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It is known that the cytoplasm is a "reducing" environment, where disulfide bonds cannot form (will soon be reduced to 2 cysteines) [I'm not putting a link as this is a fact in many biology textbooks].

The NAD+:NADH ratio is high in the cytoplasm (around 1000 (725 in this paper)), which should make the cytoplasm an oxidising environment. The NADP+:NADPH ratio is low, making a reducing environment. Also, there is Glutatione Reductase, which should also make the cytoplasm a reducing environment. Which one of these is the underlying cause of this reducing environment? Or is FAD also involved in this?

As an additional question, what does reduce disulfide bonds? Is it using the NADPH or the FADH2 or the glutathione reductase?

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The redox buffer in the cytosol is glutathione (reduced GSH and oxidized GSSG), which is present in large amounts (López-Mirabal & Winther 2008 , doi: 10.1016/j.bbamcr.2007.10.013).

Proteins with disulfide bonds are often secreted, since the disulfide bond ensures proper structure outside of the cell. These proteins are imported into the endoplasmatic reticule (ER) that contains enzymes (e.g. PDI) and an oxidative milieu for disulfide bond formation. Thioredoxin breaks down disulfide bonds in the cytosol and is regenerated by TrxR, which in turn is regenerated by NADPH (Bulleid 2012, doi: 10.1101/cshperspect.a013219)

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