Effect of Methylmercury Binding on the Peroxide-Reducing Potential of Cysteine and Selenocysteine

Abstract

Methylmercury (CH₃Hg⁺) binding to catalytically fundamental cysteine and selenocysteine of peroxide-reducing enzymes has long been postulated as the origin of its toxicological activity. Only very recently, CH₃Hg⁺ binding to the selenocysteine of thioredoxin reductase has been directly observed [Pickering, I. J. Inorg. Chem., 2020, 59, 2711-2718], but the precise influence of the toxicant on the peroxide-reducing potential of such a residue has never been investigated. In this work, we employ state-of-the-art density functional theory calculations to study the reactivity of molecular models of the free and toxified enzymes. Trends in activation energies are discussed with attention to the biological consequences and are rationalized within the chemically intuitive framework provided by the activation strain model. With respect to the free, protonated amino acids, CH₃Hg⁺ binding promotes oxidation of the S or Se nucleus, suggesting that chalcogenoxide formation might occur in the toxified enzyme, even if the actual rate of peroxide reduction is almost certainly lowered as suggested by comparison with fully deprotonated amino acids models.

Scheme 1. GPx Catalytic Cycle (Left)

The first step (1) Is the actual peroxide reduction, displaying the conversion of a selenol (−SeH) to a selenenic acid (−SeOH) that can be reduced back to a selenol by two glutathione (GSH) molecules. (Right) CH₃Hg⁺-inhibited Sec GPx mechanism of peroxide reduction, as has been postulated. The step, equivalent to step (1) of the functional GPx, displays the oxidation of a methylmercury selenolate to a selenoxide, with consequent peroxide reduction.

Scheme 2. (a) Oxidation of Cys, Sec, and Tec to Chalcogenoxide Followed by Isomerization to Chalcogenenic Acid; (b) Oxidation of MeHgCys, MeHgSec, and MeHgTec to Chalcogenoxide

Scheme 3. (a) Cys and Sec Direct Oxidation to Chalcogenenic Acid and (b) MeHgCys and MeHgSec Oxidation to Chalcogenoxide via Solvent-Assisted Proton Exchange

The proton shuttling at the transition state is depicted in parentheses.

Scheme 4. Cys^– and Sec^– Direct Oxidation to Deprotonated Chalcogenenic Acids

Figure 1

Relevant stationary points along the stepwise oxidation mechanism for Cys. Relevant interatomic distances are given in Å. Analogous structures were optimized for Sec and Tec.

Figure 2

Selected structures for Cys oxidation along the SAPE pathway. (Top left) RC; (top right) TS; (bottom left) PC; (bottom right) TS for MeHgCys oxidation. Relevant interatomic distances are given in Å. Analogous structures were optimized for Sec and MeHgSec.

Figure 3

Stationary points for Cys (blue, solid, filled circles), MeHgCys (blue, dashed, void circles), Cys^– (blue, dotted, half-filled circles), Sec (orange, solid, filled triangles), MeHgSec (orange, dashed, void triangles), and Sec^– (orange, dotted, half-filled triangles); oxidation along the minimal/anionic (top) and SAPE (bottom) pathways. Level of theory: ZORA–BLYP-D3(BJ)/TZ2P.

Figure 4

Left: ASA along the rc for the oxidation of Cys (blue) and MeHgCys (black). Right: ASA along the rc for the oxidation of Sec (orange) and MeHgSec (black). The solid lines represent IRC profiles, the dashed lines represent ΔE_strain, while the dashed-dotted lines represent ΔE_int. Filled symbols (circles, S; diamonds, Se) represent the position of the TS along the rc. Empty symbols represent the value of strain/interaction at the TS. d_O–O⁰ refers to the O–O bond length in the RC of each reaction.

Figure 5

ASA for Cys (blue), Cys, SAPE mechanism (white), MeHgCys (orange), and MeHgCys, SAPE mechanism (gray). Right: relative variations of energy values (TS–RC); left: the energy values at the transition state.

Figure 6

ASA for Sec (blue), Sec SAPE mechanism (white), MeHgSec (orange), and MeHgSec, SAPE mechanism (gray). Right: relative variations of energy values (TS–RC); left: the energy values at the transition state.