title: His26 protonation in cytochrome c triggers microsecond β-sheet formation and heme exposure: implications for apoptosis.
abstract: Cytochrome c unfolds locally and reversibly upon heating at pH 3. UV resonance Raman (UVRR) spectra reveal that instead of producing unordered structure, unfolding converts turns and some helical elements to β-sheet. It also disrupts the Met80-heme bond, and has been previously shown to induce peroxidase activity. Aromatic residues that are H-bonded to a heme propionate (Trp59 and Tyr48) alter their orientation, indicating heme displacement. T-jump/UVRR measurements give time constants of 0.2, 3.9, and 67 μs for successive phases of β-sheet formation and concomitant reorientation of Trp59. UVRR spectra reveal protonation of histidines, and specifically of His26, whose H-bond to Pro44 anchors the 40s Ω loop; this loop is known to be the least stable 'foldon' in the protein. His26 protonation is proposed to disrupt its H-bond with Pro44, triggering the extension of a short β-sheet segment at the 'neck' of the 40s Ω loop into the loop itself and back into the 60s and 70s helices. The secondary structure change displaces the heme via H-bonds from residues in the growing β-sheet, thereby exposing it to exogenous ligands, and inducing peroxidase activity. This unfolding mechanism may play a role in cardiolipin peroxidation by cyt c during apoptosis.
authors.affiliation: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Gurusamy
authors.initials: G
authors.lastname: Balakrishnan
authors.forename: Ying
authors.initials: Y
authors.lastname: Hu
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Spiro
mesh_terms: D017209:Apoptosis; D002942:Circular Dichroism; D045304:Cytochromes c / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D008958:Models, Molecular; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
chemical_list: D011522:Protons; D006418:Heme; D006639:Histidine; D045304:Cytochromes c