XYLOSE TRANSPORT BY THE ANAEROBIC THERMOPHILE THERMOANAEROBACTER-ETHANOLICUS AND THE CHARACTERIZATION OF A D-XYLOSE-BINDING PROTEIN

Thermoanaerobacter ethanolicus is a xylose-utilizing thermophilic anae robe that produces considerable amounts of ethanol. A protein in xylos e-growing cells was solubilized from cell membranes by extraction with octyl-beta-glucoside. Internal peptide sequencing revealed that the p rotein was the product of a gene, xylF, encoding a putative D-xylose-b inding protein. Metabolic labeling with C-14 palmitic acid suggested t hat this is a lipoprotein that is anchored to the cell membrane via a cysteine residue. Binding was highly specific for xylose as evident by the lack of competition by sugars with structures similar to xylose. The apparent K-d of the protein for xylose was approximately 1.5 mu M, and this value was very similar to the affinity constant determined f or xylose transport by whole cells at low substrate concentrations. Up take experiments with cells also suggested the presence of a separate low-affinity system. Binding activity varied less than 20% over a pH r ange of 4-8, and the level of activity was virtually unaffected when t emperature was varied between 40 degrees C and 80 degrees C. This is t he first biochemical characterization of a D-xylose-binding protein fr om a thermophilic organism.