Communication between active sites. Plot of guanylate cyclase activity at increasing concentrations of substrate GTP. Guanylate cyclase (5 μg) was incubated for 2 min at 24 °C with the indicated concentrations of GTP in the presence of 4 mM MnCl2 and cGMP was measured. Data were fit to the equation , where Vmax is the maximum activity, S is the concentration of GTP, S0.5 is the substrate concentration at which half-maximal velocity is reached, and n is the Hill coefficient. From the fit, Vmax = 2795 ± 117 nmoles cGMP/min/mg, S0.5 = 269 ± 26 μM, and n = 1.49 ± 0.16. A Hill coefficient greater than 1 indicates the presence of interacting active sites.