Biotechnology to Improve Shelf Life and Quality Traits of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 334

Improving Protein Composition

One molecular approach for altering the amino acid composition of seed proteins
involves the transfer of genes that encode proteins containing large amounts of the
limiting amino acid. For improving the quality of leguminous seed proteins, com-
monly deficient in the sulfur-containing amino acids, Altenbach et al. 56 have focused
on a small family of proteins found in the seeds of the Brazil nut ( Bertholletia
excelsa ) which are unusually rich in methionine and cysteine. 61 The sulfur-rich
protein from the Brazil nut is a member of a family of small, water-soluble seed
proteins that contain approximately 8% cysteine and 18% methionine. The mature
protein consists of 9-kD and 3-kD subunits linked through disulfide bridges. 62 Like
storage proteins from other seeds, the synthesis of the sulfur-rich protein is devel-
opmentally regulated; the protein is synthesized at a mid-maturation stage and
accumulates in the mature seeds. The sulfur-rich protein is synthesized initially as
a 17-kD precursor polypeptide that undergoes three proteolytic processing steps
before it attains its mature form. First, a signal peptide of about 2-kD is cleaved
from the precursor, leaving a 15-kD polypeptide precursor that is then trimmed to
a 12-kD polypeptide and finally to the 9-kD and 3-kD subunits of the sulfur-rich
protein. 62 cDNA clones representing several members of the sulfur-rich protein gene
family have been isolated and sequenced by Altenbach et al. 56 One of these cDNAs
was used to construct a chimeric gene in which the promoter region and 3

-flanking
region of the phaseolin gene from French beans were linked to cDNA sequences
encoding the 17-kD precursor from the sulfur-rich protein. They used a binary vector
system of Agrobacterium tumefaciens , and transferred the chimeric gene to tobacco
and regenerated transformed plants. 56 These authors 56 analyzed Southern blots from
leaves of the transgenic tobacco plants, and analysis of RNA isolated from devel-
oping seeds, which indicated that the chimeric gene was transcribed. They detected
that the sulfur-rich protein was expressed in the seeds at levels that approach 5% of
the total seed protein. They used tobacco as a model plant system, but the transfer
of the chimeric gene to several legumes was the next planned step.
Researchers at DuPont are modifying the amino acid content of soybeans. They
have taken a gene from Corynebacterium and expressed it in soybeans to increase
the lysine content. 54 The first transgenic material is high-lysine soybeans; commercial
plantings are still several years away. Similar technology is being used to increase
the levels of sulfur amino acids and threonine and tryptophan. Kim et al. 65 improved
the nutritional value (methionine content) and functional properties (heat-induced
gelation and emulsification) of soybean glycinin by protein engineering. Technology
is now available to use the soybean crop as a factory, using sunlight and nitrogen
to produce the desired amino acid levels, instead of production by fermentation. 54,63,64

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GRAINS

S HELF L IFE OF G RAINS

Lipids and lipid-associated components are key factors in the quality of several
grains such as maize and oats. The oat and oat products have a relatively high lipid