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pH-rate profiles at 25 ºC in MTEN buffer. A) Both S. pneumoniae DHFRs (wild type •, Sp9 DHFR □) showed maximum activities at pH 7.0. The two pKa values of S. pneumoniae DHFR were calculated to be 6.2 and 7.9 from the curve fit equation 1 (14). B) The His33Phe mutant DHFR showed a similar pH profile to that of E. coli DHFR from reference (12) with a pKa value of 8.2 when fitted to equation 2 (15).
