Carbohydrate actives enzymes derived from metagenomes: from microbial ecology to enzymology

In the last ten years, the intensive mining of various environmental metagenomes has led to the discovery of numerous new genes and corresponding putative enzymes. Some enzymes were isolated for their ability to hydrolyze carbohydrates, including starch, xylan, chitin and cellulose derivatives. The accurate characterization of these proteins highlights their variability and their biophysical adaptation in order to cope with specific environmental conditions. In this perspective, the sampling of extreme environments for metagenomic library construction resulted in the isolation of enzymes harbouring tailor made properties aimed at their implementation in various industrial processes. Although these new catalysts appear to be of particular interest for biotechnological applications, little is known about their physiological functions in their natural host.
In the field of glycosides hydrolases, different functions have been suggested including both hydrolysis and synthesis of polymers. On the one hand, indeed in the environment microorganisms compete for ecological niches by producing enzymes active against vast ranges of substrates which allow them to thrive on various carbon sources. On the other hand, production of structural (cellulose) or reserve (glycogen) polymers by bacteria such as Gluconacetobacter sp. was reported. Polysaccharides can be associated with bacterial biofilm and feed stock, compounds that are required for bacteria to live in various environments. Interestingly, the synthesis of these polymers requires enzymes which act on carbohydrate including enzymes referred to as glycoside hydrolases acting as transglycosylases.
In this chapter, a review of the representative glycoside hydrolases isolated by metagenomic and their possible physiological functions are presented.