The relationship between structure and thermostability of a nitrile hydratase from Goebacillus pallidus RAPc8
Van Wyk, Jennifer Caroline
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Nitrile hydratases (NHases) are very important biocatalysts for the enzymatic conversion of nitriles to industrially important amides such as acrylamide and nicotinamide. An “ideal” NHase should fulfil several essential criteria including, high substrate conversion rates, being able to tolerate high substrate and product concentrations as well as being highly thermostable. The NHase used in the present study was isolated from Geobacillus pallidus RAPc8, a moderate thermophile. The primary aims of this study were to use random mutagenesis to engineer the G. pallidus RAPc8 NHase towards improved thermostability and then to use X-ray crystallography to investigate the molecular mechanism(s) involved in the enhanced thermostability. Two randomly mutated libraries were constructed using MnCl2 mediated errorprone PCR. The PCR reaction was performed using 0.05 mM and 0.10 mM MnCl2 and a biased dNTP concentration. The hydroxamic acid assay was used to screen the randomly mutated libraries for NHase mutants with enhanced thermostability. Six mutants that exhibited thermostability-enhancing mutations were isolated from the randomly mutated libraries. The thermostabilised mutants contained between 3 and 7 nucleotide changes per NHase operon. The wild-type and four thermostabilised mutant NHases (7D, 8C, 9C, 9E) were over-expressed, purified, crystallised and subjected to X-ray crystallography. The resolution of the diffraction data for the all the mutant NHases were better than the 2.4Å previously obtained for the wild-type G. pallidus NHase. The best quality data was collected for mutant 9E, which diffracted to a resolution of 1.15Å. The high quality crystal structures allowed each thermostability-enhancing mutation to be viewed in detail. As most of the NHase mutants contained multiple mutations, the crystal structures were important in correlating the observed thermostabilisation with the structural effect of the mutations. Analysis of the X-ray crystal structures illustrated the importance of electrostatic interactions, particularly salt bridges and hydrogen bonds in enhancing the thermostability of the mutant NHases. The difference in the free energy of activation of thermal unfolding (DDG) was used to compare the wild-type and mutant NHases thermostability. The most improved NHase, mutant 9C, was stabilised by both a buried inter-subunit salt bridge between aR169 and bD218 and an inter-helical hydrogen bond between bK43 and bK50. The stabilisation provided by these electrostatic interactions was 7.62 kJ/mol. Mutant 8C was primarily stabilised by the introduction an electrostatic network consisting of a salt bridge between bE96 and aR28 and a hydrogen bond between bE96 and bE92. Also, an intra-helical salt bridge between aE192 and aK195 stabilised the helix consisting of a190-196 in mutant 8C by shielding the helix backbone from solvation and preventing co-operative unfolding of the a helix. However, mutant 8C was also destabilised by a mutation that disrupted a water-mediated hydrogen bond between bD167 and bK168 at the heterotetramer interface of the enzyme. Consequently, the net stabilisation energy provided as a result of stabilising and destabilising interactions was 6.16 kJ/mol. Mutant 7D was the only NHase mutant with only one possible thermostabilising mutation. This mutant was stabilised by 3.40 kJ/mol as the result of a water-mediated hydrogen bond between aS47 and bE33. Similarly, a water-mediated hydrogen bond between aS23 and bS103 provided a stabilisation energy of 4.27 kJ/mol to mutant 9E. This project has shown that moderate-frequency randomly mutated libraries can yield mutants with multiple thermostabilising interactions. Also, the importance of utilising X-ray crystallography to investigate structure-function relationships in proteins has been illustrated.