Mutant subtilisin E with enhanced protease activity obtained by site-directed mutagenesis

Bioinformatics analysis of extracellular subtilisin E from Bacillus subtilis

Bacillus spp. are the main sources of subtilisin E, which has several applications in biotechnology. The 3D structure of subtilisin E has a significant impact on its efficacy. In this study, we evaluated subtilisin E from Bacillus subtilis subsp. subtilis str. 168 by bioinformatic methods. The results revealed that the subtilisin E sequence from B. subtilis contains highly conserved amino acids, including histidine (H), aspartic acid (D) and serine (S). Subtilisin E cleaves the bonds between hydrophobic and polar amino acids in keratin-associated proteins. The effects of point mutations on the crystal structure of subtilisin E (PDB ID: 1SCJ) showed that changes of asparagine 123 (N123) to valine (V) and serine 331 (S331) to leucine (L) respectively, were the most stabilizing. Genomic analysis of the subtilisin E-coding gene (aprE) indicated that this gene and the yhfN gene are expressed through a σA promoter. The analysis of TBFs revealed AbrB, ScoC, DegU, Hpr, σA, SinR, TenA, and DegU as relevant regulators of aprE expression. Phylogenetic analysis showed that subtilisin Es have highly conserved structures among Bacillus spp., sharing a common ancestor, where their coding genes were duplicated and evolved within the Bacillus spp. As the conclusion, our in silico study demonstrated that the overexpression of the aprE gene and stability of the produced subtilisin E can be improved though system biology methods such as point mutations and identifying the involved transcription factors (TFs) or/and TBFs.Communicated by Ramaswamy H. Sarma.

Microbial Keratinase: Next Generation Green Catalyst and Prospective Applications

The search for novel renewable products over synthetics hallmarked this decade and those of the recent past. Most economies that are prospecting on biodiversity for improved bio-economy favor renewable resources over synthetics for the potential opportunity they hold. However, this field is still nascent as the bulk of the available resources are non-renewable based. Microbial metabolites, emphasis on secondary metabolites, are viable alternatives; nonetheless, vast microbial resources remain under-exploited; thus, the need for a continuum in the search for new products or bio-modifying existing products for novel functions through an efficient approach. Environmental distress syndrome has been identified as a factor that influences the emergence of genetic diversity in prokaryotes. Still, the process of how the change comes about is poorly understood. The emergence of new traits may present a high prospect for the industrially viable organism. Microbial enzymes have prominence in the bio-economic space, and proteases account for about sixty percent of all enzyme market. Microbial keratinases are versatile proteases which are continuously gaining momentum in biotechnology owing to their effective bio-conversion of recalcitrant keratin-rich wastes and sustainable implementation of cleaner production. Keratinase-assisted biodegradation of keratinous materials has revitalized the prospects for the utilization of cost-effective agro-industrial wastes, as readily available substrates, for the production of high-value products including amino acids and bioactive peptides. This review presented an overview of keratin structural complexity, the potential mechanism of keratin biodegradation, and the environmental impact of keratinous wastes. Equally, it discussed microbial keratinase; vis-à-vis sources, production, and functional properties with considerable emphasis on the ecological implication of microbial producers and catalytic tendency improvement strategies. Keratinase applications and prospective high-end use, including animal hide processing, detergent formulation, cosmetics, livestock feed, and organic fertilizer production, were also articulated.

Cloning of a fibrinolytic enzyme (subtilisin) gene from Bacillus subtilis in Escherichia coli

Several investigations are being pursued to enhance the efficacy and specificity of fibrinolytic therapy. In this regard, microbial fibrinolytic enzymes attracted much more medical interests during these decades. Subtilisin, a member of subtilases (the superfamily of subtilisin-like serine proteases) and also a fibrinolytic enzyme is quite common in Gram-positive bacteria, and Bacillus species stand out in particular, as many extracellular and even intracellular variants have been identified. In the present work, the subtilisin gene from Bacillus subtilis PTCC 1023 was cloned into the vector pET-15b and expressed in Escherichia coli strain BL21 (DE3). Total genomic DNA were isolated and used for PCR amplification of the subtilisin gene by means of the specific primers. SDS-PAGE and enzyme assay were done for characterizing the expressed protein. A ~1,100 bp of the structural subtilisin gene was amplified. The DNA and amino acid sequence alignments resulting from the BLAST search of subtilisin showed high sequence identity with the other strains of B. subtilis, whereas significantly lower identity was observed with other bacterial subtilisins. The recombinant enzyme had the same molecular weight as other reported subtilisins and the E. coli transformants showed high subtilisin activity. This study provides evidence that subtilisin can be actively expressed in E. coli. The commercial availability of subtilisin is of great importance for industrial applications and also pharmaceutical purposes as thrombolytic agent. Thus, the characterization of new recombinant subtilisin and the development of rapid, simple, and effective production methods are not only of academic interest, but also of practical importance.

