Reduction of Benzoquinones to Hydroquinones via Spontaneous Reaction with Glutathione and Enzymatic Reaction by S-Glutathionyl-Hydroquinone Reductases

Abstract

S-Glutathionyl-hydroquinone reductases (GS-HQRs) are a new class of glutathione transferases, widely present in bacteria, halobacteria, fungi, and plants. They catalyze glutathione (GSH)-dependent reduction of GS-trichloro-p-hydroquinone to trichloro-p-hydroquinone. Since GS-trichloro-p-hydroquinone is uncommon in nature, the extensive presence of GS-HQRs suggests they use common GS-hydroquinones. Here we demonstrate that several benzoquinones spontaneously reacted with GSH to form GS-hydroquinones via Michael addition, and four GS-HQRs from yeast and bacteria reduced the GS-hydroquinones to the corresponding hydroquinones. The spontaneous and enzymatic reactions led to the reduction of benzoquinones to hydroquinones with the concomitant oxidation of GSH to oxidized glutathione (GS-SG). The enzymes did not use GS-benzoquinones or other thiol-hydroquinones, for example, S-cysteinyl-hydroquinone, as substrates. Apparent kinetic parameters showed the enzymes preferred hydrophobic, bulky substrates, such as GS-menadiol. The broad substrate range and their wide distribution suggest two potential physiological roles: channeling GS-hydroquinones back to hydroquinones and reducing benzoquinones via spontaneous formation of GS-hydroquinones and then enzymatic reduction to hydroquinones. The functions are likely important in metabolic pathways with quinone intermediates.

Structural Understanding of the Glutathione-dependent Reduction Mechanism of Glutathionyl-Hydroquinone Reductases

“…8). We have shown that GSH removal can also be established by other thiol-containing compounds, such as L-cysteine, DTT, and 2-mercaptoethanol (4) or even by the reducing agent NaBH 4 (7). This reaction scheme is in agreement with our previous kinetic analysis that PcpF catalyzes GSH-dependent reduction of GS-TriCH via a ping-pong mechanism (4).…”

“…For example, PcpF is able to deglutathionylate S-(phenacyl) glutathione, but EcYqjG, CnYqjG, and ECM4 PcGHR1 do not use it as a substrate (5). In addition, the specific enzyme activity varied among these enzymes (7). Therefore, in lieu of bioremediation applications, in-depth investigation of GS-HQRs could provide a new approach for treating environmental pollutants that produce various quinone intermediates during biodegradation.…”

“…However, it is plausible that GS-HQR ability to reduce a variety of GS-HQs places this new class of GSTs in a central role for cellular redox cycling. Hydroquinones are metabolites of benzene and phenol derivatives (21) and tend to be converted into benzoquinones upon autoxidation (7,22). Considering the spontaneous reaction between benzoquinones and GSH resulting in GS-hydroquinones, accumulation of GShydroquinones can be predicted to occur during the metabolism of various benzilic and phenolic compounds.…”

“…of benzoquinones that spontaneously react with GSH to produce GS-hydroquinones via Michael addition (7). GSH does not spontaneously reduce GS-hydroquinones to hydroquinones, but GS-HQRs irreversibly catalyze this reaction (8).…”

“…GSH does not spontaneously reduce GS-hydroquinones to hydroquinones, but GS-HQRs irreversibly catalyze this reaction (8). Due to the irreversible nature, GS-hydroquinones are not detectable at the end of enzymatic reaction (7).…”