Xyloglucan breakdown by endo-xyloglucanase family 74 from Aspergillus fumigatus
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Xyloglucan is the most abundant hemicellulose in primary walls of spermatophytes except for grasses. Xyloglucan-degrading enzymes are important in lignocellulosic biomass hydrolysis because they remove xyloglucan, which is abundant in monocot-derived biomass. Fungal genomes encode numerous xyloglucanase genes, belonging to at least six glycoside hydrolase (GH) families. GH74 endo-xyloglucanases cleave xyloglucan backbones with unsubstituted glucose at the −1 subsite or prefer xylosyl-substituted residues in the −1 subsite. In this work, 137 GH74-related genes were detected by examining 293 Eurotiomycete genomes and Ascomycete fungi contained one or no GH74 xyloglucanase gene per genome. Another interesting feature is that the triad of tryptophan residues along the catalytic cleft was found to be widely conserved among Ascomycetes. The GH74 from Aspergillus fumigatus (AfXEG74) was chosen as an example to conduct comprehensive biochemical studies to determine the catalytic mechanism. AfXEG74 has no CBM and cleaves the xyloglucan backbone between the unsubstituted glucose and xylose-substituted glucose at specific positions, along the XX motif when linked to regions deprived of galactosyl branches. It resembles an endo-processive activity, which after initial random hydrolysis releases xyloglucan-oligosaccharides as major reaction products. This work provides insights on phylogenetic diversity and catalytic mechanism of GH74 xyloglucanases from Ascomycete fungi.
KeywordsFungal glucanases Xyloglucanases Xyloglucan specific Aspergillus Endo-processive
We are grateful to the State of São Paulo Research Foundation (FAPESP) for its financial support (2012/20549-4 to ARLD; 2008/58037-9 to FMS; 2014/18714-2 to FS). ARLD also received a FAPESP fellowship (2011/02169-7). This work was also financially supported by CNPq: 442333/2014-5, 310186/2014-5, 304445/2014-2, and 441912/2014-1. APS and MSB thank the Instituto Nacional de Ciência e Tecnologia do Bioetanol – INCT do Bioetanol (FAPESP 2008/57908-6 and CNPq 574002/2008-1). We would like to thank the entire team of the molecular biology laboratory from CTBE/CNPEM.
Compliance with ethical standards
This article does not contain any studies with human participants or animals performed by any of the authors.
Conflict of interest
The authors declare that they have no conflict of interest.
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