engleski

Structural and functional effects of Zn2+ and Mg2+ binding to E. coli alkaline phosphatase

Alkaline phosphatase (ALP) is a non-specific phosphomonoesterase catalysing the hydrolysis of a large number of phosphomonoesters. Although the exact biological role of ALP is not yet known, its presence in almost all forms of life implies its participation in fundamental biological processes, most likely in phosphate transport. ALP from E. coli is a dimeric metalloprotein with three metal binding sites per active centre. The crystal structure has been determined revealing direct participation of two zinc ions in substrate binding and phoshomonoester hydrolysis, while the role of Mg2+ in the kinetic mechanism has remained unexplained. Although ALP crystalises as a symmetric dimer, it displays deviation from Michaelis-Menten kinetics, supported by a model of a dimeric enzyme with unequal subunits. It has been determined that ALP contains only 1.3 ± 0.2 moles of Mg2+ per dimer indicating that negative cooperativity in Mg2+ binding could be an explanation for the observed asymmetry. The influence of the metal ion content on the catalytic properties of ALP from E.coli has been examined by kinetic analysis. Alterations in tertiary structure in dependence of the metal ion concentration were tested using limited proteolysis with trypsin. An increase in ALP activity due to catalytically relevant metal ion binding was accompanied with structural changes resulting in reduced susceptibility to proteolysis. The reduction of ALP activity in the presence of higher metal ion concentrations is probably the consequence of complete saturation of the dimer with metal ions.

alkaline phosphatase; kinetics

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