In vivo oxidation by thioredoxin regulates chloroplast enzymes activity

Abstract

Thioredoxins (Trxs) are proteins that can receive or donate electrons via reduction or oxidation, respectively, of their redox-active cysteine pair, converting a disulfide to thiol or vice versa. They are involved in multiple biological processes throughout all kingdoms of life, modulating the activity of enzymes that function, for example, in ribonucleotide reduction, nutrient assimilation, detoxification of reactive oxygen species, cell signaling, or photosynthesis (1). In plant biology, classic Trxs are especially well known for their role in reducing and thereby activating the enzymes of the Calvin–Benson cycle (refer to https://blog.aspb.org/calvin-cycle-calvin-benson-cycle-or-other/ for an explanation about not including Bassham in the name of this cycle), such as ribulose-1,5-bisphosphate carboxylase/oxygenase activase (RCA), fructose-1,6-bisphosphatase (FBPase), or phosphoribulokinase (PRK) for CO2 fixation (2). The reducing power comes from light-dependent reactions through photosystem I, ferredoxin (Fd), and ferredoxin-thioredoxin reductase (FTR) or Fd-NADP+ reductase for NADPH production. Hence, the division of chloroplast reactions into light and carbon reactions (please, correct textbooks and stop teaching it as “dark” reactions!) is due to the carbon reactions relying on illumination for reducing power to activate the Calvin–Benson cycle enzymes (3). In the recent years, an outstanding question was regarding their deactivation. What factors in the dark oxidize the Calvin–Benson enzymes? An important milestone was the in vitro demonstration that Trx-like proteins, such as Trx-like 2 (TrxL2) (4, 5), atypical Cys His-rich thioredoxin (ACHT) (6), and Trx- f (7), can play this role. The in vivo demonstration was eagerly awaited (8), and this knowledge gap is now filled by Yokochi et al. (9). Another part of the puzzle concerned the oxidation of the oxidizing factors … [↵][1]1Email: alizee.malnoe{at}umu.se. [1]: #xref-corresp-1-1