The Lactose Permease in Escherichia Coli: A Study of Its Mechanism

摘要

The unidirectional transport of galactosides into and out of the cells of (Escherichia coli) were measured under varying conditions. The rate of hydrolysis of o-nitrophenyl galactoside (oNPG) in vivo and the initial rate of accumulation of methyl thiogalactoside (TMG) were used as measures of the rate of inward transport, and the rate of loss of accumulated radioactive TMG was used as a measure of the rate of outward transport. Studies of oNPG hydrolysis at several temperatures revealed that entry via the lactose permease requires more energy than non-catalyzed entry. Evidence is presented that the inhibition of hydrolysis by alpha MG is caused by direct interaction between the glucoside and the lactose permease protein. Because of the similarity between inhibition of oNPG hydrolysis and enhancement of TMG exit by NaN3 and NaCl. A model for the mechanism of the lactose permease is proposed and discussed. It is proposed that the permease binds its substrate on one side of the membrane, and releases it on the other. The reverse reaction is prevented by the phosphorylation mechanism. (Author)