Storage proteins serve as reserves of metal ions and amino acids, which can be mobilized and utilized for the maintenance and growth of organisms. They are particularly prevalent in plant seeds, egg whites, and milk.
Perhaps the most thoroughly studied storage protein is
ferritin, which stores iron. Iron is a mineral required by all living things; it is a component of heme, which is found in the transport protein hemoglobin, as well as of cytochromes, molecules taking part in cell metabolism (including drug metabolism). Free iron in solution, on the other hand, is able to participate in free radical reactions that damage proteins, lipids, and nucleic acids. Therefore, within organisms, in addition to serving as an iron reserve, ferritin provides a safeguard against potentially harmful side effects of iron.
Ferritin is a complex of 24 polypeptide chains that form a nearly spherical shell around a core of up to 4,500 iron atoms stored as iron oxide–hydroxide (
ferrihydrite) complexes. Ferritin is able to store and release iron in a controlled fashion.
Proteins are made from amino acids, and many storage proteins serve as reserves of amino acids in embryonic and developing organisms. This is true of both animals and plants. Two well-known storage proteins in animals are casein and ovalbumin. Casein, found in mammalian milk, and oval-bumin, found in egg white, both provide a developing organism with a ready source of amino acids and organic nitrogen.
Plant storage proteins are found in high concentrations in seeds, especially in leguminous plants, in which the storage proteins constitute up to 25 percent of the dry weight of the seed. These proteins have no known enzymatic function and often exist within separate vesicles (packets) in the seeds. In addition to their importance to the germinating seed, these plant storage proteins are a valuable source of human nutrition.
Storage proteins
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