Heme-iron ligand (M80-Fe) in cytochrome c is destabilizing: combined in vitro and in silico approaches to monitor changes in structure, stability and dynamics of the protein on mutation.

Structure, stability and dynamics properties of horse cytochrome c (cyt c) and its genetically engineered M80G mutant have been investigated. The nature of the Met80 axial ligation to heme iron is believed to be the major determinant of the oxidation-reduction reactions inside and outside the cell of a particular cytochrome. This ligation has played an important role in the studies of protein structure, stability and protein folding/unfolding. To understand this ligation better, Met80 of horse cyt c has been mutated to Gly that is unable to bind to the heme iron. We have examined the effect of the M80G mutation on the structure and stability of the WT (wild type) protein by using absorbance spectroscopy, far-UV, near-UV and Soret circular dichroism, fluorescence spectroscopy and differential scanning calorimetry. We have observed that mutation caused a partial loss of secondary and tertiary structure with slightly increased overall stability of the protein. We have also measured the dynamic behavior of WT cyt c and its M80G mutant in the oxidized form (Fe3+) using the essential dynamics (ED) method. A 400 ns MD simulations were run for WT cyt c and its mutant M80G in water using GROMOS96 force field. MD results revealed that the stability and flexibility increased in mutant M80G (Fe…S (Met80) bond removed). Essential dynamics analysis revealed that the first five eigenvectors were mainly involved in overall motions of WT cyt c and its M80G mutant but the amplitude of concerted motions decreased in M80G mutant relative to WT cyt c.Communicated by Ramaswamy H. Sarma.