Fig. 6. Enzymatic properties of MsGC-ß3 compared to MsGC-ß3C338. COS-7 cells were transiently transfected with the plasmids indicated and assayed for guanylyl cyclase activity with GTP concentrations of 0.1 mmol l^-1 to 5 mmol l^-1 in the presence of 4 mmol l^-1 MgCl₂ (A,B) or 4 mmol l^-1 MnCl₂ (C,D). For each concentration the reaction was allowed to proceed for 5, 15 and 30 min and linear regression of these values used to calculate a rate of reaction. The values were also plotted as double reciprocal plots (B,D), which demonstrate that both MsGC-ß3 and MsGC-ß3C338 yield Hill coefficients of 1.0. Values for the V_max and K_m were calculated from the untransformed data using the following equation: Rate=(V_maxx[GTP])/(K_m+[GTP]) using GraphPad Prism 3.0 and are shown in Table 3.