Crystallization of a chitosanase from Streptomyces N174

Advance in chitosan hydrolysis by non-specific cellulases

Besides the specific chitinase, chitosanase and lysozyme, chitosan also could be hydrolyzed by some non-specific enzymes such as cellulase, protease, lipase and pepsin, especially cellulase, which show high activity on chitosan. Almost all the cellulases produced by different kinds of microorganisms could degrade chitosan to chitooligomers. The existence of bifunctional enzymes with cellulase and chitosanase activity is one of the reasons for cellulase on chitosan hydrolysis. The bifunctional cellulase-chitosanases mainly belong to glycoside hydrolase family 8 (GH-8), few belong to GH-5 and GH-7, according to the homogeneity analysis of amino acids sequences. Their three dimensional structures however have not been clearly determined. This paper may serve as a guide for a further study on the relationship between structure and function of chitosanolytic cellulases.

Quantitative fluorometric analysis of the protective effect of chitosan on thermal unfolding of catalytically active native and genetically-engineered chitosanases

We have taken advantage of the intrinsic fluorescence properties of chitosanases to rapidly and quantitatively evaluate the protective effect of chitosan against thermal denaturation of chitosanases. The studies were done using wild type chitosanases N174 produced by Streptomyces sp. N174 and SCO produced by Streptomyces coelicolor A3(2). In addition, two mutants of N174 genetically engineered by single amino acid substitutions (A104L and K164R) and one "consensus" (N174-CONS) chitosanase designed by multiple amino acid substitutions of N174 were analyzed. Chitosan used had a weight average molecular weight (Mw) of 220 kDa and was 85% deacetylated. Results showed a pH and concentration-dependent protective effect of chitosan in all the cases. However, the extent of thermal protection varied depending on chitosanases, suggesting that key amino acid residues contributed to resistance to heat denaturation. The transition temperatures (T(m)) of N174 were 54 degrees C and 69.5 degrees C in the absence and presence (6 g/l) of chitosan, respectively. T(m) were increased by 11.6 degrees C (N174-CONS), 13.8 degrees C (CSN-A104L), 15.6 degrees C (N174-K164R) and 25.2 degrees C (SCO) in the presence of chitosan (6 g/l). The thermal protective effect was attributed to an enzyme-ligand thermostabilization mechanism since it was not mimicked by the presence of anionic (carboxymethyl cellulose, heparin) or cationic (polyethylene imine) polymers, polyhydroxylated (glycerol, sorbitol) compounds or inorganic salts. Furthermore, the data from fluorometry experiments were in agreement with those obtained by analysis of reaction time-courses performed at 61 degrees C in which case CSN-A104L was rapidly inactivated whereas N174, N174-CONS and N174-K164R remained active over a reaction time of 90 min. This study presents evidence that (1) the fluorometric determination of T(m) in the presence of chitosan is a reliable technique for a rapid assessment of the thermal behavior of chitosanases, (2) it is applicable to structure-function studies of mutant chitosanases and, (3) it can be useful to provide an insight into the mechanism by which mutations can influence chitosanase stability.

Analysis of essential carboxylic amino acid residues for catalytic activity of fungal chitosanases by site-directed mutagenesis

A comparison of amino acid sequences of fungal chitosanases, belonging to family 75 of glycosyl hydrolases, revealed three carboxylic amino acid residues completely conserved among all of the chitosanases. To study the role of these residues in catalysis, they were replaced with other residues by site-directed mutagenesis in the chitosanase gene of Fusarium solani. The mutated genes were expressed in the yeast Saccharomyces cerevisiae and the resulting recombinant chitosanases were used in kinetic analysis. Chitosanases with Asp-175-->Asn and Glu-188-->Gln mutations were essentially inactive, whereas those with Asp-175-->Glu and Glu-188-->Asp mutations retained 25-50% specific activity as compared with the wild-type enzyme. The mutation of Asp-212-->Asn did not decrease specific activity to a large extent. Circular dichroism analysis confirmed that the mutant chitosanases had similar secondary structures to that of the wild-type enzyme. These results indicate that Asp-175 and Glu-188 are essential residues for the catalytic activity of chitosanase. Time-dependent (1)H-NMR analysis for the hydrolysis of D-glucosamine hexamer revealed that a fungal chitosanase is an inverting enzyme producing only the alpha anomeric form of reaction products.

