His26 Protonation in Cytochrome c Triggers Microsecond β-Sheet Formation and Heme Exposure: Implications for Apoptosis

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 beta-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 mu s for successive phases of beta-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 Omega 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 beta-sheet segment at the 'neck' of the 40s Omega 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 beta-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.