Probing the specifics of substrate binding for cytochrome c oxidase: a computer assisted approach

A deficiency of cytochrome c oxidase (COX) is associated with a number of diseases but details of the enzyme's mechanism of action especially the interaction with its substrate, ferrocytochrome c, remain unclear. It is known that the transfer of electrons from ferrocytochrome c to COX is facilitated by the formation of enzyme-substrate (ES) complexes which are stabilized by intermolecular salt bridges, however the identity of residues participating in the salt bridges remains obscure. Using the published structures of the two proteins, computer simulations were employed to model their interactions and to attempt to identify residues that participate in intermolecular salt bridges. The simulation process was guided in the main by cross-linking studies which proposed that Lys-13 of cytochrome c is paired with Asp-158 of COX. The initial enzyme-substrate complex, created by computer assisted manipulation of the two structures exhibited three salt bridges; following the application of energy minimization procedures, the number of salt bridges increased to seven and there were twenty-four intermolecular hydrogen bonds. The salt bridges emanated from: Glu-119 and Asp-221 of subunit I; Glu-114, Asp-115 and Asp-158 of subunit II and Asp-73 and Glu-78 of subunit VIb. These were paired with Lys-87, 8, 25, 27, 13, 22 and 100 respectively of cytochrome c. These results suggest that subunits I, II and VIb play direct roles in substrate binding. The results also suggest that hydrogen bonds contribute significantly to the stability of the ES-complex.