Structural and enzymatic analyses of a glucosyltransferase Alr3699/HepE involved in Anabaena heterocyst envelop polysaccharide biosynthesis

Glycobiology. 2016 May;26(5):520-31. doi: 10.1093/glycob/cwv167. Epub 2015 Dec 21.


Formation of the heterocyst envelope polysaccharide (HEP) is a key process for cyanobacterial heterocyst differentiation. The maturation of HEP in Anabaena sp. strain PCC 7120 is controlled by a gene cluster termed HEP island in addition to an operon alr3698-alr3699, which encodes two putative proteins termed Alr3698/HepD and Alr3699/HepE. Here we report the crystal structures of HepE in the apo-form and three complex forms that bind to UDP-glucose (UDPG), UDP&glucose, and UDP, respectively. The overall structure of HepE displays a typical GT-B fold of glycosyltransferases, comprising two separate β/α/β Rossmann-fold domains that form an inter-domain substrate-binding crevice. Structural analyses combined with enzymatic assays indicate that HepE is a glucosyltransferase using UDPG as a sugar donor. Further site-directed mutageneses enable us to assign the key residues that stabilize the sugar donor and putative acceptor. Based on the comparative structural analyses, we propose a putative catalytic cycle of HepE, which undergoes "open-closed-open" conformational changes upon binding to the substrates and release of products. These findings provide structural and catalytic insights into the first enzyme involved in the HEP biosynthesis pathway.

Keywords: GT-B fold; crystal structure; cyanobacteria; glycosyltransferase; heterocyst envelope polysaccharide.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anabaena / enzymology*
  • Anabaena / genetics
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Glucosyltransferases / chemistry*
  • Glucosyltransferases / genetics
  • Glucosyltransferases / metabolism*
  • Multigene Family
  • Polysaccharides, Bacterial / biosynthesis*
  • Polysaccharides, Bacterial / chemistry*
  • Polysaccharides, Bacterial / genetics
  • Protein Domains
  • Uridine Diphosphate Glucose / chemistry
  • Uridine Diphosphate Glucose / genetics
  • Uridine Diphosphate Glucose / metabolism


  • Bacterial Proteins
  • Polysaccharides, Bacterial
  • Glucosyltransferases
  • Uridine Diphosphate Glucose