Protein Modified Foods
Seeds, such as the bean, have been the focus of research to improve the amino acid content of legumes, the amount of protein, and the digestibility of the protein (3). Researchers inserted a naturally occurring 15 amino acid sequence from the zein storage protein found in maize (rich in methionine, but deficient in lysine) into the bean beta-phaseolin. Phaseolins represent about half of the total bean protein, and since 6 of the 15 amino acids in the inserted sequence were methionine, it was hypothesized that the addition of this sequence would increase the methionine content of the bean. However, this did not occur because the modified protein was degraded even though the developmental expression was normal.
Researchers discovered that the protein was being degraded because it did not have the appropriate 3-D structure. The correct protein structure is necessary for intracellular transportation of the protein. The insertion site that had been used for the genetic information had caused a change in the proteins structure that interfered with the Gogi-mediated transport needed before the protein could be deposited in the appropriate place (3).
The structure problem was addressed by the development of methionine-rich looping sequences that made phaseolin take on the correct three-dementional structure. Also, finding the correct place to insert the sequence was needed. The sequence for lectin, which makes up 5-10% of the protein in beans, was used to determine where to insert the methionine sequence. The engineered gene was inserted into tobacco plants, a model plant due to the knowledge about its genome, and the protein accumulated correctly in the seed (3).
Soybeans have been transformed with a gene from Brazil nuts that encodes a storage protein (2S). Brazil nuts have 18% methionine. The expression of the gene from the Brazil nut resulted in significant improvement in the amount of methionine in the soybean (1).
Despite the improvement, development of this particular plant has been discontinued. The 2S protein was tested and found to be the most likely candidate for the major allergen in Brazil nuts. While it could still be used, any products containing ingredients from the transformed soybean would have to be labeled.
The potato has been reported to be the most important noncereal food crop, ranked fourth in terms of total global food production (4). The potato is limited in the amount of lysine, tyrosine, methionine and cysteine it contains.
Amaranth Seed Albumin A seed-specific protein, amaranth seed albumin, has been used to transform potatoes (4). The amaranth seed albumin (AmA1) protein has a well-balanced amino acid profile, unlike most of its seed counterparts. In fact, its amino acid composition exceeds values recommended by the World Health Organization for a nutritionally rich protein (4). This protein was also an attractive option due to its non-allergenicity in its purified form. When the AmA1 gene was inserted into a potato, the result was a significant increase in most amino acids and an increase in the total protein in the potato (4).
DHDPS Gene Another potato transformation focused on increasing lysine levels involved use of a bacterial feedback-insensitive dihydrodipicolinate synthase (DHDPS) gene. DHDPS is an enzyme in the lysine synthesis pathway. Expression of a gene for a type of DHDPA less sensitive to inhibition in the potato resulted in a sixfold increase in lysine levels (5). Then, aspartate kinase (AK) was also added, which is the first enzyme in the aspartate family. It is feedback inhibited by lysine. Both the DHDPS and AK genes that were used were from bacterial or plant origins less sensitive to feedback inhibition by lysine. The combined effect of using both was an even greater increase in lysine.
Overall the increase in lysine was accomplished by increased activity in the lysine pathway, which usually is held in check by competition from the threonine pathway. This increase in activity results in more 3-aspartic semialdehyde being converted in lysine instead of threonine (5).
A heat stable, enzyme resistant protein (16-kDa) in rice is the cause of a condition called atopic dermatitis (AD) in some Japanese children. Inactivation of the protein to prevent this reaction had been possible only with the use of an expensive enzymatic treatment. However, another approach to the problem has been the use of chemicals to development of a mutant plant with less of the allergenic protein (1). This technique netted unpredictable results in which some plants that had less of the protein and were agronomically viable, and others were not agronomically viable even though they contained 50% less of the allergenic protein.
A more reliable technique was developed through genetic manipulation of the allergenic protein. The first step was finding the DNA sequence that encoded for the protein. Then, an antisense strand of the sequence was created. The process of using the antisense strand is founded in the principle that DNA transcription occurs from the 3’ to the 5’ end of DNA molecules. The antisense strand is a coding sequence for the protein that is inserted into the plant’s DNA backwards, or inverted. This causes transcription from that strand, resulting in antisense mRNA. The antisense mRNA is prone to binding with the sense mRNA, which interferes with the amount of mRNA that is translated into amino acids and then the protein (1). The net result is less of the allergenic protein produced by the plant. This same process has potential to decrease the allergenic proteins in foods such as peanuts, soybeans and Brazil nuts.
Enrichment of lysine in rice has been another topic of research. Beta-phaseolin, a lysine-rich bean protein, has been transformed into rice. The bean protein complements the prolamines in rice, which are deficient in lysine. The result has been increased phaseolin protein the rice at relatively high levels (4% of the total seed protein) (2).
The modifications that are being considered in wheat mimic the solutions sought for AD in rice. Celiac disease, a gluten-sensitive enteropathy, requires that patients avoid all wheat and all wheat products. The disease thought to be caused by an abnormal immune response to the gliadin proteins in wheat. The result is damage to the intestinal villi, resulting in general malabsorption problems. Removal of the gliadin proteins, using the antisense mRNA methods, could open up more dietary choice for individuals with the disease.
Research in the alteration of maize has focused on increasing its lysine and tryptophan content. The major seed storage protein, zein, is deficient in these amino acids (3). High-lysine maize has been developed using mutant genes, and these are available in the commercial market.
Two mutants with altered amino acid composition have been discovered, opaque-2 and floury-2. Both of these mutants had decreased levels of zein. Opaque-2 has an elevated lysine content associated with changes in the enzymes in the pathways for lysine synthesis and degradation (2). It was hypothesized that decreased production of zein resulted in balanced increased production of the other storage proteins. These storage proteins had higher percentages of lysine, resulting in an overall increase in the amino acid. The opaque-2 mutant did have an increased ratio of glutelin to zein, however the decrease in zein was not directly proportional to the increase in the other proteins. This resulted in softer, smaller maize kernels (2).
Now, recombinant methods, including antisense RNA technologies, are replacing traditional breeding methods. The benefits of this technology have been blocked by researcher’s incomplete knowledge about the system of protein storage compartments in the seeds. The results have so far been imperfect plants (2).