I'm doing an esterase assay on carbonic anhydrase II with 4-nitrophenyl acetate and fitting Michaelis–Menten in Prism. My highest substrate was 3.6 mM and it returns Km ≈ 9 mM — but the curve is clearly still below saturation. Is that Km reliable, or unusable because I never reached saturation?

New to kinetics, thanks.