An overview of mechanisms of redox signaling

Abstract

A principal characteristic of redox signaling is that it involves an oxidation-reduction reaction or covalent adduct formation between the sensor signaling protein and second messenger. Non-redox signaling may involve alteration of the second messenger as in hydrolysis of GTP by G proteins, modification of the signaling protein as in farnesylation, or simple non-covalent binding of an agonist or second messenger. The chemistry of redox signaling is reviewed here. Specifically we have described how among the so-called reactive oxygen species, only hydroperoxides clearly fit the role of a second messenger. Consideration of reaction kinetics and cellular location strongly suggests that for hydroperoxides, particular protein cysteines are the targets and that the requirements for redox signaling is that these cysteines are in microenvironments in which the cysteine is ionized to the thiolate, and a proton can be donated to form a leaving group. The chemistry described here is the same as occurs in the cysteine and selenocysteine peroxidases that are generally considered the primary defense against oxidative stress. But, these same enzymes can also act as the sensors and transducer for signaling. Conditions that would allow specific signaling by peroxynitrite and superoxide are also defined. Signaling by other electrophiles, which includes lipid peroxidation products, quinones formed from polyphenols and other metabolites also involves reaction with specific protein thiolates. Again, kinetics and location are the primary determinants that provide specificity required for physiological signaling although enzymatic catalysis is not likely involved. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Keywords: 4-Hydroxynonenal; Hydrogen peroxide; Hydroperoxide; Peroxidases; Reactive oxygen species; Signal transduction.

Figure 1. Scheme of the catalysis in thiol peroxidases (TPx)

The catalytic cycle of glutathione peroxidases (GPx) and, (B) of atypical two cysteines peroxiredoxin (Prdx). (A) In the active site of GPx, the catalytic moiety is a selenium (-Se) of a selenocysteine or the sulphur (-S) of a cysteine, depending on the variant. The reduced, ground state enzyme is oxidised by a hydroperoxide (ROOH) yielding an oxidized intermediate, which is believed a selenenic (sulphenic) acid derivative of the seleno(cysteine) moiety. This is stepwise reduced by two GSH via a semi-reduced intermediate yielding a (S)Se-glutathionylated GPx. (B) The active site of the atypical two cysteines Prdx invariably contains the S moiety of a peroxidatic cysteine (-S⁻ ), and a second cysteine thiol (-SH), acting as resolving. Upon oxidation by the ROOH, an intramolecular disulphide between the catalytic and the resolving cysteine is formed. One molecule of Trx regenerates the reduced enzyme via a thiol disulfide exchange, yielding a mixed disulfide with Prdx as a semi-reduced intermediate. In the typical 2 cysteine Prdx, the cycle and the substrates involved are identical, except that the resolving cysteine is in another Prdx molecule. Accordingly, an intermolecular disulfide is the oxidized species formed by the ROOH in these enzymes. Indeed, Prdx that contain one cysteine residue only, such as mammalian Prdx6, exist in mammals. These enzymes accept GSH as the reductant and, similarly to GPx, the semi-reduced intermediate is S-glutathionylated. As a peculiarity, the reduction of Prdx6 by GSH is preferentially assisted by GSTP (see section 4.2) [35]. Note that the two reductive steps are in principle reversible. The specificity for GSH of the TPxs may not always be strict. Indeed some TPx may accept protein thiols instead of GSH, yielding, theoretically, protein (mixed) disufides or S-thiolation of the protein substrate.