Proteins occupy a central position in the structure and functioning of
living matter and are intimately connected with all the metabolic
reactions that maintain life. Some proteins serve as structural elements
of the body, for instance, hair, wool, and the scleroproteins of bone
and collagen, the latter an important constituent of connective tissue.
Other proteins are enzymes, which are extremely important since they
regulate all metabolic reactions. Most of the proteins in the tissues of
actively functioning organs, such as the liver and the kidney, are
enzymes. Other proteins participate in muscular contraction, and still
others are hormones or oxygen carriers. Special proteins called histones
are associated with gene function, and the antibodies that an organism
produces to defend itself from bacteria are also proteins.
The differences in proteins, especially in enzymes, account for
differences among cells. It is now appropriate to ask what makes one
protein different from another. We know that the structure of a protein
depends upon several factors, such as the molecular weight. But the main
differences among proteins depend upon the sequence, or order, of the
amino acids that are linked together in the protein molecules.