Corn endosperm protein and starch composition

Most ‘normal’ corn hybrids have endosperm content that is about 70-80% starch and 8-10% protein after the grain is dry.  Much of the protein is a storage protein called zein. Like most proteins, minerals such as nitrogen and phosphorus are major components.  Proteins, other than zein, are major contributors to the multistage biosynthesis of starch, linking the sugar molecules into the amylose and amylopectin components of starch. These proteins, functioning as enzymes, are major factors in quickly reducing the sugar content of the endosperm, thus improving the osmotic pressure to move more carbohydrate into the endosperm.
 
The zein protein in ‘normal’ corn is devoid of two amino acids, lysine and tryptophan, essential to animal nutrition. These two amino acids can be increased by recessive mutations to two corn genes designated as Pbf and O2, resulting in a reduction in zein proteins and increase of non-zein proteins.
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Opaque corn, as the mutant o2 has been called because of its grain appearance, has been used for increased lysine and tryptophan amino acids.  It consistently has less starch and thus less grain yield than normal corn.  It has been shown that the gene O2, codes for proteins essential to the starch biosynthesis in the endosperm, probably with a regulatory function affecting the rate of conversion of sugar to starch (www.pnas.org/cgi/doi/10.1073/pnas.1613721113)
 
The complexity of starch synthesis in corn endosperm cannot be overlooked. Studies indicate that more than 1000 genes are involved in this process.  Although we have reduced many of the most detrimental mutants by yield testing and selection, remaining variability is reflected with every field yield test.