Two Tryptophans Are Better Than One in Accelerating Electron Flow through a Protein.

ABSTRACT: We have constructed and structurally characterized a Pseudomonas aeruginosa azurin mutant Re126WWCuI , where two adjacent tryptophan residues (W124 and W122, indole separation 3.6-4.1 Å) are inserted between the CuI center and a Re photosensitizer coordinated to the imidazole of H126 (ReI(H126)(CO)3(4,7-dimethyl-1,10-phenanthroline)+). CuI oxidation by the photoexcited Re label (*Re) 22.9 Å away proceeds with a ?70 ns time constant, similar to that of a single-tryptophan mutant (?40 ns) with a 19.4 Å Re-Cu distance. Time-resolved spectroscopy (luminescence, visible and IR absorption) revealed two rapid reversible electron transfer steps, W124 ? *Re (400-475 ps, K 1 ? 3.5-4) and W122 ? W124•+ (7-9 ns, K 2 ? 0.55-0.75), followed by a rate-determining (70-90 ns) CuI oxidation by W122•+ ca. 11 Å away. The photocycle is completed by 120 ?s recombination. No photochemical CuI oxidation was observed in Re126FWCuI , whereas in Re126WFCuI , the photocycle is restricted to the ReH126W124 unit and CuI remains isolated. QM/MM/MD simulations of Re126WWCuI indicate that indole solvation changes through the hopping process and W124 ? *Re electron transfer is accompanied by water fluctuations that tighten W124 solvation. Our finding that multistep tunneling (hopping) confers a ?9000-fold advantage over single-step tunneling in the double-tryptophan protein supports the proposal that hole-hopping through tryptophan/tyrosine chains protects enzymes from oxidative damage.

SUBMITTER: Takematsu K 

PROVIDER: S-EPMC6346393 | biostudies-literature | 2019 Jan

REPOSITORIES: biostudies-literature