Stabilization of substilisin E in organic solvents by site-directed mutagenesis

Subtilisin E was rationally engineered to improve its stability in polar organic solvents such as dimethylformamide (DMF). A charged surface residue, Asp248, was substituted by three amino acids of increasing hydrophobicity, Asn, Ala, and Leu; all three variants were stabilized with respect to wild type in 80% DMF. This stabilization was only observed in the presence of high concentrations of the organic solvent: no stability enhancements were observed in 40% DMF. In contrast, the mutation Asn218 --> Ser alters internal hydrogen bonding interactions and stabilizes subtilisin E in both 40% and 80% DMF. This study provides additional evidence that substitution of surface-charged residues is a generally useful mechanism for stabilizing enzymes in organic media and that the stabilizing effects of such substitutions are unique to highly altered solvent environments. The effects of the single amino acid substitutions on free energies of stabilization are additive in the Asp248 --> Asn + Asn218 --> Ser combination variant, yielding an enzyme that is 3.4 times more stable than wild type in 80% DMF.

Ca2+-dependent maturation of subtilisin from a hyperthermophilic archaeon, Thermococcus kodakaraensis: the propeptide is a potent inhibitor of the mature domain but is not required for its folding

Subtilisin from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 is a member of the subtilisin family. T. kodakaraensis subtilisin in a proform (T. kodakaraensis pro-subtilisin), as well as its propeptide (T. kodakaraensis propeptide) and mature domain (T. kodakaraensis mat-subtilisin), were independently overproduced in E. coli, purified, and biochemically characterized. T. kodakaraensis pro-subtilisin was inactive in the absence of Ca2+ but was activated upon autoprocessing and degradation of propeptide in the presence of Ca2+ at 80 degrees C. This maturation process was completed within 30 min at 80 degrees C but was bound at an intermediate stage, in which the propeptide is autoprocessed from the mature domain (T. kodakaraensis mat-subtilisin*) but forms an inactive complex with T. kodakaraensis mat-subtilisin*, at lower temperatures. At 80 degrees C, approximately 30% of T. kodakaraensis pro-subtilisin was autoprocessed into T. kodakaraensis propeptide and T. kodakaraensis mat-subtilisin*, and the other 70% was completely degraded to small fragments. Likewise, T. kodakaraensis mat-subtilisin was inactive in the absence of Ca2+ but was activated upon incubation with Ca2+ at 80 degrees C. The kinetic parameters and stability of the resultant activated protein were nearly identical to those of T. kodakaraensis mat-subtilisin*, indicating that T. kodakaraensis mat-subtilisin does not require T. kodakaraensis propeptide for folding. However, only approximately 5% of T. kodakaraensis mat-subtilisin was converted to an active form, and the other part was completely degraded to small fragments. T. kodakaraensis propeptide was shown to be a potent inhibitor of T. kodakaraensis mat-subtilisin* and noncompetitively inhibited its activity with a Ki of 25 +/- 3.0 nM at 20 degrees C. T. kodakaraensis propeptide may be required to prevent the degradation of the T. kodakaraensis mat-subtilisin molecules that are activated later by those that are activated earlier.

Expression of Bacillus protease (Protease BYA) from Bacillus sp. Y in Bacillus subtilis and enhancement of its specific activity by site-directed mutagenesis-improvement in productivity of detergent enzyme-

