Analysis of the structural specificity of the lactose permease toward sugars

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.