A protein is a folded polymer of amino acids in specific sequence,
sometimes accompanied by metal atoms and small organic molecules.
The opening illustration in this chapter showed the association
of an iron atom and an organic ring in the heme group of cytochrome
c, a small electron-carrying protein. Every protein, in every species
of living creatures, has its own unique amino acid sequence, originally
coded in a sequence of organic bases in DNA, as a part of the genetic
"library" of the organism. In principle, given the amino
acid sequence of a protein, one not only can identify the protein,
but can determine from what species it came. It is in this sense
that we describe proteins and nucleic acids as "information
Nucleic acids are the information carriers par excellence.
From one generation to the next, DNA is the source of the information
on how to synthesize proteins, and hence on how to build a living
creature. Lipids, carbohydrates, and all the other molecules that
we previously have examined are not information carriers in this
sense. Some cases are known in which one kind of molecule is used
in vertebrates for a given purpose, and a different molecule in
invertebrates; or one molecule may be peculiar to a given class
But this is a far cry from being able to say from inspection of
the molecule: This protein came from the digestive machinery of
a dog, this one from the respiratory system of a horse, and that
one from the same respiratory system in bread mold.
The "central dogma" of molecular biochemistry, so labeled
tongue-in-cheek by the men who proposed it, is "DNA makes RNA
makes protein." This is a concise way of saying that the information
contained in a protein molecule came from messenger RNA, and that
the ultimate source of the information in RNA was analogous sequences
of bases in DNA. Information flows from left to right in the illustration
on the next page. Some exceptions are known
to this simple one-way flow of information, but at least we can
say "Nucleic acid makes protein," and add "and protein
makes everything else." All of the chemical processes of living
things are under the control of enzymes, which are protein molecules.
Lipids, carbohydrates, and all the small molecules of the cell are
products of enzymatic`syntheses. They are "second-hand"
molecules, in a different category with regard to information. Carbohydrates
and lipids are the props and scenery in the living drama; proteins
and nucleic acids are the actors.