An attempt was made to express protease BYA produced by an alkalophilic Bacillus sp. Y in Bacillus subtilis by gene engineering methods. The gene encoding protease BYA was cloned from Bacillus sp. Y, and expression vector pTA71 was constructed from the amylase promoter of Bacillus licheniformis, DNA fragments encoding the open reading frame of protease BYA, and pUB110. Protease BYA was secreted at an activity level of 5100 APU/ml in the common industrial culture medium of Bacillus subtilis transformed with pTA71. We then attempted to increase the specific activity of protease BYA by site-directed mutagenesis. Amino acid residue Ala29 next to catalytic Asp30 was replaced by one of three uncharged amino acid residues (Val29, Leu29, Ile29), and each mutant enzyme was expressed and isolated from the culture medium. Val29 mutant enzyme was secreted at an activity level of greater than 7000 APU/ml in culture medium, and its specific activity was 1.5-fold higher than that of the wild-type enzyme. Other mutant enzymes had specific activity similar to that of the original one and were less stabile than the wild-type enzyme. It can be thought that the substitution at amino acid residue 29 affects the level of activity and stability of protease BYA.

Improved secretory production of recombinant proteins by random mutagenesis of hlyB, an alpha-hemolysin transporter from Escherichia coli

Fusion proteins with an alpha-hemolysin (HlyA) C-terminal signal sequence are known to be secreted by the HlyB-HlyD-TolC translocator in Escherichia coli. We aimed to establish an efficient Hly secretory expression system by random mutagenesis of hlyB and hlyD. The fusion protein of subtilisin E and the HlyA signal sequence (HlyA(218)) was used as a marker protein for evaluating secretion efficiency. Through screening of more than 1.5 x 10(4) E. coli JM109 transformants, whose hlyB and hlyD genes had been mutagenized by error-prone PCR, we succeeded in isolating two mutants that had 27- and 15-fold-higher levels of subtilisin E secretion activity than the wild type did at 23 degrees C. These mutants also exhibited increased activity levels for secretion of a single-chain antibody-HlyA(218) fusion protein at 23 and 30 degrees C but unexpectedly not at 37 degrees C, suggesting that this improvement seems to be dependent on low temperature. One mutant (AE104) was found to have seven point mutations in both HlyB and HlyD, and an L448F substitution in HlyB was responsible for the improved secretion activity. Another mutant (AE129) underwent a single amino acid substitution (G654S) in HlyB. Secretion of c-Myc-HlyA(218) was detected only in the L448F mutant (AE104F) at 23 degrees C, whereas no secretion was observed in the wild type at any temperature. Furthermore, for the PTEN-HlyA(218) fusion protein, AE104F showed a 10-fold-higher level of secretion activity than the wild type did at 37 degrees C. This result indicates that the improved secretion activity of AE104F is not always dependent on low temperature.

Reconstructing the diversification of subtilisins in the pathogenic fungus Metarhizium anisopliae

Fungi secrete subtilisin proteinases to acquire nutrients and breach host barriers. Here we sought a global characterization of the diversity of subtilisins in the insect pathogen Metarhizium anisopliae. Expressed sequence tag (EST) analyses showed that a broad host range strain of M. anisopliae sf. anisopliae (strain 2575) expressed 11 subtilisins during growth on insect cuticle, the largest number of subtilisins reported from any fungus. Polymerase chain reaction amplified 10 of their orthologs from a second strain with multiple hosts (strain 820) and seven from the locust specialist M. anisopliae sf. acridum (strain 324). Analyses based on sequence similarities and exon-intron structure grouped M. anisopliae subtilisins into four clusters-a class I ("bacterial") subtilisin (Pr1C), and three clusters of proteinase K-like class II subtilisins: extracellular subfamily 1 (Pr1A, Pr1B, Pr1G, Pr1I and Pr1K), extracellular subfamily 2 (Pr1D, Pr1E, Pr1F and Pr1J) and an endocellular subtilisin (Pr1H). Phylogenetic analysis of homologous sequences from other genera revealed that this subdivision of proteinase K-like subtilisins into three subfamilies preceded speciation of major fungal lineages. However, diversification has continued during the evolution of Metarhizium subtilisins with evidence of gene duplication events after divergence of M. anisopliae sf. anisopliae and M. anisopliae sf. acridum. Comparing alignments and nonsynonymous/synonymous rates for Pr1 isoenzymes within a lineage and between lineages showed that while overall divergence of subtilisins followed neutral expectations, amino acids involved in catalysis were under strong selective constraint. This suggests that each Pr1 paralog contributes to the pathogens fitness. Furthermore, homology modeling predicted differences between the Pr1's in their secondary substrate specificities, adsorption properties to cuticle and alkaline stability, indicative of functional differences.

