Occurrence and functional significance of secondary carbohydrate binding sites in glycoside hydrolases

Non-catalytic carbohydrate binding on independent carbohydrate-binding modules (CBMs) has been reported frequently for glycoside hydrolases (GHs) and reviewed thoroughly. However, various structural studies of GHs have revealed that non-catalytic carbohydrate binding sites can also occur on the surface of the structural unit comprising the active site. Here, the discovery of these sites, referred to as secondary binding sites (SBSs), and their putative roles in different GHs is reviewed for the first time. The majority of the SBSs have been discovered in starch-active enzymes, but there are also many reports of SBSs in various other enzymes. A wide variety of functions has been ascribed to these sites, including (1) targeting of the enzyme towards its substrate, (2) guiding the substrate into the active site groove, (3) substrate disruption, (4) enhancing processivity, (5) allosteric regulation, (6) passing on reaction products, and (7) anchoring to the cell wall of the parent microorganism. A lot of these putative functions are in agreement with the functions ascribed to non-catalytic binding in CBMs. Contrarily to CBMs, SBSs have a fixed position relative to the catalytic site, making them more or less suitable to take up specific functions.