FIG. 6.

Binding of glucose and mannose by the glucose/mannose-binding protein (GlcE). (A) GlcE at 5 μM was incubated at 70°C with ¹⁴C-labeled glucose, and the amount of bound glucose was determined by ammonium sulfate precipitation. The points obtained can be fitted with a Michaelis-Menten curve exhibiting a K_D of 0.67 μM and binding saturation at 2.1 μM, indicating a stoichiometry of 2:1 (polypeptide-substrate). (B) Mlc_Tth at 5 μM was incubated at 70°C with ¹⁴C-labeled mannose. The points obtained can be fitted with a Michaelis-Menten curve exhibiting a K_D of 47 μM and binding saturation of 2.9 μM, again indicating a 2:1 stoichiometry. The low stoichiometry may indicate that not all protein molecules are active.