I know respiration and photosynthesis are based on oxidation and reduction, and the formation of NADH and NADPH in them is very important for many cell processes. I know that NADPH, for example, is used in the CBB cycle to fix carbon. However where else are oxidation and reduction reactions needed in cells? I can think of an example for oxidation: getting rid of toxic products. For example, alcohol dehydrogenase in the liver oxidises alcohol to acetaldehyde. However I can't think of much else, and i'm especially stuck on ehy reductive power is useful in cells.
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This topic is quite broad, excluding energetic metabolism, there are still many reactions that use redox equivalents, whether NADH, NADPH, or FADH. Here are some paradigmatic illustrations.
- Oxydation of substrate : as you mentioned, alcohol to aldehyde to carboxylic acid. Generate reducing power. (The alkane oxydation shown below occurs in bacteria)
- Mono-oxygenation : hydroxylation of a substrate with $O_2$, with the other oxygen atom reduced to water. This proceeds as follow with CYP450 enzymes :
Basically every time there is a need to create/remove a double bond, or move from carbonyl to aldehyde to alcohol and back, redox equivalent are used.
- Fatty acid elongation : as this proceeds with acetyl-CoA as carbon source, intuitively one sees the need to reduce the carbonyl to obtain an alkane chain.
- HMG-CoA Reductase (HMGCoAR) catalyses the commited step in cholesterol synthesis.
- Synthesis of prostaglandins from arachidonic acid. (More generally, any lipidic mediator)
All illustrations from Biochemical Pathways, G. Michal & D. Schomburg, 2nd edition, Wiley