The Exiguobacterium sibiricum 255-15 GtfC enzyme representing a novel glycoside hydrolase 70 subfamily of 4,6-α-glucanotransferase enzymes

The Glycoside Hydrolase (GH) family 70 originally was established for glucansucrase enzymes, solely found in lactic acid bacteria, synthesizing α-glucan polysaccharides from sucrose (e.g. GtfA). In recent years we have characterized GtfB and related Lactobacillus enzymes as 4,6-α-glucanotransferase enzymes. These GtfB type of enzymes constitute a first GH70 subfamily, unable to act on sucrose as substrate, but active with maltodextrins and starch, cleaving (α1→4) linkages and synthesizing linear (α1→6)-glucan chains. The GtfB disproportionating type of activity results in conversion of malto-oligosaccharides into IsoMalto-/Malto-Polysaccharides with a relatively high percentage of (α1→6) linkages.This paper reports the identification of a second GH70 subfamily (designated GtfC enzymes) and the characterization of the Exiguobacterium sibiricum 255-15 GtfC enzyme, also inactive with sucrose and displaying 4,6-α-glucanotransferase activity with malto-oligosaccharides. GtfC differs from GtfB in synthesizing IsoMalto-/Malto-Oligosaccharides. Biochemically the GtfB and GtfC type of enzymes are related but phylogenetically they clearly constitute different GH70 subfamilies, displaying only 30% sequence identity. Whereas GtfB type of enzymes largely have the same domain order as glucansucrases (with α-amylase domains A, B, C, plus domains IV and V), this GtfC type of enzyme differs in the order of these domains and completely lacks domain V. In GtfC the sequence of the conserved regions I-IV of clan GH-H is identical to that in GH13 (I-II-III-IV) but different from that in GH70 (II-III-IV-I due to a circular permutation of the (β/α)8-barrel). The GtfC 4,6-α-glucanotransferase enzymes thus represent structurally and functionally very interesting evolutionary intermediates between α-amylase and glucansucrase enzymes.