The sugar specificity properties of the lactose permease were investigated. Free galactose was shown to competitively inhibit the lactose permease yielding a K(i) value of 7.4 mM. This value was severalfold higher than the observed K(m) for lactose (1.3 mM). A variety of other monosaccharides also showed significant inhibition of lactose transport. With regard to -OH groups along the galactose ring it appears that the relative importance is OH-3 > OH-4 > OH-6 > OH-2 > OH-1. In general, galactosides with α-linkages exhibited significantly higher affinities compared with their β-linked counterparts. An optimal size for the aglycone portion of the galactoside was reached with aglycones containing hexose residues or a benzene ring. The preferred size of the aglycone appears to be hexose, benzene ring > methyl group > no aglycone ≥ disaccharide > trisaccharide. However, neither the specific structure of the aglycone nor its relative hydrophobicity appeared to be important factors in permease recognition. For example, the hydrophobic β-nitrophenyl-galactosides had lower affinities compared with lactose (a β-galactoside), whereas the α-nitrophenylgalactosides generally had higher affinities compared with melibiose (an α-galactoside). In addition, no consistent preference was seen when considering the location of the nitro group of the benzene ring. From this work, a model is presented which depicts the binding of galactoside to the lactose permease.
|Number of pages
|Journal of Biological Chemistry
|Published - 1989