Engineering surface charges in a subtilisin

Engineering the pH-optimum of a triglyceride lipase: from predictions based on electrostatic computations to experimental results

The optimisation of enzymes for particular purposes or conditions remains an important target in virtually all protein engineering endeavours. Here, we present a successful strategy for altering the pH-optimum of the triglyceride lipase cutinase from Fusarium solani pisi. The computed electrostatic pH-dependent potentials in the active site environment are correlated with the experimentally observed enzymatic activities. At pH-optimum a distinct negative potential is present in all the lipases and esterases that we studied so far. This has prompted us to propose the "The Electrostatic Catapult Model" as a model for product release after cleavage of the ester bond. The origin of the negative potential is associated with the titration status of specific residues in the vicinity of the active site cleft. In the case of cutinase, the role of Glu44 was systematically investigated by mutations into Ala and Lys. Also, the neighbouring Thr45 was mutated into Proline, with the aim of shifting the spatial location of Glu44. All the charge mutants displayed altered titration behaviour of active site electrostatic potentials. Typically, the substitution of the residue Glu44 pushes the onset of the active site negative potential towards more alkaline conditions. We, therefore, predicted more alkaline pH optima, and this was indeed the experimentally observed. Finally, it was found that the pH-dependent computed Coulombic energy displayed a strong correlation with the observed melting temperatures of native cutinase.

Simultaneous enhancement of thermostability and catalytic activity of phospholipase A(1) by evolutionary molecular engineering

The thermal stability and catalytic activity of phospholipase A(1) from Serratia sp. strain MK1 were improved by evolutionary molecular engineering. Two thermostable mutants were isolated after sequential rounds of error-prone PCR performed to introduce random mutations and filter-based screening of the resultant mutant library; we determined that these mutants had six (mutant TA3) and seven (mutant TA13) amino acid substitutions. Different types of substitutions were found in the two mutants, and these substitutions resulted in an increase in nonpolar residues (mutant TA3) or in differences between side chains for polar or charged residues (mutant TA13). The wild-type and mutant enzymes were purified, and the effect of temperature on the stability and catalytic activity of the enzymes was investigated. The melting temperatures of the TA3 and TA13 enzymes were increased by 7 and 11 degrees C, respectively, compared with the melting temperature of the wild-type enzyme. Thus, we found that evolutionary molecular engineering was an effective and efficient approach for increasing thermostability without compromising enzyme activity.

Probing the altered specificity and catalytic properties of mutant subtilisin chemically modified at position S156C and S166C in the S1 pocket

A series of chemically modified mutants (CMMs) of subtilisin B. lentus (SBL) were generated employing the combination of site-directed mutagenesis and chemical modification. This strategy entails the mutation of a selected active site residue to cysteine and its subsequent modification with a methanethiosulfonate reagent CH3SO2S-R, where R may be infinitely variable. The present study was undertaken to evaluate the changes in specificity and pH-activity profiles that could be induced by modification of S156C and S166C in the S1 pocket of SBL with a representative range of side chain modifications, namely R=-CH3, -CH2C6H5, -CH2CH2NH3+ and CH2CH2SO3 . The side chain of S156C is surface exposed and well solvated while that of S166C points into the pocket. Kinetic evaluation of the CMMs with suc-AAPF-pNA as substrate showed that the kcat/K(M)s changed very little for the S156C CMMs, but varied by up to 11-fold for the S166C CMMs. pH-Activity profiles were also determined, and showed that a negatively or positively charged side chain modification increased or decreased respectively, the pKa of the catalytic triad histidine for both modification sites but with more dramatic changes for the interior pointing S166C than for the solvent exposed S156C site. As an additional probe of altered specificity, inhibition of the CMMs by a representative series of 5 boronic acid transition state analogue inhibitors was determined. The K(I)s observed ranged from a 3.5-fold improvement over the WT value, to a 12-fold decrease in binding. Overall, greater variability in all the parameters measured, activity, pKa, and boronic acid binding resulted from modification at the inward pointing 166 site than at the solvent-exposed 156 site.