2.15–2.19: Proteins are versatile macromolecules that serve as building blocks.

Two newborn Cattle Egret chicks; their feathers are built from proteins.

You can’t look at a living organism and not see proteins (FIGURE 2-36). Inside and out, proteins are the chief building blocks of all life. They make up skin and feathers and horns. They make up muscles and are a significant component of bone. In your bloodstream, proteins fight invading microorganisms and stop you from bleeding to death from a shaving cut. Proteins control the levels of sugar and other chemicals in your bloodstream and carry oxygen from one place in your body to another. And in just about every cell in every living organism, proteins called enzymes initiate and assist every chemical reaction that occurs.

Figure 2.36: Proteins everywhere! Proteins are the chief building blocks of all organisms.

Although proteins perform several very different types of functions, all are built in the same way and from the same raw materials in all organisms. In the English language, every sentence is made up of words and every word is formed from one or more of the 26 letters of the alphabet. With 26 letters we can write anything, from sonnets to cookbooks to biology textbooks. Proteins, too, are constructed from a sort of alphabet. Instead of 26 letters there are 20 molecules, known as amino acids. Unique combinations of these 20 amino acids are strung together, like beads on a string, and the resulting protein has a unique structure and chemical behavior.

Let’s look more closely at the structure of the amino acids in the protein alphabet. They all have the same basic two-part structure: one part is the same in all 20 amino acids, and the other part is unique, differing in each of the 20 amino acids.

Amino acids contain the same familiar atoms as carbohydrates and lipids—carbon, hydrogen, and oxygen—but differ in an important way: they also contain nitrogen. At the center of every amino acid is a carbon atom, with its four covalent bonds (FIGURE 2-37). One bond attaches the carbon to a group of atoms called a carboxyl group, which is a carbon atom bonded to two oxygen atoms (to one by a single bond and to the other by a double bond). The second bond attaches the central carbon to a single hydrogen atom. The third bond attaches the central carbon to an amino group, which is a nitrogen atom bonded to hydrogen atoms (usually two or three). These components—the central carbon with its attached hydrogen atom, carboxyl group, and amino group—are the foundation that identifies a molecule as an amino acid. As multiple amino acids are joined together, the unit formed by these three components forms the “backbone” of the protein.

Figure 2.37: Amino acid structure. Amino acids are made up of a central carbon atom attached to a hydrogen atom, an amino group, a carboxyl group, and a side chain.

The fourth bond of the central carbon atom attaches to a side chain. This side chain is the unique part of each of the 20 amino acids. In the simplest amino acid, glycine, for example, the side chain is simply a hydrogen atom. In other amino acids, the side chain is a single CH₃ group or three or four such groups. Most of the side chains include both hydrogen and carbon atoms, and a few include nitrogen or sulfur atoms. The side chain determines an amino acid’s chemical properties, such as whether the amino acid molecule is polar or nonpolar.