A tale of two charges: Distinct roles for an acidic and a basic amino acidin the structure and function of cytochrome c

Cytochrome c is a small electron transport protein found in the intermembra ne space of mitochondria. As it interacts with a number of different physio logical partners in a specific fashion, its structure varies little over eu karyotic evolutionary history. Two highly conserved residues found within i ts sequence are those at positions 13 and 90 (numbering is based on the sta ndard horse cytochrome c); with single exceptions, residue 13 is either Lys or Arg, and residue 90 is either Glu or Asp. There have been conflicting v iews on the roles to be ascribed to these residues, particularly residue 13 , so the functional properties of a number of site-directed mutants of Sacc aromyces cerevisiae iso-1 cytochrome c have been examined. Results indicate that the two residues do not interact specifically with each other; howeve r, residue 13 (Arg) is likely to be involved in interactions between cytoch rome c and other electro statically oriented physiological partners (interm olecular), whereas residue 90 (Asp) is involved in maintaining the intrinsi c structure and stability of cytochrome c (intramolecular). This is support ed by molecular dynamics simulations carried out for these mutants where re moval of the negative charge at position 90 leads to significant shifts in the conformations of neighboring residues, particularly lysine 86. Both cha rged residues appear to exert their effects through electrostatics; however , biological activity is significantly more sensitive to substitutions of r esidue 13 than of residue 90.