Mechanism of hyaluronan degradation by Streptococcus pneumoniae hyaluronate lyase. Structures of complexes with the substrate.
Hyaluronate lyase enzymes degrade hyaluronan, the main polysaccharide component of the host connective tissues, predominantly into unsaturated disaccharide units, thereby destroying the normal connective tissue structure and exposing the tissue cells to various endo- and exogenous factors, including bacterial toxins. The crystal structures of Streptococcus pneumoniae hyaluronate lyase with tetra- and hexasaccharide hyaluronan substrates bound in the active site were determined at 1.52- and 2.0-A resolution, respectively. Hexasaccharide is the longest substrate segment that binds entirely within the active site of these enzymes. The enzyme residues responsible for substrate binding, positioning, catalysis, and product release were thereby identified and their specific roles characterized. The involvement of three residues in catalysis, Asn(349), His(399), and Tyr(408), is confirmed, and the details of proton acceptance and donation within the catalytic machinery are described. The mechanism of processivity of the enzyme is analyzed. The flexibility (allosteric) behavior of the enzyme may be understood in terms of the results of flexibility analysis of this protein, which identified two modes of motion that are also proposed to be involved in the hyaluronan degradation process. The first motion describes an opening and closing of the catalytic cleft located between the alpha- and beta-domains. The second motion demonstrates the mobility of a binding cleft, which may facilitate the binding of the negatively charged hyaluronan to the enzyme.
Jedrzejas
Mark J
MJ
Children's Hospital Oakland Research Institute, Oakland, California 94609, USA. mjedrzejas@chori.org
Mello
Luciane V
LV
de Groot
Bert L
BL
Li
Songlin
S
D000595:Amino Acid Sequence; D001665:Binding Sites; D018360:Crystallography, X-Ray; D006820:Hyaluronic Acid / Q000378:metabolism*; D008958:Models, Molecular; D011133:Polysaccharide-Lyases / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D021122:Protein Subunits; D055672:Static Electricity; D013296:Streptococcus pneumoniae / Q000201:enzymology*
D021122:Protein Subunits; D006820:Hyaluronic Acid; D011133:Polysaccharide-Lyases; C100002:hyaluronate lyase