Acetyl xylan esterases (AcXEs), also termed xylan deacetylases, are broad specificity Carbohydrate-Active
Enzymes (CAZymes) that hydrolyse ester bonds to liberate acetic acid from acetylated hemicellulose (typically
polymeric xylan and xylooligosaccharides). They belong to eight families within the Carbohydrate
Esterase (CE) class of the CAZy database. AcXE classification is largely based on sequence-dependent
phylogenetic relationships, supported in some instances with substrate specificity data. However, some
sequence-based predictions of AcXE-encoding gene identity have proved to be functionally incorrect.
Such ambiguities can lead to mis-assignment of genes and enzymes during sequence data-mining,
reinforcing the necessity for the experimental confirmation of the functional properties of putative
AcXE-encoding gene products.
Although one-third of all characterized CEs within CAZy families 1–7 and 16 are AcXEs, there is a
need to expand the sequence database in order to strengthen the link between AcXE gene sequence
and specificity. Currently, most AcXEs are derived from a limited range of (mostly microbial) sources
and have been identified via culture-based bioprospecting methods, restricting current knowledge of
AcXEs to data from relatively few microbial species. More recently, the successful identification of AcXEs
via genome and metagenome mining has emphasised the huge potential of culture-independent bioprospecting
strategies. We note, however, that the functional metagenomics approach is still hampered
by screening bottlenecks.
The most relevant recent reviews of AcXEs have focused primarily on the biochemical and functional
properties of these enzymes. In this review, we focus on AcXE phylogeny, classification and the future of
metagenomic bioprospecting for novel AcXEs.