The term "protein" comes from the Greek proteios ("of the first rank"), which aptly describes their importance. These molecules, which account for about 50 percent of the organic material in the body (17 percent of the body weight), play critical roles in almost every physiological process. Proteins are composed of carbon, hydrogen, oxygen, nitrogen, and small amounts of other elements, notably sulfur. They are macromol-ecules, often containing thousands of atoms, and like most large molecules, they are formed by the linking together of a large number of small subunits to form long chains.
Amino Acid Subunits The subunits of proteins are amino acids; thus, proteins are polymers of amino acids. Every amino acid except proline has an amino (—NH2) and a carboxyl (—COOH) group linked to the terminal carbon in the molecule:
The third bond of this terminal carbon is linked to a hydrogen and the fourth to the remainder of the molecule, which is known as the amino acid side chain (R in the formula). These side chains are relatively small, ranging from a single hydrogen to 9 carbons.
The proteins of all living organisms are composed of the same set of 20 different amino acids, corresponding to 20 different side chains. The side chains may be nonpolar (8 amino acids), polar (7 amino acids), or ionized (5 amino acids) (Figure 2-13).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.