C. Wiesmann et al., CRYSTAL-STRUCTURES AND MECHANISM OF 6-PHOSPHO-BETA-GALACTOSIDASE FROMLACTOCOCCUS-LACTIS, Journal of Molecular Biology, 269(5), 1997, pp. 851-860
The initial structural model of 6-phospho-beta-galactosidase from Lact
ococcus lactis was refined to an R-factor of 16.4% (R-free = 23.6%) to
2.3 Angstrom resolution (1 Angstrom = 0.1 nm), and the structures of
three other crystal forms were solved by molecular replacement. The fo
ur structural models are essentially identical. The catalytic center o
f the enzyme is approximately at the mass center of the molecule and c
an only be reached through a 20 Angstrom long channel, which is observ
ed with an ''open'' or ''closed'' entrance. The closed entrance is pro
bably too small for the educt lactose-6-phosphate to enter, but large
enough for the first product glucose to leave. Among the presented str
uctures is a complex between an almost inactive mutant and the second
product galactose-6-phosphate, which is exclusively bound at side-chai
ns. A superposition (onto the native enzyme) of galactose-6-phosphate
as bound to the mutant suggests the geometry of a postulated covalent
intermediate. The binding mode of the educt was modeled, starting from
the bound galactose-6-phosphate. A tightly fixed tryptophan is used a
s a chopping-board for splitting the disaccharide, and several other a
romatic residues in the active center cavity are likely to participate
in substrate transport/binding. (C) 1997 Academic Press Limited.