TABLE 1.

Effect of individual alanine or glutamate substitutions at GC-A phosphorylation sites on maximal velocity and the Michaelis constant of the enzyme

Values are presented as the mean ± S.E.M. percentage of WT (n = 3).

Mutation	Vmax	Km
S497A	59 ± 8*	389 ± 174*
S497E	119 ± 3	184 ± 79
T500A	45 ± 4*	150 ± 56
T500E	93 ± 7	107 ± 38
S502A	70 ± 9*	70 ± 41
S502E	107 ± 12	113 ± 54
S506A	87 ± 4*	221 ± 44*
S506E	119 ± 4*	196 ± 28*
S510A	58 ± 11*	195 ± 151
S510E	113 ± 14	162 ± 82
T513A	80 ± 8	165 ± 70
T513E	152 ± 10*	111 ± 33
S487A	107 ± 14	99 ± 58
S487E	130 ± 16	144 ± 77
S473A	91 ± 3	88 ± 16
S473E	83 ± 8	116 ± 58

293 cells were transiently transfected with plasmids expressing the indicated mutant and assayed for GC activity in the presence 1 μM ANP, 1 mM ATP, and increasing concentrations of Mg²⁺ GTP. Average V_max and K_m values for WT GC-A were 16.5 ± 3.9 nmol cGMP/mg protein per 5 minutes and 54.8 ± 1.4 μM GTP, respectively.

^*P < 0.0125 (significantly different from WT-GC-A).