Virtually everything cells do depends upon proteins. We are all, regardless of build, made up of water plus more or less equal amounts of fat and protein. Although the DNA in our cells contains the information necessary to make our bodies, DNA itself is not a significant part of our body mass. Nor is it a chemically interesting molecule, in the sense that one length of DNA is much the same as another in terms of shape and chemical reactivity. The simplicity of DNA arises because it is a polymer made up of only four fairly similar monomers, and this is appropriate because the function of DNA is simply to remain as a record and to be read during transcription. In contrast, proteins made using the instructions in DNA vary enormously in physical characteristics and function and can be considered as constituting the much more complex proteome (page 178). Silk, hair, the lens of an eye, an immunoassay (such as found in a pregnancy test kit), and cottage cheese are all just protein plus more or less water, but they are different because the proteins they contain are different. Proteins carry out almost all of the functions of the living cell including, of course, the synthesis of new DNA. Neither growth nor development would be possible without proteins.
Most proteins have functions that depend on their ability to recognize other molecules by binding. This recognition depends on specific three-dimensional binding sites that make multiple interactions with the ligand, the molecule being bound. To do this a protein must itself have a specific three-dimensional structure. Each of the huge number of protein functions demands its own protein structure. Evolution has produced this diversity by using a palette of 20 amino acid monomers, each with its own unique shape and chemical properties, as the building blocks of proteins. Huge numbers of very different structures are therefore possible.
Cell Biology: A Short Course, Second Edition, by Stephen R. Bolsover, Jeremy S. Hyams, Elizabeth A. Shephard, Hugh A. White, Claudia G. Wiedemann ISBN 0-471-26393-1 Copyright © 2004 by John Wiley & Sons, Inc.
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