Crystal structure of Pedobacter heparinus heparin lyase Hep III with the active site in a deep cleft

Pedobacter heparinus (formerly known as Flavobacterium heparinum) is a typical glycosaminoglycan-degrading bacterium that produces three heparin lyases, Hep I, Hep II, and Hep III, which act on heparins with 1,4-glycoside bonds between uronate and amino sugar residues. Being different from Hep I and Hep II, Hep III is specific for heparan sulfate. Here we describe the crystal structure of Hep III with the active site located in a deep cleft. The X-ray crystallographic structure of Hep III was determined at 2.20 Å resolution using single-wavelength anomalous diffraction. This enzyme comprised an N-terminal α/α-barrel domain and a C-terminal antiparallel β-sheet domain as its basic scaffold. Overall structures of Hep II and Hep III were similar, although Hep III exhibited an open form compared with the closed form of Hep II. Superimposition of Hep III and heparin tetrasaccharide-bound Hep II suggested that an active site of Hep III was located in the deep cleft at the interface between its two domains. Three mutants (N240A, Y294F, and H424A) with mutations at the active site had significantly reduced enzyme activity. This is the first report of the structure-function relationship of P. heparinus Hep III.