Structure and Function of Proteins

<h1>Text Preview</h1> <p>The final group of macromolecules we'll look at is proteins. Eggs, meat, and legumes are all high-protein foods. Proteins are polymers of amino acids, joined together in condensation reactions. The carboxyl group of one amino acid gets linked to the amino group of another amino acid, and a molecule of water is removed. The new bond is sometimes called a peptide bond, and the chain of amino acids is called a polypeptide. The chain is sometimes called the polypeptide backbone, or just the backbone, when we're focused on the atoms in the peptide bonds rather than the amino acid side chains. Some proteins are made of a single polypeptide, but as we'll see in a moment, other proteins are made from more than one polypeptide.</p> <p>The sequence of amino acids is called the primary structure. Proteins aren't stretched out flat like this diagram might suggest. Instead, the carbonyl oxygen and the amide hydrogen can form hydrogen bonds that link the amino acids together. These hydrogen bonds cause the polypeptide chain to fold in repeating patterns called secondary structures. </p> <p>A protein's secondary structure is the three dimensional arrangement of the backbone atoms in a small region of the protein. The most common secondary structures are the alpha helix, on the left, and the beta sheet, on the right. The dots between the amide hydrogen atoms, shown in black, and the carbonyl oxygen atoms, shown in red, are the hydrogen bonds that hold the secondary structures together. It's easier to see the secondary structures in a protein when we represent the alpha helices as twisted ribbons and the beta strands as flat ribbons. A group of two or more beta strands is called a beta sheet. </p> <p>In a protein, the secondary structure elements fold in a very specific way to give the protein its tertiary structure. The tertiary structure is the three dimensional structure of a complete polypeptide chain. Noncovalent interactions between the amino acid side chains help hold the tertiary structure together.</p> <p>This is the tertiary structure of lysozyme. Lysozyme is an enzyme, or biological catalyst, that degrades certain polysaccharide chains. It's of historical importance because it's the first enzyme whose structure was determined by X-ray crystallography. It contains several short helices and a section of beta sheet. Notice that much of the protein doesn't adopt a secondary structure. </p> <p>Hemoglobin is the main oxygen-carrying protein in blood. Hemoglobin is made of more than one polypeptide chain. Each chain folds independently and is called a SUBUNIT. The three-dimensional arrangement of the subunits is the protein's quaternary structure.</p> <p>Proteins can be grouped into two broad categories, based on their three-dimensional structure: globular proteins and fibrous proteins. Let's look at globular proteins first. Lysozyme and hemoglobin are both examples of globular proteins. When they fold into compact shapes, the hydrophobic amino acids fold to the inside and the hydrophilic amino acids are on the outside. This arrangement lets the protein be soluble in the aqueous environment of the cell. </p> <p>Most proteins involved in metabolism are globular proteins. That's because protein folding creates special pockets where chemical reactions can occur or where small molecules can bind. In enzymes like lysozyme, these pockets are called active sites. That's where reactant molecules bind and are chemically converted to product molecules. In other globular proteins, the pocket is called the binding site. Transport proteins bind small molecules in their binding site and carry them to their destination. For example, the glucose transporter binds a molecule of glucose on the outside of the membrane, carries it through the membrane, and releases it inside the cell, where it can be used for energy. As we've seen, hemoglobin, another transport protein, carries oxygen from the lungs to the tissues.</p> <p>These examples show us that the structure of a globular protein is closely related to its function. The relationship between structure and function is equally important for the fibrous proteins. Their elongated form makes them ideal for structural support in animal cells.</p> <p>The major type of protein in hair and fingernails is alpha-keratin. A single alpha-keratin molecule is one large alpha helix. Pairs of helices coil around each other, then two coils wrap around each other to form a protofibril. Finally, groups of protofibrils associate to form the microfibrils we know as strands of hair.</p> <p>Another fibrous protein, collagen, plays an important structural role in bone, tendons, and skin. Like alpha keratin, collagen forms helical strands. But notice that a collagen strand is made of a single triple helix, rather than 4 helices in a coiled-coil arrangement. </p> <p>Fibrous proteins play a limited number of roles because of their similar structures. Compare this to the wide variety of tertiary structures in the globular proteins, which correlates with the wide range of metabolic functions they perform.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>