Microbial alkaline proteases: from a bioindustrial viewpoint

Alkaline proteases are of considerable interest in view of their activity and stability at alkaline pH. This review describes the proteases that can resist extreme alkaline environments produced by a wide range of alkalophilic microorganisms. Different isolation methods are discussed which enable the screening and selection of promising organisms for industrial production. Further, strain improvement using mutagenesis and/or recombinant DNA technology can be applied to augment the efficiency of the producer strain to a commercial status. The various nutritional and environmental parameters affecting the production of alkaline proteases are delineated. The purification and properties of these proteases is discussed, and the use of alkaline proteases in diverse industrial applications is highlighted.

Functional analysis of the propeptides of subtilisin E and aqualysin I as intramolecular chaperones

Several proteases require propeptides for the correct folding of their own protease domain. We have recently found that the propeptide from a thermostable subtilisin homolog aqualysin I can refold subtilisin BPN' when added in trans. Here, we constructed chimeric genes with subtilisin E and aqualysin I to attempt the in cis folding of subtilisin E by means of the propeptide of aqualysin I. Our results indicate that the propeptide of aqualysin I can to some extent chaperone the intramolecular folding of the denatured subtilisin E. These results suggest that propeptides in the subtilisin family, despite their sequence diversity, have similar functions. Further, some enzymatic properties of some chimeras in which the subtilisin mature domain is partly swapped with that of aqualysin I were shown to be more similar to those of aqualysin I.

Active subtilisin-like protease from a hyperthermophilic archaeon in a form with a putative prosequence

The gene encoding subtilisin-like protease T. kodakaraensis subtilisin was cloned from a hyperthermophilic archaeon Thermococcus kodakaraensis KOD1. T. kodakaraensis subtilisin is a member of the subtilisin family and composed of 422 amino acid residues with a molecular weight of 43,783. It consists of a putative presequence, prosequence, and catalytic domain. Like bacterial subtilisins, T. kodakaraensis subtilisin was overproduced in Escherichia coli in a form with a putative prosequence in inclusion bodies, solubilized in the presence of 8 M urea, and refolded and converted to an active molecule. However, unlike bacterial subtilisins, in which the prosequence was removed from the catalytic domain by autoprocessing upon refolding, T. kodakaraensis subtilisin was refolded in a form with a putative prosequence. This refolded protein of recombinant T. kodakaraensis subtilisin which is composed of 398 amino acid residues (Gly(-82) to Gly(316)), was purified to give a single band on a sodium dodecyl sulfate (SDS)-polyacrylamide gel and characterized for biochemical and enzymatic properties. The good agreement of the molecular weights estimated by SDS-polyacrylamide gel electrophoresis (44,000) and gel filtration (40,000) suggests that T. kodakaraensis subtilisin exists in a monomeric form. T. kodakaraensis subtilisin hydrolyzed the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide only in the presence of the Ca(2+) ion with an optimal pH and temperature of pH 9.5 and 80 degrees C. Like bacterial subtilisins, it showed a broad substrate specificity, with a preference for aromatic or large nonpolar P1 substrate residues. However, it was much more stable than bacterial subtilisins against heat inactivation and lost activity with half-lives of >60 min at 80 degrees C, 20 min at 90 degrees C, and 7 min at 100 degrees C.

Protein engineering of subtilisin

The serine protease subtilisin is an important industrial enzyme as well as a model for understanding the enormous rate enhancements affected by enzymes. For these reasons along with the timely cloning of the gene, ease of expression and purification and availability of atomic resolution structures, subtilisin became a model system for protein engineering studies in the 1980s. Fifteen years later, mutations in well over 50% of the 275 amino acids of subtilisin have been reported in the scientific literature. Most subtilisin engineering has involved catalytic amino acids, substrate binding regions and stabilizing mutations. Stability has been the property of subtilisin which has been most amenable to enhancement, yet perhaps least understood. This review will give a brief overview of the subtilisin engineering field, critically review what has been learned about subtilisin stability from protein engineering experiments and conclude with some speculation about the prospects for future subtilisin engineering.