Exploration of glycosyl hydrolase family 75, a chitosanase from Aspergillus fumigatus

A powerful endo-chitosanase (CSN) previously described for a large scale preparation of chito-oligosaccharides (Cheng, C.-Y., and Li, Y.-K. (2000) Biotechnol. Appl. Biochem. 32, 197-203) was cloned from Aspergillus fumigatus and further identified as a member of glycosyl hydrolase family 75. We report here a study of gene expression, functional characterization, and mutation analysis of this enzyme. Gene cloning was accomplished by reverse transcription-PCR and inverse PCR. Within the 1382-bp Aspergillus gene (GenBank accession number AY190324), two introns (67 and 82 bp) and an open reading frame encoding a 238-residue protein containing a 17-residue signal peptide were characterized. The recombinant mature protein was overexpressed as an inclusion body in Escherichia coli, rescued by treatment with 5 m urea, and subsequently purified by cation exchange chromatography. A time course 1H NMR study on the enzymatic formation of chito-oligosaccharides confirmed that this A. fumigatus CSN is an inverting enzyme. Tandem mass spectrum analysis of the enzymatic hydrolysate revealed that the recombinant CSN can cleave linkages of GlcNAc-GlcN and GlcN-GlcN in its substrate, suggesting that it is a subclass I chitosanase. In addition, an extensive site-directed mutagenesis study on 10 conserved carboxylic amino acids of glycosyl hydrolase family 75 was performed. This showed that among these various mutants, D160N and E169Q lost nearly all activity. Further investigation using circular dichroism measurements of D160N, E169Q, wild-type CSN, and other active mutants showed similar spectra, indicating that the loss of enzymatic activity in D160N and E169Q was not because of changes in protein structure but was caused by loss of the catalytic essential residue. We conclude that Asp160 and Glu169 are the essential residues for the action of A. fumigatus endo-chitosanase.

COG3926 and COG5526: a tale of two new lysozyme-like protein families

We have identified two new lysozyme-like protein families by using a combination of sequence similarity searches, domain architecture analysis, and structural predictions. First, the P5 protein from bacteriophage phi8, which belongs to COG3926 and Pfam family DUF847, is predicted to have a new lysozyme-like domain. This assignment is consistent with the lytic function of P5 proteins observed in several related double-stranded RNA bacteriophages. Domain architecture analysis reveals two lysozyme-associated transmembrane modules (LATM1 and LATM2) in a few COG3926/DUF847 members. LATM2 is also present in two proteins containing a peptidoglycan binding domain (PGB) and an N-terminal region that corresponds to COG5526 with uncharacterized function. Second, structure prediction and sequence analysis suggest that COG5526 represents another new lysozyme-like family. Our analysis offers fold and active-site assignments for COG3926/DUF847 and COG5526. The predicted enzymatic activity is consistent with an experimental study on the zliS gene product from Zymomonas mobilis, suggesting that bacterial COG3926/DUF847 members might be activators of macromolecular secretion.

X-ray structure of an anti-fungal chitosanase from streptomyces N174

We report the 2.4 A X-ray crystal structure of a protein with chitosan endo-hydrolase activity isolated from Streptomyces N174. The structure was solved using phases acquired by SIRAS from a two-site methyl mercury derivative combined with solvent flattening and non-crystallographic two-fold symmetry averaging, and refined to an R-factor of 18.5%. The mostly alpha-helical fold reveals a structural core shared with several classes of lysozyme and barley endochitinase, in spite of a lack of shared sequence. Based on this structural similarity we postulate a putative active site, mechanism of action and mode of substrate recognition. It appears that Glu 22 acts as an acid and Asp 40 serves as a general base to activate a water molecule for an SN2 attack on the glycosidic bond. A series of amino-acid side chains and backbone carbonyl groups may bind the polycationic chitosan substrate in a deep electronegative binding cleft.

