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.
Cheng
Chih-Yu
CY
Center for Interdisciplinary Molecular Science and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan.
Chang
Chu-Han
CH
Wu
Yue-Jin
YJ
Li
Yaw-Kuen
YK
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry; D001232:Aspergillus fumigatus / Q000201:enzymology*; D002384:Catalysis; D020134:Catalytic Domain; D002852:Chromatography, Ion Exchange; D002853:Chromatography, Liquid; D002942:Circular Dichroism; D003001:Cloning, Molecular; D004247:DNA / Q000378:metabolism; D017931:DNA Primers / Q000737:chemistry; D018076:DNA, Complementary / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D015723:Gene Library; D018698:Glutamic Acid / Q000737:chemistry; D006868:Hydrolysis; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008957:Models, Genetic; D008969:Molecular Sequence Data; D005810:Multigene Family; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009699:N-Glycosyl Hydrolases / Q000737:chemistry* / Q000235:genetics; D009844:Oligosaccharides / Q000737:chemistry; D010957:Plasmids / Q000378:metabolism; D016133:Polymerase Chain Reaction; D021382:Protein Sorting Signals; D012313:RNA / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D013696:Temperature; D014508:Urea / Q000737:chemistry
D000596:Amino Acids; D017931:DNA Primers; D018076:DNA, Complementary; D009844:Oligosaccharides; D021382:Protein Sorting Signals; D011994:Recombinant Proteins; D001224:Aspartic Acid; D018698:Glutamic Acid; D012313:RNA; D014508:Urea; D004247:DNA; D009699:N-Glycosyl Hydrolases