Figure 2.17

A peptide bond between two amino acids.

sources, or chemical messengers (hormones). Other proteins combine with carbohydrates (glycoproteins) and function as receptors on cell surfaces that bind to particular kinds of molecules. Yet others act as weapons (antibodies) against substances that are foreign to the body. Many proteins play vital roles in metabolic processes as enzymes. Enzymes are molecules that act as catalysts in living systems. That is, they speed specific chemical reactions without being consumed in the process. (Enzymes are discussed in chapter 4, p. 110.)

Like carbohydrates and lipids, proteins consist of atoms of carbon, hydrogen, and oxygen. In addition, proteins always contain nitrogen atoms and sometimes contain sulfur atoms as well. The building blocks of proteins are smaller molecules called amino acids.

Twenty kinds of amino acids comprise proteins in organisms. Amino acid molecules have an amino group (—NH2) at one end and a carboxyl group (—COOH) at the other end. Between these groups is a single carbon atom. This central carbon is bonded to a hydrogen atom and to another group of atoms called a side chain or R group ("R" may be thought of as the "Rest of the molecule"). The composition of the R group distinguishes one type of amino acid from another (fig. 2.16).

Proteins have complex shapes, yet the way they are put together is surprisingly simple. Amino acids are connected by peptide bonds—which are covalent bonds that link the amino end of one amino acid with the carboxyl end of another. Figure 2.17 shows two amino acids connected by a peptide bond. The resulting molecule is a dipeptide. Adding a third amino acid creates a tripep-tide. Many amino acids connected in this way constitute a polypeptide (fig. 2.18a).

Proteins have four levels of structure: primary, secondary, tertiary and quaternary. The primary structure is the amino acid sequence of the polypeptide chain. Depending on the protein, the primary structure may range from fewer than 100 to more than 5,000 amino acids. The amino acid sequence is characteristic of a particular protein. Thus, the blood protein hemoglobin and the muscle protein myosin have different amino acid sequences.

In the secondary structure, the polypeptide chain either forms a springlike coil (alpha helix, fig. 2.18fo), or it folds back and forth on itself (beta-pleated sheet, fig. 2.18c). Secondary structure is due to hydrogen bonding. Recall that polar molecules result when electrons are not shared evenly in certain covalent bonds. In amino acids, this results in slightly negative oxygen and nitrogen atoms and slightly positive hydrogen atoms. Hydrogen bonding between oxygen and hydrogen atoms in different parts of the molecule determines the secondary structure.

Hydrogen bonding and even covalent bonding between atoms in different parts of a polypeptide can cause yet another level of folding, the tertiary structure. As a result, proteins have distinct three-dimensional shapes, or conformations (fig. 2.18d), which determine function. Some proteins are long and fibrous, such as the keratins that form hair, and the threads of fibrin that knit a blood clot. Many proteins are globular. Myoglobin and hemoglobin, which transport oxygen in muscle and blood, respectively, are globular, as are many enzymes.

Various treatments can cause the secondary and tertiary structures of a protein's conformation to fall apart, or denature. Because the primary structure (amino acid sequence) remains, sometimes the protein can regain its

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Essentials of Human Physiology

Essentials of Human Physiology

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