Abstract
Canavalin is the major storage protein in sword beans (Canavalia gladiata) and belongs to the 7S seed globulin class. Canavalin solubility can be reversibly altered via the addition of MgCl2 and CaCl2 at different concentrations; specifically, it is insoluble at lower concentrations and soluble at higher concentrations. However, the addition of NaCl does not induce the insolubilization of canavalin. In this study, it was determined that the addition of NaCl causes the nearly complete solubilization of MgCl2-precipitated canavalin in the presence of high concentrations. Moreover, using gel filtration we examined the quaternary structures of soluble canavalin in the bean extract and in the presence of high-concentration salts. Results indicated that canavalin was present in the monomer form within crude extracts with distilled water. Alternatively, we identified trimeric soluble canavalin in the presence of high concentrations of NaCl or MgCl2. Our study revealed that the quaternary structures of sword bean soluble proteins differ in crude extract compared to those in high-concentration salt solutions. The three-dimensional structure of β-conglycinin, which is a typical 7S-seed globulin in soybean, has a trimer form in the presence of high concentrations of NaCl. However, it remains unclear whether β-conglycinin is present as trimers in soybean seed. Our findings serve as an important reference to analyze 7S globulin characteristics.
Highlights
The sword bean (Canavalia gladiata) is an edible leguminous plant that originated either from southern Asia or Africa (Purseglove, 1968)
Canavalin that is extracted in distilled water is soluble and becomes precipitated following the addition of low concentrations of divalent cations; this effect is lost in the presence of higher concentrations of divalent cations
Soluble canavalin gradually increased with increasing NaCl concentrations until it reached a maximum (102.5 Æ 7.9%) in the presence of 180 mM NaCl. These results indicated that the solubility of MgCl2-precipitated canavalin was NaCl concentration-dependent and that the precipitated canavalin was almost entirely solubilized at high concentrations of NaCl
Summary
The sword bean (Canavalia gladiata) is an edible leguminous plant that originated either from southern Asia or Africa (Purseglove, 1968). Sword bean seeds are highly nutritious and contain ~26% protein (Vadivel and Janardhanan, 2005). We established a method to extract the proteins from dried sword bean seeds in distilled water (Nishizawa et al, 2016). Canavalin that is extracted in distilled water is soluble and becomes precipitated following the addition of low concentrations of divalent cations; this effect is lost in the presence of higher concentrations of divalent cations. NaCl can maintain canavalin in the soluble form in sword bean extracts, regardless of the NaCl concentration (0–400 mM) (Nishizawa and Arii, 2016). The variable solubility of canavalin is an interesting physicochemical property It is unclear whether the addition of NaCl can solubilize MgCl2-precipitated canavalin and whether the structures of these soluble proteins differ
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