Can an engineered bacterial protein imitate a light-driven electron transfer used in photosynthesis?

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The central photochemical reaction in photosystem II of green algae and plants, and in the reaction center of some photosynthetic bacteria, transfers one electron from a light-activated chlorin complex to a bound quinone molecule. In a laboratory protein-engineering experiment, researchers modified a protein to mimic this reaction.

They engineered a unique quinone-binding site into Escherichia coli cytochrome b(562) by introducing a cysteine into the protein's hydrophobic interior. Various quinones, including p-benzoquinone and 2,3-dimethoxy-5-methyl-1,4-benzoquinone, were covalently attached to the protein through an addition reaction between the cysteine sulfur and the quinone ring. The cysteine was positioned to bind the quinone approximately 10 Å from the edge of the bound porphyrin.

Fluorescence measurements confirmed that the bound hydroquinone sits toward the protein's hydrophobic interior and is partially shielded from the solvent. The bound quinones remained redox-active: at neutral pH, they could be oxidized and reduced again in a two-electron process. At high pH, a one-electron reduction could generate semiquinone, and binding different quinones to the protein could adjust the midpoint potential for this process by approximately 500 mV.

The researchers then reconstituted the modified cytochrome's heme-binding site with the chlorophyll analogue zinc chlorin e(6). Using EPR and fast optical techniques, they showed that the various chlorin–protein–quinone complexes could undergo light-induced electron transfer from the chlorin to the bound oxidized quinone, but not to hydroquinone. Electron transfer rates were on the order of 10⁸ s⁻¹.