Peptide bond

(a) Primary structure

(b) Secondary structure

(dotted red lines show hydrogen bonding)

Pleated sheet

(c) Tertiary structure

(d) Quaternary structure

Figure 2.18

Levels of protein structure.

Peptide bond

Pleated sheet

Peptide Bonds The Hair

shape when normal conditions return. High temperature, radiation, pH changes, and certain chemicals (such as urea) can denature proteins.

A familiar example of irreversible protein denatura-tion is the response of the protein albumin to heat (for example, cooking an egg white). A permanent wave that curls hair also results from protein denaturation. Chemicals first break apart the tertiary structure formed when sulfur-containing amino acids attract each other within keratin molecules. This relaxes the hair. When the chemicals are washed out and the hair set, the sulfur bonds reform, but in different places, changing the appearance of the hair.

Not all proteins are single polypeptide chains. Sometimes several polypeptide chains are connected in a quaternary structure to form a very large protein (fig. 2.18e). Hemoglobin is a quaternary protein made up of four separate polypeptide chains.

A protein's conformation determines its function. The amino acid sequence and interactions between the amino acids in a protein determine the conformation. Thus, it is the amino acid sequence of a protein that determines its role in the body. Genes, made of nucleic acid, contain the information for the amino acid sequences of all the body's proteins in a form that the cell can decode.

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Essentials of Human Physiology

Essentials of Human Physiology

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