The nicotinamide moiety is the business end of the NADH and NADPH cofactors. It can take two forms which look like this:



       H     H   O                      H      O
        \   /   ||                      |     ||
         \ /    ||                      |     ||
         / \   /  \                    / \\  /  \
        /   \ /    \                  /   \\/    \
       ||   ||      N-H              ||    |      N-H
       ||   ||      |                ||    |      |
        \   /       H                 \   //      H
         \ /                           \ //
          N                             N (+)
          |                             |
          |                             | 
          R                             R




The molecule on the left is reduced nicotinamide and the molecule on the right is oxidized nicotinamide. The cofactors transfer their energy to other molecules in the cell by the transfer of a hydride (one proton plus two electrons), becoming oxidized in the process (the oxidized cofactors are called NAD and NADP. The redox midpoint potential for this two-electron process, an important measure of its electron-donating properties, is -320 mV at pH 7.0. About half the time the molecule which receives the hydride is a flavoprotein.

Whether NADH and NADPH transfer their hydrides all at once (a concerted mechanism) or in an electron-proton-electron sequential process is currently a topic of much contention in biochemistry and biophysics.

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