The major chemical components of cells include the nucleic acids RNA and DNA, polysaccharides (carbohydrates), fatty materials (lipids), and many thousands of different proteins. Proteins catalyze most of the metabolism, the network of chemical reactions by which cells construct their own substance and by which they obtain and utilize energy for all life processes. Proteins, whose structure is dictated by the DNA of the corresponding genes, are preciselyconstructed. As submicroscopicmachines theyhave moving parts and apparatus for recognizing and binding to other molecules, both large and small. Some coenzymes cooperate with the proteins by carrying electrons, atoms, or molecular fragments. Others help the proteins to catalyze reactions that are difficult or impossible for the reactive groups provided by the amino acid side chains of the proteins.
The B-vitamin-containing coenzymes provide a logical starting point for our discussion. As mentioned in Section I, thiamin, nicotinamide, and riboflavin were recognized early in the 20th century as participants in energy metabolism in both animal and yeast cells. Panthothenic acid, biotin, and vitamin B12 were soon added to this list. We now know, in part from complete genome sequences, that all living creatures depend upon these coenzymes to help catalyze a series of central pathways of metabolism. One of these pathways is utilized by aerobic organisms, from bacteria to human beings, for the oxidation of fatty acids (Fig. 12).
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