Problems 1071Summary
Cofactorsassist enzymes in catalyzing a variety of reac-
tions that cannot be catalyzed solely by their amino acid
side chains. Cofactors can be metal ions or organic mole-
cules. An enzyme with a tightly bound metal ion is called a
metalloenzyme. Cofactors that are organic molecules are
called coenzymesand these are derived from vitamins. A
vitamin is a substance the body cannot synthesize that is
needed in small amounts for normal body function. All the
water-soluble vitamins except vitamin C function as coen-
zymes. Vitamin K is the only water-insoluble vitamin cur-
rently recognized to function as a coenzyme.
Coenzymes play a variety of chemical roles that the
amino acid side chains of enzymes cannot play: Some func-
tion as oxidizing and reducing agents; some allow electrons
to be delocalized; some activate groups for further reaction;
and some provide good nucleophiles or strong bases needed
for a reaction. Molecular recognitionallows the enzyme to
bind the substrate and the coenzyme in the proper orienta-
tion for reaction. Coenzymes are recycled. An enzyme plus
its cofactor is called a holoenzyme. An enzyme that has had
its cofactor removed is called an apoenzyme.
Metabolism—the set of reactions that living organisms
carry out to obtain energy and to synthesize the compounds
they require—can be divided into catabolism and
anabolism. Catabolic reactionsbreak down complex mol-
ecules to provide energy and simple molecules. Anabolic
reactionsrequire energy and lead to the synthesis of com-
plex biomolecules.
The coenzymes used by enzymes to catalyze oxidation re-
actions are NAD,NADP,FA D, and FMN; those used to
catalyze reduction reactions are NADH,NADPH,
and Many enzymes that catalyze oxidation reac-
tions are called dehydrogenases. All the redox chemistry of
the pyridine nucleotide coenzymestakes place at the 4-po-
sition of the pyridine ring. All the redox chemistry of the
flavin coenzymestakes place on the flavin ring.
Thiamine pyrophosphate (TPP)is the coenzyme re-
quired by enzymes that catalyze the transfer of a two-car-
bon fragment. Biotinis the coenzyme required by enzymes
that catalyze carboxylation of a carbon adjacent to a car-
bonyl group. Pyridoxal phosphate (PLP)is the coenzyme
required by enzymes that catalyze certain transformations
of amino acids: decarboxylation, transamination, racemiza-
tion, bond cleavage, and In a
transimination reaction, one imine is converted into an-
other imine; in a transamination reaction, the amino
group is removed from a substrate and transferred to anoth-
er molecule.
In a coenzyme –requiring reaction, a group bonded
to one carbon changes places with a hydrogen bonded to
an adjacent carbon. Tetrahydrofolate (THF)is the coen-
zyme used by enzymes catalyzing reactions that transfer
a group containing a single carbon—methyl, methylene,
or formyl—to their substrates. Vitamin is the coen-
zyme for the enzyme that catalyzes the carboxylation of
the of glutamate side chains—a reaction re-
quired for blood clotting. A suicide inhibitorinactivates
an enzyme by taking part in the normal catalytic mecha-
nism. Competitive inhibitorscompete with the substrate
for binding at the active site of the enzyme.g-carbonKH 2B 12Ca¬Cb a,b-elimination.FMNH 2.FADH 2 ,Key Terms
anabolism (p. 1035)
apoenzyme (p. 1034)
biotin (p. 1052)
catabolism (p. 1035)
coenzyme (p. 1033)
coenzyme A (CoASH) (p. 1050)
coenzyme (p. 1061)
cofactor (p. 1033)
competitive inhibitor (p. 1067)
dehydrogenase (p. 1040)
electron sink (p. 1048)
B 12flavin adenine dinucleotide (FAD) (p. 1044)
flavin mononucleotide (FMN) (p. 1044)
holoenzyme (p. 1034)
lipoate (p. 1050)
mechanism-based inhibitor (p. 1067)
metabolism (p. 1035)
metalloenzyme (p. 1033)
nicotinamide adenine dinucleotide
( ) (p. 1039)
nicotinamide adenine dinucleotide
phosphate (NADP+) (p. 1039)NAD+nucleotide (p. 1039)
pyridoxal phosphate (PLP) (p. 1054)
suicide inhibitor (p. 1067)
tetrahydrofolate (THF) (p. 1064)
thiamine pyrophosphate (TPP) (p. 1048)
transamination (p. 1056)
transimination (p. 1055)
vitamin (p. 1033)
vitamin (p. 1068)KH 2Problems
- Answer the following questions:
a. What six cofactors act as oxidizing agents?
b. What are the cofactors that donate one-carbon groups?Author: term is boldface in text. Add to
key terms? Or make lightface in text?heterocycle (p. 1039)