Special proteins protect seeds from being eaten by animals
The protein bodies of some seeds contain additional proteins, which, although also acting as storage proteins, protect the seeds from being eaten. Some examples are: the storage protein vicilin has a defense function as it binds to the chitin matrix of fungi and insects. In some insects, vicilin interferes with the development of the larvae. The seeds of some legumes contain lectins, which bind to sugar residues, irrespective of whether these are free sugars or constituents of glycolipids or glycoproteins. When these seeds are consumed by animals, the lectins bind to glycoproteins in the intestine and thus interfere with the absorption of food. The seeds of some legumes and other plants also contain proteinase inhibitors, which block the digestion of proteins by inhibiting proteinases in the animal digestive tract. Because of their content of lectins and proteinase inhibitors, many beans and other plant products are suitable for human consumption only after being denatured by cooking. This is one reason why humans have learned to cook. Castor beans contain the extremely toxic protein ricin of which a few milligrams are sufficient to kill a human being. Beans also contain amylase inhibitors, which specifically inhibit the hydrolysis of starch by amylases in the digestive tract of certain insects. Using genetic engineering, -amylase inhibitors from beans have been successfully expressed in the seeds of pea. Whereas the larvae of the pea beetle normally cause large losses during storage of peas, the peas from the genetically modified plants were protected against these losses.
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