| Abstract: | The dielectric constant in the active site cleft of subtilisin from Bacillus amyloliquefaciens has been probed by mutating charged residues on the rim and measuring the effect on the pKa value of the active site histidine (His64) by kinetics. Mutation of a negatively charged surface residue, which is 12 to 13 A from His64, to an uncharged one Asp----Ser99) lowers the pKa of the histidine by up to 0.4 unit at low ionic strength (0.005 to 0.01 M). This corresponds to an apparent dielectric constant of about 40 to 50 between Asp99 and His64. The mutation is in an external loop that is known to tolerate a serine at position 99 from homologies with subtilisins from other bacilli. The environment between His64 and Asp99 is predominantly protein. Another charged residue that is at a similar distance from His64 (14 to 15 A) and is also in an external loop that is known to tolerate a serine residue is Glu156, at the opposite side of the active site. There is only water in a direct line between His64 and Glu156. Mutation of Glu----Ser156 also lowers the pKa of His64 by up to 0.4 unit at low ionic strength. This change again corresponds to an apparent dielectric constant of about 40 to 50. The pKa values were determined from the pH dependence of kcat/KM for the hydrolysis of peptide substrates, with a precision of typically +/- 0.02 unit. The following suggests that the changes in pKa are real and not artefacts of experimental conditions: Hill plots of the data for pKa determination have gradients (h) of -1.00(+/- 0.02), showing that there are negligible systematic deviations from theoretical ionization curves involving a monobasic acid: the pH dependence for the hydrolysis of two different substrates (succinyl-L-alanyl-L-alanyl-L-prolyl-L-phenylalanyl p-nitroanilide and benzoyl-L-valyl-L-glycyl-L-arginyl p-nitroanilide) gives identical results so that the pKa is independent of substrate; the pH dependence is unaffected by changing the concentration of enzyme, so that aggregation is not affecting the results; the shift in pKa is masked by high ionic strength, as expected qualitatively for ionic shielding of electrostatic interactions. |
