Figure 2. Catalytic activity of SpGH38 α−mannosidase and inhibition by swainsonine.

(A) Activity on 4-methylumbelliferyl α−D mannoside (4-MeUMB) and (B) α−1,3 mannobiose (see text for details) Substrate insolubility under the conditions used precluded higher [S] values. (C) The K _i for swainsonine was determined using α−1,3 mannobiose as substrate with [S] < < K_m and [I] straddling the K _i. V₀ and V_i are the rates of the reaction in the absence and presence of inhibitor, respectively. The K _i for a competitive inhibitor is derived from the gradient of 1/K _i (see Methods ); here 18±0.5 µM.