The double mutant, Val-177/Asn-322, was investigated with regard to its ability to transport H+ and galactosides. In downhill lactose transport assays, the wild-type strain had a K(m) value for lactose uptake of 0.9 mM and a V(max) of 0.65 μmol lactose/min·mg protein while the mutant had a significantly higher K(m) value of 1.9 mM but a similar V(max) of 0.49 μmol/min·mg protein. In spite of its moderate ability to transport lactose downhill, the Val-177/Asn-322 mutant exhibited the striking property of being completely defective in the uphill accumulation of lactose or methyl-β-D-thiogalactopyranoside. Direct measurements of H+ transport, however, showed that the mutant's defect in active acumulation is not due to a defect in the ability to transport H+ with lactose or methyl-β-D-thiogalactopyranoside. the Val-177/Asn-322 mutant strain had a H+:lactose stoichiometry of 0.84 which was similar to that measured in the wild-type strain (0.68). These results are discussed with regard to the role His-322 plays in H+ transport, active accumulation of sugars, and sugar recognition.
|Original language||English (US)|
|Number of pages||6|
|Journal||Journal of Biological Chemistry|
|State||Published - 1990|