Biomedical Engineering Reference
In-Depth Information
unlike storage carbohydrates, they should be easier to manipulate. In terms of total
storage protein production for cereals, wheat, maize, rice, and barley, it is much
more significant than that produced by legumes, except for soybean. 63,64 Lysine is
the first limiting amino acid in wheat, barley, maize, sorghum, and triticale; threonine
(barley, sorghum) or tryptophan (maize) is the second. Thus, in a pure cereal diet
in which lysine is limiting, the quality will be poor because the grain protein will
not be metabolized efficiently by humans and animals. 91
The discovery that the maize opaque2 ( o2 ) mutation dramatically increases the
lysine content of the grain 92 led to the development of high lysine corn. 93 However,
the soft, starchy endosperm of this mutant, which causes the kernel to be susceptible
to pests and mechanical damage, 94 prevented significant utilization of the mutation.
After the initial characterization of o2 , genes that alter the mutant phenotype were
identified, giving it a normal appearance. These genes designated o2 modifiers 95
were subsequently used by plant breeders at the Centro Internacional de Mejo-
ramiento de MaĆ­z y Trigo (CIMMYT) 96 to develop o2 varieties with normal kernel
hardness and protein content, as well as an enhanced percentage of lysine. These
modified o2 mutants are called quality protein maize (QPM). 97-99
The major storage fraction of most cereals are prolamins, which are given trivial
names such as gliadin (wheat), zein (maize), hordein (barley), and secalin (rye). In
order to increase the limiting amino acids in cereals, two approaches have been
suggested: 100 (1) insertion of extra codons for lysine, threonine, or tryptophan into
cloned storage protein genomic DNA, followed by reintroduction of the gene into
the plant; and (2) modification of the expression of existing genes so that proteins
rich in limiting amino acids are preferentially synthesized. A specific problem is that
prolamins are coded by multigene families (e.g., for zein, possibly up to 150 closely
related genes), so replacement of a single modified copy would have little effect.
Some likely approaches to circumvent this problem may include the following:
(1) introduction of a modified gene into a recipient that has a deletion lacking part of
the gene family; (2) inactivation of normal gene expression (without deletion), with
expression of introduced modified genes; (3) insertion of a modified gene with a strong
promoter such that it is transcribed more frequently than natural genes; and (4) insertion
of multiple copies of the modified gene, perhaps combined with approaches 1 through
3. Eggum et al. 101 have developed several rice mutants for prolamin and glutelin in
order to improve the nutritional properties of rice protein. They obtained mutants
with a higher lysine content and a higher net protein utilization (NPU).
Soluble Amino Acids
It is also possible to improve the nutritional quality of cereals by increasing specific
soluble amino acid levels; there has been some success in producing mutants with
feedback-insensitive regulatory pathways, particularly those of lysine biosynthesis. 91
The amino acids lysine, threonine, methionine, and isoleucine are derived from aspartic
acid, and it is known for barley that there is a negative feedback to three isozymes of
aspartate kinase, the first enzyme in the pathway, by the end products lysine, threonine,
and S-adenosyl methionine. Thus, cloning of the genes that encode for these isozymes
is being developed in order to increase the production of such amino acids. 63
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