2.2 The stroma has multiple sulfhydryl oxidation systems
The majority of research on redox regulation in chloroplasts has focused on light-dependent reduction of regulatory thiols in the light. More recently, attention has shifted to the mechanism of “thiol oxidation”, the “reversal” of light-induced reduction important for determining the steady-state redox poise of regulatory thiols.
The thiol-modulated enzymes in the stroma have redox potentials relatively close to those of the major redox carrier, NADPH (see also below), so that the reaction should be thermodynamically reversible (Kramer et al. 1990). This means that a net reduction or oxidation of NADPH, should result in concomitant changes in the thiol regulatory status. For instance, relative changes in the rates of the light and assimilatory reactions should alter redox balance, in turn affecting the activation of a range of thiol-modulated enzymes (Kramer et al. 1990; Ort et al. 1990; Stitt 2004). The extent to which such modulation occurs, however, will depend on the relative redox potentials of the target enzymes. Some of these (e.g. FBPase) have relatively negative redox potentials and should be more rapidly reversed, whereas others, notably the chloroplast ATP synthase, have less negative potentials, and will only be reoxidized after extensive oxidation of NADPH.
This simple, redox quasi-equilibrium model has, in recent years, been challenged by the identification of redox proteins that specifically oxidize specific sets of regulatory thiols. For example, the newly discovered 2-Cys peroxiredoxin (2CP) is involved in oxidizing reductively activated proteins in the dark (Fig. 1B) (Yoshida, Hara, Sugiura, Fukaya & Hisabori 2018; Yoshida, Yokochi & Hisabori 2019; Vaseghi et al. 2018; Ojeda, Pérez-Ruiz & Cejudo 2018; Yokochi, Fukushi, Wakabayashi, Yoshida & Hisabori 2021). The proposed mediators for thiol oxidation of target proteins are Trx-f, Trx-like2 (TrxL2), atypical Cys His-rich Trx (ACHT), which have a less negative redox potential (more oxidizing) than the thiols on typical regulatory proteins (Eliyahu, Rog, Inbal & Danon 2015; Yoshida et al. 2018, 2019; Vaseghiet al. 2018; Ojeda et al. 2018; Yokochi et al.2019, 2021). Those mediators transfer reducing power from redox-regulated proteins to 2CP. Reduced 2CP then reduces H2O2 to H2O. The protein-oxidizing activity of mediators such as TrxL2 is strongly dependent on 2CP and H2O2(Yoshidaet al. 2018, 2019; Vaseghi et al. 2018; Ojeda et al. 2018; Yokochi et al. 2021).
Finally, other forms of peroxiredoxins, such as Prx IIE and Prx Q, did not show a similar role to 2CP in the dark transition. However, there are contradictory results for PrxQ (Yoshidaet al. 2018, 2019; Vaseghi et al. 2018; Ojeda et al. 2018; Telman, Liebthal & Dietz 2020; Yokochi et al. 2021). It was also found that NTRC-modulated 2CP contributes to the control of chloroplast redox homeostasis (Pérez-Ruiz, Naranjo, Ojeda, Guinea & Cejudo 2017). Several thiol-regulated enzymes were revealed from these studies to be oxidized by 2CP, including CF1-γ, FBPase, SBPase, Rca, and NADPH-malate dehydrogenase (MDH) (Yoshidaet al. 2018, 2019; Vaseghi et al. 2018; Ojeda et al. 2018; Yokochi et al. 2021).