Division of the large and multifunctional glycoside hydrolase family 2: high functional specificity and biochemical assays in the uncharacterized subfamilies

Background Glycoside Hydrolase family 2 (GH2) is one of the largest and most functionally diverse carbohydrate-active enzyme families. This functional diversity is an obstacle to accurate functional prediction by family assignment and has led to the accumulation of erroneous annotations in non-curated databases.
Results We explored the sequence space of the GH2 family using Sequence-Similarity Networks coupled with closeness centrality to identify 23 subfamilies. The analysis suggests that the GH2 family evolved via multiple duplications followed by neofunctionalization events, with two main activities, β-glucuronidase and β-galacturonidase, re-emerging from likely flexible/reversible ancestors, while an early diverging branch gave birth to several subfamilies with unique activities. To increase the predictive power of subfamily assignments, we biochemically characterized seven members of four of the five subfamilies without previously reported activity. 
Conclusions The GH2 subfamilies showing high functional homogeneity will enable more precise functional predictions, while our work highlights subfamilies that require further biochemical and structural investigations.