Slightly off topic, but for a year or so now, I've been extremely confused when it comes to NAD vs. NADH vs. NADPH. SO confusing…
@nandixon, can you explain these as if you were talking to a first grader?
NAD comes from nicotinic acid or niacin, but that's about as far as I understand it, then my brain goes to mush.
Thank you.
Not sure how simple I can make it, but I guess to start with, there really isn't a significant difference between NAD+/NADH and NADP+/NADPH from a general chemistry point of view, in terms of their abilities to perform reduction-oxidation reactions. (You probably know that the "+" versions are the oxidized forms; the "H" versions the reduced forms.)
So while the extra phosphate group in NADPH (or NADP+) doesn't affect its redox abilities relative to NADH (or NAD+), what it does do is give a molecule of NADPH a different shape/binding property from that of NADH, so that NADPH and NADH can be recognized differently and bind as substrates to different sets of enzymes.
This is important so that the cell containing them can regulate NADH and NADPH independently. (Generally speaking, enzymes that use NAD+/NADH don't use NADP+/NADPH, and vice versa.)
So, for example, normally the ratio of NAD+ (the oxidized form) to NADH (the reduced form) inside the cell is kept high (i.e., more oxidizing), while the ratio of NADP+ to NADPH is kept low (i.e., more reducing).
Thus, the role of NADH is mostly in catabolic reactions (breaking down molecules), where NAD+ is needed as an oxidizing agent. During this process, NAD+ receives electrons and hydrogen and is converted to NADH.
Critically, the NADH that is formed can transfer those electrons to the electron transport chain (ETC) to generate ATP. (NADH is converted back to NAD+ with this transfer.)
On the other hand, the role of NADPH is mostly in anabolic reactions (biosynthesis), where NADPH is needed as a reducing agent. So it donates electrons and hydrogen to make new compounds like cholesterol, fatty acids, and others.
Critically, this reducing action of NADPH includes regenerating glutathione that has been oxidized.
NAD+/NADH also has other roles, including reductive synthesis like NADPH. NADP+/NADPH also has other roles too, including oxidative related ones. So things are not black and white by any means.