Journal of Food, Agriculture and Environment

Study on the purification and properties of glutamine synthetase from sugar beet (Beta vulgaris L.)

Author(s):

Cai F. Li ¹*, Yi F. Zhang ^{1, 2}, Song Yu ^{1, 2}, Duo Y. Zhang ^{1, 3}, Feng M. Ma ¹

Recieved Date: 2013-06-11, Accepted Date: 2013-10-30

Abstract:

More than one-third of the sugar (sucrose) consumed by human is obtained from sugar beet (Beta vulgaris L.), and beet root is highly regarded as a potential biomass feedstock for fuel-ethanol production. Applying nitrogen (N) fertilizer is one of the important measures to improve the yields of sugar beet, however, unreasonable application of N fertilizer not only leads to beet yield and quality decreasing, but also brings about low renewable energy output/fossil fuel input ratio and a series of current environmental issues. Glutamine synthetase (GS; EC: 6.3.1.2) plays a critical role in the assimilation of nitrogen in sugar beet. GS from leaves and roots of sugar beet have been purified to homogeneity by ammonium sulfate and ion exchange chromatography. The isoforms of the enzyme show a differential distribution in leaf and root tissues. The results of denaturing SDS-PAGE revealed that GS is composed of eight subunits with a subunit molecular weight of 50 kD each. Moreover, the properties of GS were also studied and the results showed that only GS2 was present in the leaf of sugar beet, while both GS1 and GS2 were observed in the root when the seedlings reached 31-d-old. The activity of GS was reached to the highest level under the conditions of Mg²⁺ concentration of 0.2 M at pH 7.2, and temperatures of 35^oC in the leaf and 40^oC in the root. The results of the Lineweaver-Burk plot showed that the K_m of sugar beet leaf GS was 7.3 mM for Glu-Na and 5.6 mM for NH₄⁺ and the K_m of root GS was 5.8 mM and 7.1 mM for Glu-Na and NH₄⁺, respectively. The optimization study results of GS assay indicated that total GS content in roots was lower than that in leaves, and GS2 content was less than GS1 in root. Moreover, GS was regulated by nitrogen form. GS1 was reached to the highest content when the ratio of NO₃^–/NH₄⁺ was 3:1 in leaf. In root, GS1 content was highest at a NO₃^–/NH₄⁺ ratio of 4:1 and GS2 content was highest in 3:1, and root total GS content was highest at a NO₃^–/NH₄⁺ ratio of 4:1. These results will be used for theoretical basis of N fertilization practices efficiency in sugar beet cultivation, aid in evaluation of sugar beet germplasm and selecting high ammonia-assimilating lines, and promote the sustainable development of beet sugar manufacturing, energy and ecological environment.

Keywords:

Beta vulgaris L., glutamine synthetase, pH, temperature, K_m, nitrogen form

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Journal: Journal of Food, Agriculture and Environment
Year: 2013
Volume: 11
Issue: 3&4
Category: Agriculture
Pages: 710-716

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