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.