Binding characteristics of Mn2+, Co2+ and Mg2+ ions with several D-xylose isomerases.
D-Xylose isomerases are metal-ion (Mn2+, Co2+, Mg2+)-requiring tetrameric enzymes. Both the stoichiometry and the binding constants have been determined by titrating the metal-ion-free enzymes from five organisms (Actinomycetaceae and more divergent bacteria) with the respective metal ions using the enzyme activity as indicator of active complex-formation. The following characteristics have been observed for each specific isomerase: (i) two essential metal ion sites (one structural and one catalytic) exist per subunit; (ii) the metal ion binding at one site does not affect the binding at the other site; (iii) of the four possible configurations E, aE, Eb and aEb, only the double-occupied enzyme is active; (iv) the metal ion activation is a time-dependent process; (v) the dissociation constants for both the structural and catalytic sites may be identical or may differ by one or higher orders of magnitude; (vi) metal ion binding is stronger in the order Mn2+ greater than Co2+ much greater than Mg2+; (vii) pronounced increases in Km values concomitant with decreasing equivalents of metal ion added are only observed in the presence of Mg2+ ions.
Van Bastelaere
P B
PB
Laboratory of Biochemistry, University of Ghent, Belgium.
Callens
M
M
Vangrysperre
W A
WA
Kersters-Hilderson
H L
HL
D019747:Aldose-Ketose Isomerases; D001407:Bacillus / Q000201:enzymology; D001665:Binding Sites; D002238:Carbohydrate Epimerases / Q000378:metabolism*; D002413:Cations, Divalent; D003035:Cobalt / Q000378:metabolism*; D007778:Lactobacillus / Q000201:enzymology; D008274:Magnesium / Q000378:metabolism*; D008345:Manganese / Q000378:metabolism*; D013302:Streptomyces / Q000201:enzymology
D002413:Cations, Divalent; D003035:Cobalt; D008345:Manganese; D002238:Carbohydrate Epimerases; D019747:Aldose-Ketose Isomerases; C023298:xylose isomerase; D008274:Magnesium