Enhancement of Amino Acid Availability in Corn Grain

Abstract

As modern corn hybrids were bred for higher yields, the composition of the grain has inadvertently trended to higher starch content at the expense of protein (Scott et al. 2006). Moreover since corn grain protein is deficient in certain nutritionally essential amino acids, this reduction in grain protein level has further reduced the nutritional quality of the grain. One approach to address this problem is to increase the nutritional quality of corn grain protein, particularly by enhancing the content of essential amino acids, such as lysine and tryptophan. The most limiting amino acid in corn grain, with respect to the dietary needs of monogastric animals, is lysine. Therefore, enhancement of lysine content is a primary target for improving grain quality. The poor nutritional quality of corn protein is mostly caused by the amino acid composition of endosperm proteins. Corn protein has a lysine content of 2.7%, which is well below the recommendation by FAO (FAO/WHO/UNU 1985) for human nutrition. Although the germ protein has an adequate lysine content (5.4%) in whole grain, this is diluted by the much more abundant endosperm proteins, which have an average lysine content of only about 1.9%. This is because 60–70% of endosperm protein consists of zeins, which contain few or no lysine residues (Coleman and Larkins 1999). Similarly, the absence of tryptophan residues in zein proteins is the reason for the low tryptophan content of corn protein. Therefore, modification of the grain protein profile through approaches such as zein reduction and expression of lysine-rich proteins could significantly improve the balance of amino acids. Alternatively, the lysine content of the grain could be increased by elevating the level of free lysine in the kernel.