Further proof for the catalytic role of the larger subunit in the spinach leaf ribulose-1,5-diphosphate carboxylase |
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Authors: | M Nishimura T Akazawa |
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Institution: | Research Institute for Biochemical Regulation Nagoya University, School of Agriculture Chikusa, Nagoya (464), Japan |
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Abstract: | The catalytically active oligomeric form of the larger subunit, Am, obtained from spinach leaf ribulose-1,5-diphosphate carboxylase by pretreatment with p-mercuribenzoate at pH 7.5 followed by incubation at pH 9.0, was free of the smaller subunit based on C-terminal amino acid analyses. Valine was the predominant C-terminus of the Am preparations, the release of tyrosine being negligibly small cf. Sugiyama and Akazawa, 9 (1970) 4499]. The pH optimum of the ribulose-1,5-diphosphate carboxylase reaction by Am was about 8.5, in comparison to the native enzyme which showed an alkaline pH optimum only in the absence of Mg2+. The substrate saturation curve of the catalytic subunit with respect to bicarbonate followed the Michaelis-Menten equation, as contrasted to the anomalous reaction kinetics of the native ribulose-1,5-diphosphate carboxylase molecule reported previously. These overall results indicate that the allosteric properties of spinach ribulose-1,5-diphosphate carboxylase are possibly conveyed by a unique structural conformation that requires the presence of the smaller subunit in association with the larger catalytic subunit component of the enzyme molecule. |
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