Fig. 3.

Schematic representation of HA biosynthesis in Class I and Class II Hyaluronic Acid Synthases (HAS). a Class I HAS are integral membrane proteins that catalyse the UDP-sugar addition to growing HA chain and may transport the hydrophilic HA polymer across the cell membrane of eukaryotes or Gram-positive bacteria. Lipid molecules (yellow circles) facilitate the HAS activity which allows it to direct HA translocation. b Class II HAS is a peripheral protein that also catalyses HA elongation. This enzyme is a hybrid of two glycosyltransferases that transfers GlcNAc-UDP and GlcUA-UDP at the non-reducing end of the HA chain. It was suggested that this protein may interact with other cell membrane proteins (capsular polysaccharide transport protein–purple block) in order to translocate HA across the cell membrane of Gram-negative bacteria (P. multocida). Blue and red dots represent GlcUA and GlcNAc, respectively. Orange triangle represents the UDP component of the sugar