Just as some fatty acids are essential to a diet, some amino acids also are necessary. For example, the letter V or the three-letter symbol val represent valine. Proline is an exception to the amino acid's standard structure since its amino group is not separate from the side chain ( Figure 3.23).Ī single upper case letter or a three-letter abbreviation represents amino acids. Proline has an R group that is linked to the amino group, forming a ring-like structure. The side chains of lysine and arginine are positively charged, and therefore these amino acids are also basic amino acids. Amino acids such as valine, methionine, and alanine are nonpolar or hydrophobic in nature, while amino acids such as serine, threonine, and cysteine are polar and have hydrophilic side chains. For example, the amino acid glycine has a hydrogen atom as the R group. The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic, polar, or nonpolar). Which categories of amino acid would you expect to find on a soluble protein's surface and which would you expect to find in the interior? What distribution of amino acids would you expect to find in a protein embedded in a lipid bilayer? Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group ( Figure 3.22).įigure 3.23 There are 20 common amino acids commonly found in proteins, each with a different R group (variant group) that determines its chemical nature. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha ( α) carbon, bonded to an amino group (NH 2), a carboxyl group (COOH), and to a hydrogen atom. Amino AcidsĪmino acids are the monomers that comprise proteins. Two rare new amino acids were discovered recently (selenocystein and pirrolysine), and additional new discoveries may be added to the list. Different arrangements of the same 20 types of amino acids comprise all proteins. Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the protein's shape, leading to loss of function, or denaturation. Protein shape is critical to its function, and many different types of chemical bonds maintain this shape. For example, hemoglobin is a globular protein, but collagen, located in our skin, is a fibrous protein. Some proteins are globular in shape whereas, others are fibrous in nature. Proteins have different shapes and molecular weights. Table 3.1 lists the primary types and functions of proteins. For example, insulin is a protein hormone that helps regulate the blood glucose level. Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. An example of an enzyme is salivary amylase, which hydrolyzes its substrate amylose, a component of starch. Note that all enzymes increase the reaction rate and, therefore, are organic catalysts. Those that build more complex molecules from their substrates are anabolic enzymes, and enzymes that affect the rate of reaction are catalytic enzymes. We call enzymes that break down their substrates catabolic enzymes. The enzyme may help in breakdown, rearrangement, or synthesis reactions. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) upon which it acts. Types and Functions of ProteinsĮnzymes, which living cells produce, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. They are all, however, amino acid polymers arranged in a linear sequence. Their structures, like their functions, vary greatly. Each cell in a living system may contain thousands of proteins, each with a unique function. They may serve in transport, storage, or membranes or they may be toxins or enzymes. Proteins may be structural, regulatory, contractile, or protective. Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Describe the ways in which protein shape and function are linked.Explain the four levels of protein organization.Discuss the relationship between amino acids and proteins.Describe the functions proteins perform in the cell and in tissues.By the end of this section, you will be able to do the following:
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