Three-dimensional structure of galactose oxidase: an enzyme with a built-in secondary cofactor.
Galactose oxidase is a copper-containing enzyme, which catalyses stereospecific oxidation of primary alcohols. The three-dimensional structure of the enzyme has been determined in this study by X-ray crystallography at high resolution. The molecule is almost entirely composed of beta-structures and consists of three domains. The arrangement of 28 beta-strands in the second domain is of particular interest, having seven four-stranded antiparallel beta-sheets with pseudo-sevenfold symmetry. The copper site has square-pyramidal coordination with two histidines, one tyrosine and one exogenous ligand at the equatorial sites and another tyrosine at the axial site. The most intriguing structural feature is a covalent bond between C epsilon 1 of Tyr-272, which is one of the equatorial ligands, and S gamma of Cys-228. This unexpected thioether bond, and Trp-290 stacked above it, strongly supports the presence of a tyrosine free radical in the enzyme as a 'built-in' secondary cofactor. Calculation of the molecular surface shows a small pocket at the copper site and suggests a substrate-binding model, which can explain the substrate specificity. A model for the catalytic mechanism, involving a tyrosine free radical and basic tryptophan, is also proposed.
Ito
N
N
Department of Biochemistry and Molecular Biology University of Leeds, UK.
Phillips
S E
SE
Stevens
C
C
Ogel
Z B
ZB
McPherson
M J
MJ
Keen
J N
JN
Yadav
K D
KD
Knowles
P F
PF
D000595:Amino Acid Sequence; D005692:Galactose Oxidase / Q000737:chemistry* / Q000378:metabolism*; D008956:Models, Chemical; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D014961:X-Ray Diffraction
D005692:Galactose Oxidase