Agriculture Biotechnology / The use of molecular genetics to improve food properties

Amino acid content of proteins

Biochemical regulatory mechanism proposed to regulate the synthesis of the amino acid Lysine, in higher plants, derived from the amino acid Aspartate. Enzyme activities, aspartate kinase (AK) and dihidrodipicolinate synthase (DHDPS) are indicated. Broken lines indicate the inhibition of these enzymes by Lysine.
Biochemical regulatory mechanism proposed to regulate the synthesis of the amino acid Lysine, in higher plants, derived from the amino acid Aspartate. Enzyme activities, aspartate kinase (AK) and dihidrodipicolinate synthase (DHDPS) are indicated. Broken lines indicate the inhibition of these enzymes by Lysine.
An early application of biotechnology for improving the nutritional value of foods has involved changing the amino acid composition of some common proteins of the human diet. It has been long known that humans cannot live on a protein-free diet. The reason is that we are incapable of synthesising half of the 20 standard amino acids present in proteins. These are known as essential amino acids and must be provided in the diet. Consequently, the nutritive value of a protein-based diet is directly related to the content of these essential amino acids. In general, cereals have proteins with a low content of the essential amino acids lysine, threonine and tryptophan, while legume proteins have a deficiency of cystein, methionine and triptophane. Among the most valuable sources of vegetable protein are the grain legumes.
Biochemical regulatory mechanism proposed to regulate the synthesis of the amino acid Lysine, in higher plants, derived from the amino acid Aspartate. Enzyme activities, aspartate kinase (AK) and dihidrodipicolinate synthase (DHDPS) are indicated. Broken lines indicate the inhibition of these enzymes by Lysine.
Biochemical regulatory mechanism proposed to regulate the synthesis of the amino acid Lysine, in higher plants, derived from the amino acid Aspartate. Enzyme activities, aspartate kinase (AK) and dihidrodipicolinate synthase (DHDPS) are indicated. Broken lines indicate the inhibition of these enzymes by Lysine.
In one of these, soybean, genetic engineering has been applied to increase the content of the essential amino acid lysine in the seed proteins. The rationale was that increasing the synthesis of lysine in the seeds of soybean would increase the synthesis of proteins with high lysine content. Lysine synthesis in this species is finely regulated by a feed-back mechanism, i.e., when the lysine content is high there is an inhibition of two of the enzymesinvolved in the metabolicpathwayleadingto the synthesisof this aminoacid (Fig. 6.1).

The strategyconsistedof the integrationin the genomeof soybeanof genes from otherspeciesencodingfor enzymeswithout thefeed-backmechanism. The transformation of soybeanwith the gene lysCM4 from Escherichia coli (encodingAK) andthe dapA genefrom Corynebacterium (encodingDHDPS), both insensitiveto the feed-backinhibition by lysine, resultedin a transgenic soybeanplant with a duplicatedpathwayof lysine biosynthesis,one sensitive and the other insensitive to lysine. As a result, the lysine content of the transgenicsoybeanplants was over 100-fold the value of the untransformed plants.Other plant specieslike corn, wheatandcanolahavebeensubjectedto the samegenetic manipulationto increasetheir lysine content with similar resultsto thoseobtainedin soybean.

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