2.17: A protein’s function is influenced by its three-dimensional shape.

Proteins are formed by linking individual amino acids together with a peptide bond, in which the amino group of one amino acid is bonded to the carboxyl group of another. Two amino acids joined together form a dipeptide, and several amino acids joined together form a polypeptide. The sequence of amino acids in the polypeptide chain is called the primary structure of the protein and can be compared to the sequence of letters that spells a specific word (FIGURE 2-39).

Amino acids in a polypeptide chain don’t remain in a simple straight line like beads on a string. The chain begins to fold as side chains come together and hydrogen bonds form between various atoms in the chain. The two most common patterns of hydrogen bonding between amino acids cause a segment of the chain to either twist in a corkscrew-like shape or form a zigzag folding pattern. The distribution of corkscrews and zigzags within a protein gives a protein its secondary structure.

The secondary structure itself continues to fold and bend, bringing together amino acids that then form bonds such as hydrogen bonds or covalent sulfur-sulfur bonds (see Figure 2-39). Eventually, the protein folds into a unique and complex three-dimensional shape called its tertiary structure.

Some protein molecules have a quaternary structure in which two or more polypeptide chains are held together by hydrogen bonds and other non-peptide bonds between amino acids in the different chains. An example of a protein with quaternary structure is hemoglobin, the protein molecule that carries oxygen from the lungs to the cells where it is needed. Hemoglobin is made from four polypeptide chains, two “alpha” chains and two “beta” chains.

Some proteins are attached to other types of macromolecules. Lipoproteins, for example, circulate in the bloodstream carrying fats. They are formed when molecules of cholesterol and a triglyceride (both lipids) combine with a protein. Glycoproteins are combinations of carbohydrates and proteins. These are found on the surfaces of nearly all animal cells and play a role in helping the immune system distinguish between your own cells and foreign cells. (We learn more about glycoproteins in the next chapter, which discusses cells.)

The overall shape of a protein molecule determines its function—how it behaves and the other molecules it interacts with. For proteins to function properly, they must retain their three-dimensional shape. When their shapes are deformed, they usually lose their ability to function. We can see proteins deforming when we fry an egg. The heat breaks the hydrogen bonds that give the proteins their shape. The proteins in the clear egg white unfold, losing their secondary and tertiary structure. This disruption of protein folding is called denaturation (FIGURE 2-40).

Almost any extreme environment will denature a protein. Take a raw egg, for instance, and crack it into a dish containing baking soda or rubbing alcohol. Both chemicals are sufficiently extreme to turn the clear protein opaque white, as in fried egg whites.

Hair is a protein whose shape most of us have modified at one time or another. Styling hair—whether curling or straightening it—involves altering some of the hydrogen bonds between the amino acids that make up the hair protein, changing its tertiary structure. When your hair gets wet, the water is able to disrupt some of the hydrogen bonds, causing some amino acids in the protein to form hydrogen bonds with the water molecules instead. Thus, if you style your hair while it’s wet, you can change your hair’s shape—making it straighter or, if you manipulate it around curlers, making it curlier. The hair can then hold this shape when it dries, because as the water evaporates the hydrogen bonds to water are replaced by hydrogen bonds between amino acids of the hair protein. Once your hair gets wet again, however, unless it is combed, brushed, or wrapped in a different style, it will return to its natural shape.

Whether your hair is straight or curly or somewhere in between depends on your hair protein’s amino acid sequence and the three-dimensional shape it confers (FIGURE 2-41). This amino acid sequence is something you’re born with (that is, it’s genetically determined). The chains are more or less coiled, depending on the extent of covalent and hydrogen bonding between different parts of the coil. Many hair salons make use of the ability to alter covalent bonds to change hair texture semi-permanently. They are able to do this in three simple steps. First, the bonds are broken chemically. Second, the hair is wrapped around curlers to hold the polypeptide chains in a different position. And third, chemicals are put on the hair to create new covalent bonds between parts of the polypeptide chains. The hair thus becomes locked in a new position. (New hair will continue to grow with its genetically determined texture, of course, requiring the procedure to be repeated regularly.)