Site-directed mutagenesis of evolutionary conserved carboxylic amino acids in the chitosanase from Streptomyces sp. N174 reveals two residues essential for catalysis

The comparison of four sequences of prokaryotic chitosanases, belonging to the family 46 of glycosyl hydrolases, revealed a conserved N-terminal module of 50 residues, including five invariant carboxylic residues. To verify if some of these residues are important for catalytic activity in the chitosanase from Streptomyces sp. N174, these 5 residues were replaced by site-directed mutagenesis. Substitutions of Glu-22 or Asp-40 with sterically conservative (E22Q, D40N) or functionally conservative (E22D, D40E) residues reduced drastically specific activity and kcat, while Km was only slightly changed. The other residues examined, Asp-6, Glu-36, and Asp-37, retained significant activity after mutation. Circular dichroism studies of the mutant chitosanases confirmed that the observed effects are not due to changes in secondary structure. These results suggested that Glu-22 and Asp-40 are directly involved in the catalytic center of the chitosanase and the other residues are not essential for catalytic activity.

Reaction mechanism of chitosanase from Streptomyces sp. N174

Chitosanase was produced by the strain of Streptomyces lividans TK24 bearing the csn gene from Streptomyces sp. N174, and purified by S-Sepharose and Bio-Gel A column chromatography. Partially (25-35%) N-acetylated chitosan was digested by the purified chitosanase, and structures of the products were analysed by NMR spectroscopy. The chitosanase produced heterooligosaccharides consisting of D-GlcN and GlcNAc in addition to glucosamine oligosaccharides [(GlcN)n, n = 1, 2 and 3]. The reducing- and non-reducing-end residues of the heterooligosaccharide products were GlcNAc and GlcN respectively, indicating that the chitosanase can split the GlcNAc-GlcN linkage in addition to that of GlcN-GlcN. Time-dependent 1H-NMR spectra showing hydrolysis of (GlcN)6 by the chitosanase were obtained in order to determine the anomeric form of the reaction products. The chitosanase was found to produce only the alpha-form; therefore it is an inverting enzyme. Separation and quantification of (GlcN)n was achieved by HPLC, and the time course of the reaction catalysed by the chitosanase was studied using (GlcN)n (n = 4, 5 and 6) as the substrate. The chitosanase hydrolysed (GlcN)6 in an endo-splitting manner producing (GlcN)2, (GlcN)3 and (GlcN)4, and did not catalyse transglycosylation. Product distribution was (GlcN)3 >> (GlcN)2 > (GlcN)4. Cleavage to (GlcN)3 + (GlcN)3 predominated over that to (GlcN)2 + (GlcN)4. Time courses showed a decrease in rate of substrate degradation from (GlcN)6 to (GlcN)5 to (GlcN)4. It is most likely that the substrate-binding cleft of the chitosanase can accommodate at least six GlcN residues, and that the cleavage point is located at the midpoint of the binding cleft.

Primary sequence of the chitosanase from Streptomyces sp. strain N174 and comparison with other endoglycosidases

A 1.6-kb DNA fragment from the soil actinomycete, Streptomyces sp. strain N174, containing the gene (csn) encoding an extracellular chitosanase (CSN), has been isolated and its complete nucleotide sequence determined. The gene was expressed in Escherichia coli and Streptomyces lividans using appropriate vectors. The sequence was found to contain one large ORF which encodes a protein of 238 amino acids (aa). The deduced aa sequence begins with a signal peptide which has an unusual C-terminal segment, well recognized by the Streptomyces secretion system, but poorly in Escherichia coli. The strain N174 CSN aa sequence was compared with those of other proteins and significant homology was found only with the Bacillus circulans MH-K1 CSN but not with known chitinases or lysozymes. This suggests that CSN form a new enzyme family distinct from other endoglycosidases.