Molecular and biotechnological aspects of microbial proteases

Proteases represent the class of enzymes which occupy a pivotal position with respect to their physiological roles as well as their commercial applications. They perform both degradative and synthetic functions. Since they are physiologically necessary for living organisms, proteases occur ubiquitously in a wide diversity of sources such as plants, animals, and microorganisms. Microbes are an attractive source of proteases owing to the limited space required for their cultivation and their ready susceptibility to genetic manipulation. Proteases are divided into exo- and endopeptidases based on their action at or away from the termini, respectively. They are also classified as serine proteases, aspartic proteases, cysteine proteases, and metalloproteases depending on the nature of the functional group at the active site. Proteases play a critical role in many physiological and pathophysiological processes. Based on their classification, four different types of catalytic mechanisms are operative. Proteases find extensive applications in the food and dairy industries. Alkaline proteases hold a great potential for application in the detergent and leather industries due to the increasing trend to develop environmentally friendly technologies. There is a renaissance of interest in using proteolytic enzymes as targets for developing therapeutic agents. Protease genes from several bacteria, fungi, and viruses have been cloned and sequenced with the prime aims of (i) overproduction of the enzyme by gene amplification, (ii) delineation of the role of the enzyme in pathogenecity, and (iii) alteration in enzyme properties to suit its commercial application. Protein engineering techniques have been exploited to obtain proteases which show unique specificity and/or enhanced stability at high temperature or pH or in the presence of detergents and to understand the structure-function relationships of the enzyme. Protein sequences of acidic, alkaline, and neutral proteases from diverse origins have been analyzed with the aim of studying their evolutionary relationships. Despite the extensive research on several aspects of proteases, there is a paucity of knowledge about the roles that govern the diverse specificity of these enzymes. Deciphering these secrets would enable us to exploit proteases for their applications in biotechnology.

A serine alkaline protease from the fungus Conidiobolus coronatus with a distinctly different structure than the serine protease subtilisin Carlsberg

In view of the functional similarities between subtilisin Carlsberg and the alkaline protease from Conidiobolus coronatus, the biochemical and structural properties of the two enzymes were compared. In spite of their similar biochemical properties, e.g., pH optima, heat stability, molecular mass, pI, esterase activity, and inhibition by diisopropyl fluorophosphate and phenylmethlysulfonylfluoride, the proteases were structurally dissimilar as revealed by (1) their amino acid compositions, (2) their inhibition by subtilisin inhibitor, (3) their immunological response to specific anti-Conidiobolus protease antibody, and (4) their tryptic peptide maps. Our results demonstrate that although they are functionally analogous, the Conidiobolus protease is structurally distinct from subtilisin Carlsberg. The Conidiobolus protease was also different from other bacterial and animal proteases (e.g. pronase, protease K, trypsin, and chymotrypsin) as evidenced by their lack of response to anti-Conidiobolus protease antibody in double diffusion and in neutralization assays. The Conidiobolus serine protease fails to obey the general rule that proteins with similar functions have similar primary sequences and, thus, are evolutionarily related. Our results strengthen the concept of convergent evolution for serine proteases and provide basis for research in evolutionary relationships among fungal, bacterial, and animal proteases.

Restriction of substrate specificity of subtilisin E by introduction of a side chain into a conserved glycine residue

Substitution of the conserved Gly127 for residues having a side chain markedly changed the substrate specificity of subtilisin E from Bacillus subtilis. The crystallographic findings suggested that Gly127 is responsible for accepting even the large P1 substrates, and the marked change of specificity was attributed to the introduction of a side chain in this position. To test this hypothesis, Gly127 was replaced with 3 non-charged amino acids, Ala, Ser and Val. When assayed with synthetic peptide substrates, all mutants purified from the periplasmic space in Escherichia coli showed a marked preference for small P1 substrate up to 150-fold relative to the wild-type. The kinetic data and molecular modeling analysis suggest that large hydrophobic P1 residues were unable to access the binding pocket due to steric hindrance.

