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Type: Journal article
Title: Substrate specificity and catalytic mechanism of a xyloglucan xyloglucosyl transferase HvXET6 from barley (Hordeum vulgare L.)
Author: Hrmova, M.
Farkas, V.
Harvey, A.
Lahnstein, J.
Wischmann, B.
Kaewthai, N.
Ezcurra, I.
Teeri, T.
Fincher, G.
Citation: The Federation of European Biochemical Societies (FEBS) Journal, 2009; 276(2):437-456
Publisher: Blackwell Publishing Ltd
Issue Date: 2009
ISSN: 1742-464X
Statement of
Maria Hrmova, Vladimir Farkas, Andrew J. Harvey, Jelle Lahnstein, Bente Wischmann, Nomchit Kaewthai, Inés Ezcurra, Tuula T. Teeri and Geoffrey B. Fincher
Abstract: A family 16 glycoside hydrolase, xyloglucan xyloglucosyl transferase (EC, also known as xyloglucan endotransglycosylase (XET), and designated isoenzyme HvXET6, was purified approximately 400-fold from extracts of young barley seedlings. The complete amino acid sequence of HvXET6 was deduced from the nucleotide sequence of a near full-length cDNA, in combination with tryptic peptide mapping. An additional five to six isoforms or post-translationally modified XET enzymes were detected in crude seedling extracts of barley. The HvXET6 isoenzyme was expressed in Pichia pastoris, characterized and compared with the previously purified native HvXET5 isoform. Barley HvXET6 has a similar apparent molecular mass of 33-35 kDa to the previously purified HvXET5 isoenzyme, but the two isoenzymes differ in their isoelectric points, pH optima, kinetic properties and substrate specificities. The HvXET6 isoenzyme catalyses transfer reactions between xyloglucans and soluble cellulosic substrates, using oligo-xyloglucosides as acceptors, but at rates that are significantly different from those observed for HvXET5. No hydrolytic activity could be detected with either isoenzyme. Comparisons of the reaction rates using xyloglucan or hydroxyethyl cellulose as donors and a series of cellodextrins as acceptors indicated that the acceptor site of HvXET can accommodate five glucosyl residues. Molecular modelling supported this conclusion and further confirmed the ability of the enzyme's active site to accommodate xyloglucan and cellulosic substrates. The two HvXETs followed a ping-pong (Bi, Bi) rather than a sequential reaction mechanism.
Keywords: glycoside hydrolase family 16
molecular modelling
phylogenetic analyses
plant cell walls
reaction mechanisms
DOI: 10.1111/j.1742-4658.2008.06791.x
Grant ID: ARC
Appears in Collections:Agriculture, Food and Wine publications
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