Reduction Potential Tuning at a Type 1 Copper Site Does Not Compromise Electron Transfer Reactivity

摘要

Type 1 (T1) copper sites promote biological electron transfer (ET) and typically possess a weakly coordinated thioether sulfur from an axial Met [Cu(Ⅱ)—S~δ ～ 2.6 to 3.3 A] along with the conserved His_2Cys equatorial ligands. A strong axial bond [Cu(Ⅱ)-O~(ε1) ～ 2.2 A] is sometimes provided by a Gln (as in the stellacyanins), and the axial ligand can be absent (a Val, Leu or Phe in the axial position) as in ceruloplasmin, Fet3p, fungal laccases and some plantacyanins (PLTs). Cucumber basic protein (CBP) is a PLT which has a relatively short Cu(Ⅱ)-S(Met89) axial bond (2.6 A). The Met89Gln variant of CBP has an electron self-exchange (ESE) rate constant (k_(ese), a measure of intrinsic ET reactivity) ～7 times lower than that of the wild-type protein. The Met89Val mutation to CBP results in a 2-fold increase in k_(ese). As the axial interaction decreases from strong O~(ε1) of Gln to relatively weak S~δ of Met to no ligand (Val), ESE reactivity is therefore enhanced by ～1 order of magnitude while the reduction potential increases by ～350 mV. The variable coordination position at this ubiquitous ET site provides a mechanism for tuning the driving force to optimize ET with the correct partner without significantly compromising intrinsic reactivity. The enhanced reactivity of a three-coordinate T1 copper site will facilitate intramolecular ET in fungal laccases and Fet3p.