Structural basis of hyaluronan degradation by Streptococcus pneumoniae hyaluronate lyase

Streptococcus pneumoniae hyaluronate lyase (spnHL) is a pathogenic bacteria l spreading factor and cleaves hyaluronan, an important constituent of the extracellular matrix of connective tissues, through an enzymatic beta-elimi nation process, different from the hyaluronan degradation by hydrolases in animals. The mechanism of hyaluronan binding and degradation was proposed b ased on the 1.56 Angstrom resolution crystal structure, substrate modeling and mutagenesis studies on spnHL, Five mutants, R243V, N349A, H399A, Y408F and N580G, were constructed and their activities confirmed our mechanism hy pothesis. The important roles of Tyr408, Asn349 and His399 in enzyme cataly sis were proposed, explained and confirmed by mutant studies. The remaining weak enzymatic activity of the H399A mutant, the role of the free carboxyl ate group on the glucuronate residue, the enzymatic behavior on chondroitin and chondroitin sulfate, and the small activity increase in the N580G muta nt were explained based on this mechanism. A possible function of the C-ter minal beta-sheet domain is to modulate enzyme activity through binding to c alcium ions.