Figure 5. Thermostability and cooperativity of hUGP.

(a) Normalized temperature-dependent fluorescence (depicted as relative fluorescence intensity, y-axis) of the octameric wt hUGP1 and nearly inactive mutants displaying octameric (R287L), tetrameric (I498D*) and dimeric (Δ501-508*) quaternary structures. Inflection points of the curves, representing the melting temperature of the proteins, are indicated by dashed vertical lines. Data points represent means of triplicates ± s.e.m. *Oligomerization status of the corresponding hUGP2 mutants determined in solution²⁴. (b) In vitro activity of wt hUGP1 in the reverse reaction in dependence of PP_i concentration. Specific activity (y-axis) is depicted in dependence of substrate concentration (x-axis). Data points represent means of six replicates ± s.e.m. The solid line represents curve fitting to the function y = V_max · x^H/(K^H + x^H). Kinetic parameters V_max, K_0.5 and H are given as inset. The dashed black line represents curve fit using the Michaelis-Menten-equation y = V_max · x/(K_m + x).