Structures of vertebrate hyaluronidases and their unique enzymatic mechanism of hydrolysis

Human hyaluronidases (Hyals) are a group of five endo‐β‐acetyl‐hexosaminidase enzymes, Hyal‐1, ‐2, ‐3, ‐4, and PH‐20, which degrade hyaluronan using a hydrolytic mechanism of action. Catalysis by these Hyals has been shown to follow a double‐displacement scheme. This involves a single Glu residue within the enzyme, the only catalytic residue, as the proton donor (acid). Also involved is a carbonyl group of the hyaluronan (HA) N ‐acetyl‐ D ‐glucosamine as a unique type of nucleophile. Thus the substrate participates in the mechanism of action of its own catalysis. An oxocarbonium ion transition state is postulated, but there is no formation of a covalent enzyme–glycan intermediate, as found in most such reactions. The major domain is catalytic and has a distorted (β/α) 8 triose phosphate isomerase (TIM) barrel fold. The C‐terminal domain is separated by a peptide linker. Each Hyal has a different C‐terminal sequence and structure, the function of which is unknown. These unique C‐termini may participate in the additional function(s) associated with these multifunctional enzymes. Proteins 2005. © 2005 Wiley‐Liss, Inc.