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Encyclopedia of Physical Science and Technology EN017G-116 August 2, 2001 18:14

528 Vitamins and Coenzymes

FIGURE 23 Structures of a few recently discovered coenzymes or prosthetic groups.

in Fig. 23. The coenzyme PQQ is a hydrogen carrier that pounds which, like topaquinone and MIO, are made from
replacesNADincertainbacterialalcoholdehydrogenases. side chains of amino acids. The list could be continued for
Topaquinone is a cofactor for amino oxidases found in pages.
bacterial, plant, and human tissues. It is a prosthetic group
that is formed by oxidation of a specific tyrosine in the
active site. It functions together with a nearby copper ion SEE ALSO THE FOLLOWING ARTICLES
which binds the O 2 substrate. The same copper center
and O 2 are apparently involved in the spontaneous syn-
ENZYME MECHANISMS • NATURAL ANTIOXIDANTS IN
thesis of the prosthetic group. Formation of topaquinone
FOODS • PHARMACEUTICALS • PROTEIN STRUCTURE •
is only one case of many, in which prosthetic groups are
PROTEIN SYNTHESIS
self-assembled using components of the protein that car-
ries the group. Another recently discovered example is
formationof4-methylidene-imidazole-5-one(MIO),from BIBLIOGRAPHY
glycine and serine units of a protein. The prosthetic group
is involved in a previously hard-to-explain isomerization
Metzler, D. E. (2001). Biochemistry, The Chemical Reactions of Living
of histidine and phenylalanine. A parallel reaction with
Cells, 2nd ed. Vol. 1, Harcourt/Academic Press, San Diego.
phenylalanine initiates the major pathway of synthesis of Shils, E., Olson, J. A., Shike, M., and Ross, A. C., eds. (1999). Mod-
thousands of aromatic compounds in plants. ern Nutrition in Health and Disease, 9th ed., Williams & Wilkins,
Among specialized coenzymes are a collection of Baltimore.
compounds (which include the previously mentioned McCormick, D. B., and Chen, H. (1999). J. Nutrition 129, 325–
327.
methanopterin) needed by methane-forming bacteria and
Iriarte, A., Kagan, H. M., Martinez-Carrion, M., eds. (2000). Biochem-
compounds such as the light-emitting luciferin of fireflies. istry and Molecular Biology of Vitamin B 6 and PQQ-Dependent Pro-
Luminous jellyfish make different light-emitting com- teins, Birkh¨auser Verlag, Basel, Switzerland.