Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CATALYSIS AND THE USE OF ENERGY BY CELLS
67
NADH and NADPH Are Important Electron Carriers
Other important activated carrier molecules participate in oxidation–reduction
reactions and are commonly part of coupled reactions in cells. These activated
carriers are specialized to carry electrons held at a high energy level (sometimes
called “high-energy” electrons) and hydrogen atoms. The most important of these
electron carriers are NAD + (nicotinamide adenine dinucleotide) and the closely
related molecule NADP + (nicotinamide adenine dinucleotide phosphate). Each
picks up a “packet of energy” corresponding to two electrons plus a proton (H + ),
and they are thereby converted to NADH (reduced nicotinamide adenine dinucleotide)
and NADPH (reduced nicotinamide adenine dinucleotide phosphate),
respectively (Figure 2–36). These molecules can therefore be regarded as carriers
of hydride ions (the H + plus two electrons, or H – ).
Like ATP, NADPH is an activated carrier that participates in many important
biosynthetic reactions that would otherwise be energetically unfavorable. The
NADPH is produced according to the general scheme shown in Figure 2–36A.
During a special set of energy-yielding catabolic reactions, two hydrogen atoms
are removed from a substrate molecule. Both electrons but just one proton (that is,
a hydride ion, H – ) are added to the nicotinamide ring of NADP + to form NADPH;
the second proton (H + ) is released into solution. This is a typical oxidation–reduction
reaction, in which the substrate is oxidized and NADP + is reduced.
NADPH readily gives up the hydride ion it carries in a subsequent oxidation–reduction
reaction, because the nicotinamide ring can achieve a more
stable arrangement of electrons without it. In this subsequent reaction, which
(A)
H
C
C
OH
O
oxidation of
molecule 1
NADP +
NADPH
+ H +
H
H
C
C
C
C
reduction of
molecule 2
Figure 2–36 NADPH, an important carrier of electrons.
(A) NADPH is produced in reactions of the general type shown on
the left, in which two hydrogen atoms are removed from a substrate.
The oxidized form of the carrier molecule, NADP + , receives one
hydrogen atom plus an electron (a hydride ion); the proton (H + ) from
the other H atom is released into solution. Because NADPH holds
its hydride ion in a high-energy linkage, the hydride ion can easily
be transferred to other molecules, as shown on the right. (B) and
(C) The structures of NADP + and NADPH. The part of the NADP +
molecule known as the nicotinamide ring accepts the hydride ion,
H – , forming NADPH. The molecules NAD + and NADH are identical in
structure to NADP + and NADPH, respectively, except that they lack
the indicated phosphate group.
(B)
(C)
NADP +
oxidized form
NADPH
reduced form
H
O
H
H
O
nicotinamide
ring
+
N
C
NH 2
N
C
NH 2
P
O
RIBOSE
H –
P
O
RIBOSE
ADENINE
ADENINE
P
O
RIBOSE
P
O
RIBOSE
O
O
P
this phosphate group is
missing in NAD + and NADH
P