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Figure 1.
Fig. 1. Effect of residue 225 on the Na^+ specificity of
thrombin, measured as the ratio of the k[cat]/K[m] values for
the hydrolysis of H-D-Phe-Pro-Arg-p-nitroanilide (FPR) in the
presence of 200 mM NaCl or choline chloride, 5 mM Tris, 0.1%
polyethylene glycol, pH 8.0 at 25°C (6). Binding of Na^+ to
wild type (Tyr) enhances k[cat]/K[m] nearly 25-fold. A
significant enhancement also is observed for Phe, His, Met, Gln,
Trp, and Ser, in decreasing order. All other residues show no
significant difference between Na^+ and choline.
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