Properties of spinach chloroplast fructose-1,6-diphosphatase |
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Authors: | Juan J Lázaro Ana Chueca Julio López Gorgé Federico Mayor |
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Affiliation: | Department of Biochemistry, Estación Experimental del Zaidin C.S.I.C., Granada, Spain |
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Abstract: | Photosynthetic fructose-1,6-diphosphatase (FDPase) fractions I and II, earlier purified from spinach leaves, show a similar amino acid composition, with the exception of a higher glutamic acid content in the latter. In both fractions glutamic and aspartic acids are the main amino acids. pH activity profiles of fractions I and II are similar, with optima at 8·65–8·70, both showing a high specificity for fructose- 1,6-diphosphate. These two fractions are Mg2+-dependent for activity, with an Optimum Mg2+ concentration of 10 mM in standard conditions, which shifts to 5 mM when the MG2+/EDTA ratio is increased to 10; Mn2+ and Co2+ are slightly active. EDTA enhances FDPase activity slightly, with an optimum at 0·4–0·8 mM. Cysteine has no activating effect, and acts as an inhibitor above 10 mM. Both I and II have an optimum substrate concentration of 4 mM, and the substrate inhibits at concns above this value. Kinetic velocity curves are sigmoidal, with the concave zone located in the range of physiological substrate concns. (Hill coefficient 1·75 for both). This suggests a strong regulatory role of fructose-1,6-diphosphate. Km values are 1·4 × 10−3 M (fraction I) and 1·1 × 10−3 M (fraction II). The highest activity rate occurs at 60°, in accordance with the high thermostability of both fractions; the activation energies are 14·3 kcal/mol (fraction I) and 13·0 kcal/mol (fraction II). |
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Keywords: | Chenopodiaceae spinach fructose-1 6-diphosphatase kinetic photosynthesis |
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