Dataset Information

Solvent and Temperature Probes of the Long-Range Electron-Transfer Step in Tyramine ?-Monooxygenase: Demonstration of a Long-Range Proton-Coupled Electron-Transfer Mechanism.

ABSTRACT: Tyramine ?-monooxygenase (T?M) belongs to a family of physiologically important dinuclear copper monooxygenases that function with a solvent-exposed active site. To accomplish each enzymatic turnover, an electron transfer (ET) must occur between two solvent-separated copper centers. In wild-type T?M, this event is too fast to be rate limiting. However, we have recently shown [Osborne, R. L.; et al. Biochemistry 2013, 52, 1179] that the Tyr216Ala variant of T?M leads to rate-limiting ET. In this study, we present a pH-rate profile study of Tyr216Ala, together with deuterium oxide solvent kinetic isotope effects (KIEs). A solvent KIE of 2 on kcat is found in a region where kcat is pH/pD independent. As a control, the variant Tyr216Trp, for which ET is not rate determining, displays a solvent KIE of unity. We conclude, therefore, that the observed solvent KIE arises from the rate-limiting ET step in the Tyr216Ala variant, and show how small solvent KIEs (ca. 2) can be fully accommodated from equilibrium effects within the Marcus equation. To gain insight into the role of the enzyme in the long-range ET step, a temperature dependence study was also pursued. The small enthalpic barrier of ET (Ea = 3.6 kcal/mol) implicates a significant entropic barrier, which is attributed to the requirement for extensive rearrangement of the inter-copper environment during PCET catalyzed by the Tyr216Ala variant. The data lead to the proposal of a distinct inter-domain pathway for PCET in the dinuclear copper monooxygenases.

SUBMITTER: Zhu H

PROVIDER: S-EPMC4970857 | biostudies-literature | 2015 May

REPOSITORIES: biostudies-literature

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Publications

Solvent and Temperature Probes of the Long-Range Electron-Transfer Step in Tyramine β-Monooxygenase: Demonstration of a Long-Range Proton-Coupled Electron-Transfer Mechanism.

Zhu Hui H Sommerhalter Monika M Nguy Andy K L AK Klinman Judith P JP

Journal of the American Chemical Society 20150428 17

Tyramine β-monooxygenase (TβM) belongs to a family of physiologically important dinuclear copper monooxygenases that function with a solvent-exposed active site. To accomplish each enzymatic turnover, an electron transfer (ET) must occur between two solvent-separated copper centers. In wild-type TβM, this event is too fast to be rate limiting. However, we have recently shown [Osborne, R. L.; et al. Biochemistry 2013, 52, 1179] that the Tyr216Ala variant of TβM leads to rate-limiting ET. In this ...[more]

PMID: 25919134

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Project description:Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science 2019, 364, 471-475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene-phenol-pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (-ΔG°) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (ΔG*CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in ΔG° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B 1999, 103, 9130-9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both ΔG°CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

Dataset Information

Solvent and Temperature Probes of the Long-Range Electron-Transfer Step in Tyramine ?-Monooxygenase: Demonstration of a Long-Range Proton-Coupled Electron-Transfer Mechanism.

Publications

Solvent and Temperature Probes of the Long-Range Electron-Transfer Step in Tyramine β-Monooxygenase: Demonstration of a Long-Range Proton-Coupled Electron-Transfer Mechanism.

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