Crystal structure and enzymatic properties of a bacterial family 19 chitinase reveal differences from plant enzymes.
We describe the cloning, overexpression, purification, characterization and crystal structure of chitinase G, a single-domain family 19 chitinase from the Gram-positive bacterium Streptomyces coelicolor A3(2). Although chitinase G was not capable of releasing 4-methylumbelliferyl from artificial chitooligosaccharide substrates, it was capable of degrading longer chitooligosaccharides at rates similar to those observed for other chitinases. The enzyme was also capable of degrading a colored colloidal chitin substrate (carboxymethyl-chitin-remazol-brilliant violet) and a small, presumably amorphous, subfraction of alpha-chitin and beta-chitin, but was not capable of degrading crystalline chitin completely. The crystal structures of chitinase G and a related Streptomyces chitinase, chitinase C [Kezuka Y, Ohishi M, Itoh Y, Watanabe J, Mitsutomi M, Watanabe T & Nonaka T (2006) J Mol Biol358, 472-484], showed that these bacterial family 19 chitinases lack several loops that extend the substrate-binding grooves in family 19 chitinases from plants. In accordance with these structural features, detailed analysis of the degradation of chitooligosaccharides by chitinase G showed that the enzyme has only four subsites (- 2 to + 2), as opposed to six (- 3 to + 3) for plant enzymes. The most prominent structural difference leading to reduced size of the substrate-binding groove is the deletion of a 13-residue loop between the two putatively catalytic glutamates. The importance of these two residues for catalysis was confirmed by a site-directed mutagenesis study.
Hoell
Ingunn A
IA
Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, 1432 As, Norway.
Dalhus
Bjørn
B
Heggset
Ellinor B
EB
Aspmo
Stein I
SI
Eijsink
Vincent G H
VG
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D020134:Catalytic Domain; D002686:Chitin / Q000378:metabolism; D002688:Chitinases / Q000737:chemistry* / Q000235:genetics; D003460:Crystallization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D009844:Oligosaccharides / Q000737:chemistry; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D048372:Streptomyces coelicolor / Q000201:enzymology*; D013379:Substrate Specificity
D001426:Bacterial Proteins; D009844:Oligosaccharides; D010940:Plant Proteins; D011994:Recombinant Proteins; D002686:Chitin; D002688:Chitinases