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PL8s are active on a variety of uronic acid-containing polysaccharides including: hyaluronan (EC 4.2.2.1) [4)-β-D-Glucuronate-1,3-β-D-N-Acetyl-Glucosamine(1]n, chondroitin AC (EC 4.2.2.5) [4)-β-D-Glucuronate-1,3-β-D-N-Acetyl-GalactosamineΔ4,6S(1]n, xanthan (EC 4.2.2.12) [4)-β-D-Glucuronate-1,4-β-D-Glucuronate (1]n, and chondroitin ABC (EC 4.2.2.20) [chondroitin AC and chondroitin B (aka. dermatan sulfate: 4)-β-L-Iduronate2S-1,3-β-D-N-Acetyl-Galactosamine4S(1]n.
Kinetics and Mechanism
One of the major unresolved controversies around the PL8 catalytic mechanism is the candidate of the general base. Jedrzejas et al. proposed that in the Streptococcus pneumoniae hyaluronidase, His399 acts as the general base, Asn349 acts to neutralize the C5-carboxylate group, and Tyr408 is the proton donor [1, 2]. However, for two other PL8 family members: the Bacillus sp. GL1 xanthanase and Streptomyces coelicolor A3 hyaluronidase, it was suggested that an equivalent tyrosine residue served as the general acid and general base throughout the reaction [3, 4]. Combined quantum mechanical and molecular mechanical (QM/MM) simulations of the S. pneumoniae enzyme suggests the latter hypothesis is favored, with His399 participating in the neutralization of the C5-carboxylate group [5]. Molecular dynamic simulations of the pneumococcal hyaluronidase with hyaluronan fragments suggest that a combination of opening/closing and twisting domain motions of the (α/α)6 barrel with respect to the anti-parallel β-sheet domain underlies processive substrate translocation [6].
Catalytic Residues
In S. pneumoniae, mutagenesis and kinetic analysis of the HysA mutant suggested three residues were involved in catalysis Asn249, His399, Tyr408 and that two residues, Arg243 and Asn580 were responsible for substrate binding and translocation [1]. However, there is some question over what the identity is over the general base (please see Elmabrouk or Zheng et al. for discussions [4, 5]. With respect to chondroitin AC and chondroitin ABC substrate specificity; structural comparisons of the Flavobacterium heparinum chondroitin AC lyase with the Proteus vulgaris chondroitin ABC lyase suggests that an Asp444 for an Asn differential in the Proteus vulgaris active centre provides the mechanism for enzymatic distinguishing between the two epimers [7, 8].
Three dimensional structure
The enzymatic PL8 domain is comprised by an N-terminal α-helical and C-terminal β-sheet domain, which constitute incomplete α5/α5-barrel and anti-parallel β-sheet structures, respectively. A deep cleft is located in the N-terminal α-helical domain facing the interface between the two domains that accommodates the substrate.
Three dimensional structures by activity:
Chondroitin AC lyase (EC 4.2.2.5) - F. heparinum [7](PDB 1CB8).
Chondroitin ABC lyase (EC 4.2.2.20) – P. vulgaris [8](PDB 1HN0).
Hyaluronan lyase (EC 4.2.2.1) – S. pneumoniae R6 [2](PDB 1OJM).
Xanthan lyase (EC 4.2.2.12) – Bacillus sp GL1 [9](PDB 1J0M).
Family Firsts
First catalytic activity
The hydrolysis of hyaluronan was attributed to a pneumococcal hyaluronidase from Pneumococcus type II strain D39R [10].
