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The thermostability of neutral proteases has been shown to depend on autolysis which presumably occurs in flexible regions of the protein. In an attempt to rigidify such a region in the neutral protease of Bacillus stearothermophilus, residues in the solvent-exposed 63–69 loop were replaced by proline. The mutations caused large positive (Ser-65 → Pro, Ala-69 → Pro) or negative (Thr-63 → Pro, Tyr-66 → Pro) changes in thermostability, which were explained on the basis of molecular modelling of the mutant proteins. The data show that the introduction of prolines at carefully selected positions in the protein can be a powerful method for stabilization.

The active site of thermolysin-like proteases (TLPs) is located at the bottom of a cleft between the N- and C-terminal domains. Crystallographic studies have shown that the active-site cleft is more closed in Ligand-binding TLPs than in Ligand-free TLPs. Accordingly, it has been proposed that TLPs undergo a hinge-bending motion during catalysis resulting in "closure" and "opening" of the active-site cleft. Two hinge regions have been proposed. One is located around a conserved glycine 78; the second involves residues 135 and 136. The importance of conserved glycine residues in these hinge regions was studied experimentally by analyzing the effects of Gly --> Ala mutations on catalytic activity. Eight such mutations were made in the TLP of Bacillus stearothermophilus (TLP-ste) and their effects on activity toward casein and various peptide substrates were determined. Only the Gly78Ala, Gly136Ala, and Gly135Ala + Gly136Ala mutants decreased catalytic activity significantly. These mutants displayed a reduction in k(cat)/K-m for 3-(2-furylacryloyl)-L-glycyl-L-leucine amide of 73%, 62%, and 96%, respectively. Comparisons of effects on k(cat)/K-m for various substrates with effects on the K-i for phosphoramidon suggested that the mutation at position 78 primarily had an effect on substrate binding, whereas the mutations at positions 135 and 136 primarily influence k(cat). The apparent importance of conserved glycine residues in proposed hinge-bending regions for TLP activity supports the idea that hinge-bending is an essential part of catalysis.

Autolytic degradation of the thermolysin-like proteinase of Bacillus subtilis (TLP-sub) is responsible for the irreversible inactivation of the enzyme at elevated temperatures. Previously we have reported five cleavage sites in Tip-sub [Van den Burg et al, (1990) Biochem. J. 272, 93-97]. In an attempt to render the enzyme less susceptible to autolytic breakdown, one of the fission sites, located between Leu-156 and Ile-157, was modified by replacing Ile-157, C-terminally located with respect to the fission site, by an Asp residue. Aspartic acid is less preferred at this position with respect to the substrate preference of TLP-sub, Modelling studies indicated that this mutation was unlikely to cause conformational changes in the enzyme. Although the 156-157 fission was not observed in the mutant enzyme, a new fission site, between Gly-148 and Val-149, was now observed.

A 10 residue beta-hairpin, which is characteristic of thermostable Bacillus neutral proteases, was engineered into the thermolabile neutral protease of Bacillus subtilis. The recipient enzyme remained fully active after introduction of the loop. However, the mutant protein exhibited autocatalytic nicking and a 0.4 degree C decrease in thermostability. Two additional point mutations designed to improve the interactions between the enzyme surface and the introduced beta-hairpin resulted in reduced nicking and increased thermostability. After the introduction of both additional mutations in the loop-containing mutant, nicking was largely prevented and an increase in thermostability of 1.1 degrees C was achieved.