Agriculture Reference
In-Depth Information
storage proteins. Main soybean proteins are the storage proteins 11S globulin (glycinin) and
7S globulin (β-conglycinin).
Glycinin consists of a hexamer with a molecular weight of 320-380 kDa. Glycinin has
five non-allelic genes, Gy1, Gy2, Gy3, Gy4, and Gy5, which code for five glycinin protein
precursor molecules, G1, G2, G3, G4, and G5, respectively. Each protein precursor subunit
consists of two or three chains which are cleaved post-translationally. Based on the homology
in amino acid sequences, these five genetic variants are classified into two major groups:
group I consisting of G1 (A 1a Bx), G2 (A 2 B 1a ), and G3 (A 1a B 1b ) variants and group II that
includes G4 (A 5 A 4 B 3 ) and G5 (A 3 B 4 ) variants. Each glycinin variant consists of an acidic (Mr
~ 32 kDa) and a basic (Mr ~ 20 kDa) polypeptide linked together through disulfide bonds (A-
S-S-B, where A represent the acidic polypeptide and B de basic polypeptide). For each
subunit there is more than 84% homology within the group and 45-49% between groups [1,
2].
β-conglycinin is a trimeric glycoprotein with a molecular weight close to 180 kDa which
is composed of three different subunits, α-, α'-, and β- of 76, 72, and 53 kDa, respectively. In
addition, it contains a 5% of carbohydrates, mainly mannose type.
Glycinin and β-conglycinin differ in quantity observing a protein ratio of glycinin to β-
conglycinin ranging from 1.6 to 2.5 among soybean varieties [3]. Moreover, despite soybean
proteins are, in general, limited in sulfur containing amino acids, glycinin presents a higher
content in this kind of amino acids than β-conglycinin which is practically devoid of
methionine content [4]. Nevertheless, the composition and content in these proteins not only
varies with the cultivar but also with the environmental and growing conditions [3, 5].
The differences regarding the structure, composition, and quantity between these two
main protein constituents of soybean made their contribution to nutritional and physical
properties in different soybean cultivars was not identical. Thus, while glycinin is more
advantageous from a nutritional point of view due to their higher methionine content, from a
functional perspective, glycinin is a better gel former and β-conglycinin possesses greater
emulsifying properties [6]. Different works have contributed to the discovery of the
differences in content and subunit composition of glycinin and β-conglycinin in different
soybean cultivars [7-15].
S OYBEAN B REEDING
Breeding consists of combining different cultivars with desirable characteristics to obtain
a new one with improved features. During the past 30 years, soybean breeding programs have
developed varieties for their adaptation to different geographical areas [16, 17] and for
improving seed characteristics: seeds with a higher protein or oil contents, with improved
protein quality, with improved oil quality, with reduced allergenicity, with genetic resistance
to plagues, with higher yield, etc. [18-22].
Important efforts have been performed for the production of high protein seeds of more
commercial value. Cultivars with a 10 to 20 % higher protein content have been developed at
expense of cultivar yield and oil content [23, 24]. On the other hand, despite soybean proteins
are superior in quality to other vegetable proteins, they are of lower quality than animal
proteins mainly due to their limitation in sulfur containing amino acids being the
Search WWH ::




Custom Search