>> Life depends on proteins. These large molecules make up almost all the structures of a cell and mediate most all of its functions. Proteins are responsible for just about everything you do. For instance, enzymes are proteins that catalyze and regulate all the chemical reactions of your body. Other proteins called transport proteins move molecules into and out of your cells across the cell membrane. Some proteins act as signaling molecules that bind to protein receptors on cells and help to coordinate your body's activities. For example, the hormone insulin is a signal that causes cells to take up glucose. And the contractile proteins that make up your muscles let you dance. How can proteins perform these and so many other functions? What can you observe about the differences between these proteins? Right. They obviously differ in shape. A protein's function depends on its specific shape. Let's explore how that shape arises. A protein is made from amino acids joined by peptide bonds to form a polypeptide. The proteins of your body are composed of 20 different amino acids. You can see the different kinds indicated here by their 3-letter abbreviations. Let's take a closer look at the makeup of an individual amino acid. Each amino acid has an amino group, a carboxyl group, a hydrogen atom, and a variable group symbolized by R, and all are attached to a central carbon atom. The R groups of the various amino acids differ in their size and composition. An R group can be hydrophobic, meaning water fearing, or hydrophilic, meaning water loving. These properties help determine what shape a protein assumes. Note that leucine's R group is nonpolar and thus hydrophobic. Hydrophilic R groups may be polar, as shown here with serine, or even charged, as you can see in aspartic acid. So let's build a protein. The final functional shape of a protein is based on several superimposed levels of structure. Primary, secondary, tertiary, and in many but not all proteins, quaternary. Primary structure is the sequence of amino acids. This specific sequence is determined by instructions written in a cell's DNA. Added levels of structure, however, are required to turn a linear string of amino acids into the functional shape of a protein. There are two types of secondary structures. Portions of the chain may coil into an alpha helix. Other segments may fold back and forth, forming beta pleated sheets. These secondary structures are held together by hydrogen bonds, shown here as dotted lines between hydrogen and oxygen atoms of the polypeptide backbone. You can see secondary structures compacted in the globular three-dimensional shape of this polypeptide. The overall shape of a protein is called its tertiary structure. This shape results from interactions between R groups of the various amino acids. Hydrophobic R groups cluster in the center of a protein out of contact with water. R groups that have positive or negative charges may form ionic bonds. Hydrogen bonds can connect polar R groups, and covalent bonds may form between sulfur-containing R groups. These disulfide bridges reinforce the shape of some proteins. You just learned that various types of interactions between the amino acids that make up a polypeptide chain produce the secondary and tertiary structures that determine a protein's shape. But can changes in the chemical or physical environment disrupt a protein's shape and thus its function? For example, could an increase in temperature or a change in acidity affect a protein? Yes. A physical or chemical change in the protein's environment may disrupt the chemical interactions responsible for secondary and tertiary structure, causing the protein to unravel. This process is called denaturation. Without its specific shape, the denatured protein loses its function. Many proteins consist of a single polypeptide and thus have just primary, secondary, and tertiary structure. But some proteins have a quaternary structure in which two or more polypeptide chains are aggregated into one functional protein, such as transthyretin, as seen here. In summary, a protein's functional shape results from four levels of structure: primary, secondary, tertiary, and sometimes quaternary. [ Silence ]