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I know NADH is used in cellular respiration and NADPH is used in photosynthesis. What difference does the phosphate group make that the same one isn't or can't be used for both? Is there a greater reason for this separation or is it just coincidental? Why can't the two be interchanged?

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NADH provides energy for Catabolic reactions and as for NADPH it provides energy for anabolic reactions (Synthesis ) –  user3293 Mar 26 '13 at 1:45

3 Answers 3

The phosphate group in NADPH doesn't affect the redox abilities of the molecule, it is too far away from the part of the molecule involved in the electron transfer. What the phosphate group does is to allow enzymes to discriminate between NADH and NADPH, which allows the cell to regulate both independently.

The ratio of NAD+ to NADH inside the cell is high, while the ratio of NADP+ to NADPH is kept low. The role of NADPH is mostly anabolic reactions, where NADPH is needed as a reducing agent, the role of NADH is mostly in catabolic reactions, where NAD+ is needed as a oxidizing agent.

You'll find some more information about this in chapter 2 of "Molecular Biology of the Cell by Alberts et al.

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too bad I only have one upvote to give. A direct link to the section in Alberts that talks about NADH/NADPH was great! –  Steve Lianoglou Jan 26 '12 at 13:50
    
Great use of the link to the chapter - I wish more authors did this. –  Mac Cowell Sep 18 '13 at 2:30

NADPH is found in the cytosol and stroma (chloroplast) of eukaryotes. NADH is more ubiquitous, but mostly found in bacteria and in mitichondria, possibly evidence for the endosymbosis of bacteria in eukaryotes. Neither can pass easily through a membrane.

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Just to clear out some things:

As stateted above, NADH is produced in catabolic reactions and is later used in the electron transport chain to obtain energy by converting NADH back to NAD+.

NADPH is primarily produced in the oxidative part of the pentose phosphate pathway. NADPH is used in a) anabolic syntheses to produce cholesterol, fatty acids, transmittor substances and nucleotides. b) detoxifying processes as an antioxidant. NADPH is for example an essential part of CYP450 in the liver and rereduces gluthatione (one of the most potent antioxidants in nature) in order to make it active once again.

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It would be great if you provided some references for your response! –  Bez Oct 15 at 19:01

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