Proteins are vital components of all forms of life, and are important to living cells both structurally and functionally. They are made up of amino acids linked together in a chain by a sequence of peptide bonds. A peptide bond links the amino group (-NH 2) of one amino acid with the carboxyl group (-COOH) of the next amino acid in the chain. Most simple proteins contain from 100 to 300 amino acid units and have molecular weights ranging from 12,000 to 36,000. Elaborate proteins with more than one polypeptide chain may have molecular weights of several hundred thousand.
The central role of proteins in living organisms has been recognized for nearly 200 years. They make up about 90% of the dry weight of blood, 80% of the dry weight of muscle, and lesser amounts of other animal tissues and organs. They are also important constituents of plant and bacterial cells. Although they are important quantitatively, because of their relative abundance, their functional importance is even more crucial. Enzymes, the all-important cellular catalysts often present in minute amounts, are entirely or mostly protein.
Twenty different amino acids are commonly found in proteins. They all have the general formula RCH(NH2)COOH, where C is carbon, H is hydrogen, N is nitrogen, and O is oxygen. The R represents a group called a side chain, and is different for each type of amino acid. Individual proteins contain their own characteristic component of amino acids arranged in a unique sequence. The properties of a protein are determined by the nature of its amino acid side chains, and the sequence in which the amino acids are arranged. The sequence of amino acids is called the primary structure of the protein, and by the interactions of side chains with each other and with the solvent (usually water), the shape and function of the protein is determined. Twelve of the amino acids can be made in cells; the eight remaining, which are called essential amino acids, are obtainedthrough nutrition.
Proteins often exhibit a secondary, tertiary, and even quaternary structure. They develop a secondary structure due to the angles formed by the peptide bonds linking the amino acids. The bond angles are determined by weak links called hydrogen bonds that form between a nitrogen-bound hydrogen atom of one amino acid and an oxygen atom of another. Very often, the result is a helical secondary structure like a string spirally arranged around and imaginary cylinder. Proteins develop a tertiary structure by a bending and folding of the primary chain back upon itself. Common globular proteins, with a more or less spherical shape, are formed in this way. The side chains of amino acids are usually responsible for the patterns leading to tertiary structure. When side chains are large they interfere with the usual helical secondary structure of the protein causing bends and kinks. When they carry opposite electrical charges they attract one another, and when they have like charges they repel each other. Side chains containing carboxyl or hydroxyl groups (both hydrophilic) tend to form on the outside of the protein molecule in contact with an aqueous environment, whereas hydrophobic groups tend to avoid water by folding into the interior of the molecule. Some especially complex proteins are formed from more than one polypeptide chain, or subunit. The subunits are held together by the same kinds of forces involved in tertiary structure. Hemoglobin is a well known example of a protein with quaternary structure.
Although it has long been known that the properties of proteins are determined by their complement of amino acids and sequence in which the amino acids are arranged, the mechanism that determines which amino acids are incorporated into a given protein, and in what order, was discovered only recently. Not surprisingly, both the nature and sequence are determined by the genetic nature of the organism. Portions of the cellular DNA are able to code for a needed protein by the sequence of nucleotides found in the applicable part of the DNA. The DNA code for the protein is transcribed into a complementary RNA nucleotide sequence. The segments of RNA serve as templates for protein synthesis with the aid of cellular components called ribosomes. Each group of three RNA nucleotides designates a specific amino acid. The sequence of amino acids is determined by the sequence of nucleotide triplets in the RNA molecule.
Proteins are classified as either simple or conjugated depending on whether they contain nonprotein components. Simple proteins are made up entirely of amino acids, whereas conjugated proteins contain a prosthetic group in addition to the polypeptide chain. The prosthetic group may be as simple as an ion, or as complex as a nucleotide. Proteins may also be classified according to their role. Two main types are structural proteins and functionally active proteins. Examples of the former are proteins found in skin, tendons, ligaments, hair, and nails. Functionally active proteins include enzymes, immunoglobulins (or antibodies), and protein hormones.
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