of Molecular, Cellular and Animal Biology, University of Camerino, Via F. Camerini 2, I-62032 Camerino, Italy.
authors.forename: Giampiero
authors.initials: G
authors.lastname: De Sanctis
authors.forename: Chiara
authors.initials: C
authors.lastname: Ciaccio
authors.forename: Giovanni Francesco
authors.initials: GF
authors.lastname: Fasciglione
authors.forename: Laura
authors.initials: L
authors.lastname: Fiorucci
authors.forename: Magda
authors.initials: M
authors.lastname: Gioia
authors.forename: Federica
authors.initials: F
authors.lastname: Sinibaldi
authors.forename: Stefano
authors.initials: S
authors.lastname: Marini
authors.forename: Roberto
authors.initials: R
authors.lastname: Santucci
authors.forename: Massimo
authors.initials: M
authors.lastname: Coletta
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D003580:Cytochromes / Q000737:chemistry; D045304:Cytochromes c / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011522:Protons; D013053:Spectrophotometry; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D003580:Cytochromes; D011522:Protons; D006418:Heme; D006639:Histidine; D045304:Cytochromes c