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Mannanase A from Pseudomonas fluorescens ssp. cellulosa is a retaining glycosyl hydrolase in which E212 and E320 are the putative catalytic residues

Lookup NU author(s): Dr David Bolam, Dr Richard Virden, Professor Jeremy Lakey, Professor Harry Gilbert

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Abstract

Mannanase A (MANA) from Pseudomonas fluorescens, a member of glycosyl hydrolase family 26, was hyperexpressed in Escherichia coli and purified to homogeneity. Analysis of the stereochemical course of mannotetraose hydrolysis by purified MANA showed that the configuration of the anomeric carbon was retained on cleavage of the middle glycosidic bond. These data suggest that the mannanase hydrolyzes mannooligosaccharides by a double- displacement general acid-base mechanism. By hydrophobic cluster analysis (HCA), two glutamate and two aspartate residues were shown to he conserved in all of the glycosyl hydrolase family 26 enzymes analyzed. In addition, HCA suggested that family 26 was related to the GH-A clan (families 1, 2, 5, 10, 30, 35, 39, and 42) of (α/β)8-barrel glycosyl hydrolases, which led to the prediction that E320 and E212 constitute the catalytic nucleophile and acid-base residues, respectively. To investigate the role of these amino acids, site-directed mutagenesis was used to replace the two aspartates with alanine and glutamate, while the two conserved glutamates were changed to alanine and aspartate. The mutant enzymes were purified and their biochemical properties were analyzed. The data showed that neither the D → A nor the D → E mutation resulted in a dramatic decrease in enzyme activity, suggesting that the two aspartate residues did not play a pivotal role in catalysis. In contrast, modification of either of the glutamate residues to alanine caused a dramatic decrease in k(cat) against carob galactomannan, azo-carob galactomannan, mannotetraose and 2,4-dinitrophenyl β-mannobioside (2,4-DNPM). The E320A mutation did not alter the apparent K(m) (K(m)') of MANA against these substrates, while E212A resulted in a 27- fold decrease in K(m)' against 2,4-DNPM. Pre-steady-state kinetics of 2,4- DNPM hydrolysis by E212A showed that there was a rapid burst of 2,4- dinitrophenol release. Circular dichroism and fluorescence spectroscopy indicated that there were no significant differences between the structures of the mutant and wild-type forms of MANA. These data are consistent with E212 and E320 constituting the catalytic acid-base and nucleophile residues of MANA, respectively.


Publication metadata

Author(s): Bolam DN, Hughes N, Virden R, Lakey JH, Hazlewood G, Henrissat B, Braithwaite K, Gilbert HJ

Publication type: Article

Publication status: Published

Journal: Biochemistry

Year: 1996

Volume: 35

Issue: 50

Pages: 16195-16204

Print publication date: 17/12/1996

ISSN (print): 0006-2960

ISSN (electronic): 1943-295X

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/bi961866d

DOI: 10.1021/bi961866d

PubMed id: 8973192


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