BIOLOGICAL ELECTRON-TUNNELING THROUGH NATIVE PROTEIN MEDIA

Nature has engineered a universe of redox proteins to efficiently cont rol the oxidation and reduction of substrates and to convert redox ene rgy into a delocalized transmembrane proton gradient power source. Som e rapid physiologically relevant electron transfers are rate limited b y electron tunneling. Distance appears to be the principle means natur ally selected to control the speed of electron tunneling; free energy and reorganization energy can play important auxiliary roles. Thus, an electron from a biological redox center can tunnel in any direction a nd is likely to reduce the closest redox center with a favorable free energy. Although it is clearly possible to facilitate electron tunneli ng by designing covalent bridges in the regions between donors and acc epters, this does not seem to be a strategy that evolution has used. E volutionary mutagenic adjustment of a bridge-like quality of the amino acid medium may be difficult in the face of heavy selection on the fo lding, stability and other properties of the protein medium. Repositio ning cofactors by even a few angstroms has more profound effects on pr omoting and retarding rates, independent of the structure of the amino acid medium.