Section 25.2 Niacin: The Vitamin Needed for Many Redox Reactions 1039NADand NADPare oxidizing agents.25.2 Niacin: The Vitamin Needed for Many
Redox Reactions
An enzyme that catalyzes an oxidation reaction or a reduction reaction requires a
coenzyme because none of the amino acid side chains can act as oxidizing or reducing
agents. The coenzyme is the oxidizing or reducing agent. The enzyme’s role is to hold
the substrate and coenzyme together so that the oxidation or reduction reaction can
take place.
The coenzymes most commonly used by enzymes to catalyze oxidation reactions are
nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide
phosphate. (You were introduced to in Section 20.11.)
When (or ) oxidizes a substrate, the coenzyme is reduced to NADH
(or NADPH). NADH and NADPH are reducing agents; they are used as coenzymes by
enzymes that catalyze reduction reactions. Enzymes that catalyze oxidation reactions
bind (or ) more tightly than they bind NADH (or NADPH). When the
oxidation reaction is over, the relatively loosely bound NADH (or NADPH) dissoci-
ates from the enzyme. Likewise, enzymes that catalyze reduction reactions bind
NADH (or NADPH) more tightly than they bind (or ). When the re-
duction reaction is over, the relatively loosely bound (or ) dissociates
from the enzyme.
is composed of two nucleotides linked together through their phosphate
groups. A nucleotideconsists of a heterocycle attached, in a to C-1 of
a phosphorylated ribose (Section 27.1). A heterocycleis a cyclic compound in which
one or more of the ring atoms is an atom other than carbon (p. 872).
b-configuration,NAD+substratereduced NAD+enzyme
+ substrateoxidized + NADH + H+substratereduced NADP+enzyme
+ substrateoxidized + NADPH + H+NAD+ NADP+NAD+ NADP+NAD+ NADP+NAD+ NADP+ON+CNH 2HP
−ONAD+ X = H
NADP+ X = PO 32 −adeninepyridinenicotinamideNADH X = H
NADPH X = PO 32 −OHO OHOON NN NNH 2P
−OOHO OXOONCNH 2HHOOP
−OOHO OHOON NN NNH 2P
−OOHO OXOOO(NADP) NAD+(NAD)NADH and NADPH are reducing agents.