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 Ki value of 7.4 mM. This value was severalfold higher than the observed Km 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 greater than OH-4 greater than OH-6 greater than OH-2 greater than OH-1. In general, galactosides with alpha-linkages exhibited significantly higher affinities compared with their beta-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 greater than methyl group greater than no aglycone much greater than disaccharide greater than 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 beta-nitrophenyl-galactosides had lower affinities compared with lactose (a beta-galactoside), whereas the alpha-nitrophenylgalactosides generally had higher affinities compared with melibiose (an alpha-galactoside). In addition, no consistent preference was seen when considering the location of the nitro group on the benzene ring. From this work, a model is presented which depicts the binding of galactosides to the lactose permease.