Proteins as food. To you, these may not look appetizing (or they might), but they do provide a nice supply of amino acids, the monomers of proteins. Proteins have many important roles, from transporting, signaling, receiving, and catalyzing to storing, defending, and allowing for movement. Where do you get the amino acids needed so your cells can make their own proteins? If you cannot make it, you must eat it.
A protein is an organic compound made up of small molecules called amino acids, the monomers of proteins. There are 20 different amino acids commonly found in the proteins of living organisms. Small proteins may contain just a few hundred amino acids, whereas large proteins may contain thousands of amino acids. The largest known proteins are titins, found in muscle and composed of over 27,000 amino acids.
The Figure below shows the general structure of all amino acids. Only the side chain, R, varies from one amino acid to another. In the amino acid glycine, for example, the side chain is simply hydrogen (H). In glutamic acid, in contrast, the side chain is CH2CH2COOH. Variable side chains give amino acids different chemical properties. The order and properties of amino acids determine the shape of the protein. The shape of the protein determines the function of the protein.
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When amino acids bind together, they may form short chains of two or just a few amino acids. These short chains are called peptides. When amino acids form long chains, the chains are called polypeptides. A protein consists of one or more polypeptide chains. A protein may have up to four levels of structure. The lowest level, a protein’s primary structure, is its sequence of amino acids. Higher levels of protein structure are described in the animation below. The complex structures of different proteins give them unique properties, which they need to carry out their various jobs in living organisms.
Proteins play many important roles in living things. Some proteins help cells keep their shape (structural proteins), some, such as connective and motor proteins, make up muscle tissues, and some transport items in and out of cells (transport proteins). Some proteins act as signals, and other proteins receive those signals. Enzymes are proteins that speed up chemical reactions in cells. Other proteins are antibodies, which bind to foreign substances such as bacteria and target them for destruction. Still other proteins carry messages or transport materials. For example, human red blood cells contain a protein called hemoglobin, which binds with oxygen. Hemoglobin allows the blood to carry oxygen from the lungs to cells throughout the body. Proteins have many vital roles in living things, from transporting, signaling, receiving, and catalyzing to storing, defending, and allowing movement.
Proteins in the diet are necessary for life. Dietary proteins are broken down into their component amino acids when food is digested. Cells can then use the components to build new proteins. Humans are able to synthesize all but nine of the twenty common amino acids. The nine amino acids, called essential amino acids, must be consumed in foods. Like dietary carbohydrates and lipids, dietary proteins can also be broken down to provide cells with energy.
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Science Friday: The Medical Wonders of Worm Spit
How useful is worm spit? It turns out that worm spit, also known as silk, is a very useful material in medicine. In this video by Science Friday, Dr. David Kaplan describes how silk is used in a variety of medical applications.
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