Function and Shape
Proteins are the stuff of life. As enzymes, they control metabolism and literally make life functions possible. The genetic information found in DNA codes for production of proteins—nothing more and nothing less. Proteins are important structural components, particularly in animals. They are involved in transport, immunity, storage, and function as hormones. The shape of a protein gives it its unique function. The chemical composition of the protein determines its shape.
Proteins are composed of amino acid monomers, which always have a nitrogen-based amine group and a carboxyl group bonded to a carbon or hydrocarbon side chain. The unique side chain of each amino acid is usually designated R. There are 20 unique amino acids. The R group of an amino acid affects its reactivity. Some amino acids have polar side chains, while others have non-polar side chains. Some act as acids, others as bases. The sequence of amino acids in a primary protein determines its ultimate shape.
Amino acids undergo dehydration synthesis reactions to form polypeptide bonds. Polypeptide chains represent the primary level of protein structure.
The secondary structure of a protein results from hydrogen bonding between amino acids in the peptide chain. This leads to twisting or folding of the chain into the alpha helix and the beta pleated sheet shapes.
Tertiary structure results from the hydrophobic effect: a folding of the polypeptide chain due to positioning of polar and non-polar amino acids. Non-polar amino acids are typically inside the three-dimensional structure and polar side chains are on the outer surface. Hydrogen bonds and disulfide bonds stabilize tertiary structure.
Quaternary structure involves a complex grouping of two or more polypeptide chains into a stable, multi-subunit structure. They are stabilized by hydrogen bonding, van der Waals interactions and ionic bonding, and occasionally by disulfide bonds. RUBISCO, the most abundant protein on earth, has 16 subunits. Not all proteins have quaternary structure, and some function as simple primary polypeptide chains.
Proteins are important structural elements, especially in animals. The proteins actin and myosin are the main components of the cytoskeleton. These proteins function in cell movement and support. Actin and myosin are the main component of muscle cells and their interaction results in muscle contraction. Much of the non-cellular matrix of our bodies is composed of protein.
Proteins are also important structural components of cell membranes. Proteins embedded in cell membranes allow the control of movement of materials in and out of cells. Many membrane-bound proteins act as enzymes.
Proteins function in transport. Hemoglobin, the protein found in red blood cells, transports oxygen around the body. Sickle cell anemia is a disease caused by a misshapen hemoglobin protein, which is less efficient at transporting oxygen. Lipoproteins are important in transport of lipids. Within cells, molecules are transported along proteinaceous cytoskeletal elements.
Many hormones are proteins. Insulin and human growth hormone (HGH) are proteins that are now produced by genetically engineered bacteria for use in treating human disease. Protein hormones function by interacting with specific chemical receptors on cells. Hormone receptor reactions are called signal transduction pathways and depend on the physical fit of the molecules.
The immune system produces proteins called antibodies to neutralize invading cells or pathogens, such as viruses. Antibodies function by having the correct fit to the invader it is designed to neutralize. When the immune system encounters a pathogen, it remembers how to produce the correctly shaped antibody to fit that particular pathogen.
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