Page 113 - Color Atlas of Biochemistry
P. 113
104 Metabolism
+
Coenzymes 1 metabolism. In contrast, reduced NADP is the
most important reductant involved in biosyn-
thesis (see p. 112).
A. Coenzymes: definitions
The flavin coenzymes FMN and FAD (2, 3)
In many enzyme-catalyzed reactions, elec- contain flavin (isoalloxazine) as a redox-active
trons or groups of atoms are transferred group. This is a three-membered, N-contain-
from one substrate to another. This type of ingringsystem that can accept a maximum of
reaction always also involves additional mol- two electrons and two protons during reduc-
ecules, which temporarily accept the group tion. FMN carries the phosphorylated sugar
being transferred. Helper molecules of this alcohol ribitol at the flavin ring. FAD arises
type are called coenzymes. As they are not from FMN through bonding with AMP. The
catalytically active themselves, the less fre- two coenzymes are functionally similar.
quently used term “cosubstrate” would be They are found in dehydrogenases, oxidases,
more appropriate. In contrast to substrates and monooxygenases.In contrastto the pyri-
for which a given enzyme is usually specific dine nucleotides, flavin reactions give rise to
(see p. 88), coenzymes cooperate with many radical intermediates (see p. 32). To prevent
enzymes of varying substrate specificity. We damage to cell components, the flavins al-
have rather arbitrarily divided the coenzymes ways remain bound as prosthetic groups in
here into group-transferring and redox coen- theenzymeprotein.
zymes. Strictly speaking, redox coenzymes Theroleof ubiquinone (coenzyme Q, 4)in
also transfer groups—namely, reducing equiv- transferring reducing equivalents in the res-
alents (see p. 32). piratory chain is discussed on p. 140. During
Depending on the type of interaction with reduction, the quinone is converted into the
the enzyme, a distinction is made between hydroquinone (ubiquinol). The isoprenoid side
soluble coenzymes and prosthetic groups. chain of ubiquinone can have various lengths.
Soluble coenzymes (1) are bound like It holds the molecule in the membrane, where
substrates during a reaction, undergo a chem- it is freely mobile. Similar coenzymes are also
ical change, and are then released again.The found in photosynthesis (plastoquinone; see
original form of the coenzyme is regenerated p. 132). Vitamins E and K (see p. 52) also be-
by a second, independent reaction. Prosthetic long to the quinone/hydroquinone systems.
groups (2), on the other hand, are coenzymes L-Ascorbic acid (vitamin C, 5)is a powerful
that are tightly bound to the enzyme and re- reducing agent. As an antioxidant,it provides
main associated with it during the reaction. nonspecific protection against oxidative dam-
Thepartof the substratebound by thecoen- age (see p. 284), but it is also an essential
zyme is later transferred to another substrate cofactor for various monooxygenases and di-
or coenzyme of the same enzyme (not shown oxygenases. Ascorbic acid is involved in the
in Fig. 2). hydroxylation of proline and lysine residues
during the biosynthesis of collagen (see
p. 344), in the synthesis of catecholamines
B. Redox coenzymes 1
(see p. 352) and bile acids (see p. 314), as
All oxidoreductases (see p. 88) require coen- well as in the breakdown of tyrosine (see
zymes. The most important of these redox p. 415). Thereduced form of thecoenzyme
coenzymes are shown here. They can act in is a relatively strong acid and forms salts,
soluble form (S) or prosthetically (P). Their the ascorbates. The oxidized form is known
normal potentials E 0 are shown in addition as dehydroascorbic acid. The stimulation of
to the type of reducing equivalent that they theimmunesystem caused byascorbicacid
transfer (see p. 18). has not yet been fully explained.
+
The pyridine nucleotides NAD and NADP +
(1) are widely distributed as coenzymes of
dehydrogenases. They transport hydride ions
–
+
(2e and 1 H ; see p. 32) and always act in
+
soluble form. NAD transfers reducing equiv-
alents from catabolic pathways to the respi-
ratory chain and thus contributes to energy
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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