Focusing of alkaline proteases (subtilisins) in pH 10-12 immobilized gradients

Isoelectric focusing in very alkaline immobilized pH gradients (IPG) was adopted for checking the purity and assessing the pI value of two strongly alkaline proteases: Savinase and Durazym. The first enzyme (known to be the most alkaline) contains 5 Asp, 5 Glu, 7 His, 7 Tyr, 5 Lys, no Cys and 8 Arg residues and should have a theoretical pI of 9.7. Yet, when focused in a pH 9-11 IPG interval, it was lost in the cathodic compartment. After repeated attempts at creating even more alkaline pH intervals, a pH 10-12 IPG range was finally optimized and proved successful in focusing both enzymes midway between the two electrodic compartments. The pI of Savinase was measured as 11.15 +/- 0.15; that of Durazym as 10.95 +/- 0.20 and that of the pI marker cytochrome c as 10.6 +/- 0.17. Both enzymes (and a number of minor components in each preparation) were proven to be active by an in situ zymogram consisting of a casein/agar overlay. The discrepancy between theoretical and experimental pI values could not be fully reconciled: when correcting for pK values of amino acids in proteins at 10 degrees C, instead of the tabulated values at 25 degrees C, the pI should increase to a value of 10. Differential UV spectra showed that ca. 1/2 Tyr are buried in the protein interior and are thus unable to contribute to surface charge. This further increases the pI value by 0.3 pH units to a value of 10.3, still quite removed from the experimentally assessed pI value (in the gel, at 10 degrees C) of 11.15.

Probing the mechanism and improving the rate of substrate-assisted catalysis in subtilisin BPN'

A mutant of the serine protease, subtilisin BPN', in which the catalytic His64 is replaced by Ala (H64A), is very specific for substrates containing a histidine, presumably by the substrate-bound histidine assisting in catalysis [Carter, P., & Wells, J.A. (1987) Science (Washington, D.C.) 237, 394-399]. Here we probe the catalytic mechanism of H64A subtilisin for cleaving His and non-His substrates. We show that the ratio of aminolysis to hydrolysis is the same for ester and amide substrates as catalyzed by the H64A subtilisin. This is consistent with formation of a common acyl-enzyme intermediate for H64A subtilisin, analogous to the mechanism of the wild-type enzyme. However, the catalytic efficiencies (kcat/KM) for amidase and esterase activities with His-containing substrates are reduced by 5000-fold and 14-fold, respectively, relative to wild-type subtilisin BPN, suggesting that acylation is more compromised than deacylation in the H64A mutant. High concentrations of imidazole are much less effective than His substrates in promoting hydrolysis by the H64A variant, suggesting that the His residue on the bound (not free) substrate is involved in catalysis. The reduction in catalytic efficiency kcat/KM for hydrolysis of the amide substrate upon replacement of the oxyanion stabilizing asparagine (N155G) is only 7-fold greater for wild-type than H64A subtilisin. In contrast, the reductions in kcat/KM upon replacement of the catalytic serine (S221A) or aspartate (D32A) are about 3000-fold greater for wild-type than H64A subtilisin, suggesting that the functional interactions between the Asp32 and Ser221 with the substrate histidine are more compromised in substrate-assisted catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

A highly active and oxidation-resistant subtilisin-like enzyme produced by a combination of site-directed mutagenesis and chemical modification

The subtilisins are known to be susceptible to chemical oxidation due to the conversion of Met222 into the corresponding sulfoxide. A number of derivatives with resistance towards oxidation have previously been prepared by replacement of this group with the other 19 amino acid residues. Unfortunately, the activities of these enzymes were of the order of 1-10% of that obtained with the wild-type enzyme. In contrast, the oxidation-labile cysteine mutant exhibited much higher activity, suggesting that this is associated with the presence of a sulphur atom in the amino acid at position 222. It is shown here that it is possible to maintain a sulphur atom in the amino acid at position 222 without the enzyme becoming labile towards oxidation. A subtilisin from Bacillus lentus, subtilisin 309, in which Met222 was replaced with a cysteinyl residue by site-directed mutagenesis was modified with thioalkylating reagents. Treatment of such enzyme derivatives with H2O2 revealed that their stabilities towards oxidation had increased significantly compared to both wild-type and unmodified [Cys222]subtilisin. One of the chemically modified enzyme derivatives, [Me-S-Cys222]subtilisin, exhibited a kcat/Km value of 56% of that obtained with the wild-type enzyme when assayed against the substrate Suc-Ala-Ala-Pro-Phe-NH-Ph-NO2 (Suc, succinyl) and it exhibited 89% activity when tested in an assay with dimethyl casein as a substrate. The corresponding values obtained for unmodified [Cys222]subtilisin were lower, i.e. 39% for the dimethyl casein activity and 46% for the kcat/Km for the hydrolysis of Suc-Ala-Ala-Pro-Phe-NH-Ph-NO2. This demonstrates the feasibility of replacing the oxidation-labile methionyl residue group in a subtilisin enzyme with a group stable towards oxidation without substantially reducing the activity.