Towards an understanding of the structural basis of 'forbidden' transport pathways in the Escherichia coli lactose carrier: mutations probing the energy barriers to uncoupled transport.

Recent progress in the analysis of mutants of the Escherichia coli lactose carrier function is reviewed, with special emphasis on the structural basis for energy barriers which prevent 'forbidden' conformational changes. Mutations which break down the barriers to forbidden isomerizations involving the binary carrier:sugar (CS) and carrier:proton (CH) complexes have been obtained in several laboratories. These mutants allow uncoupled transport of H+ or galactoside in the lactose carrier which normally couples cation and sugar movement in a 1:1 stoichiometry. These uncoupled mutants appear to be associated with changes in both sugar and cation recognition, suggesting that the physical interactions forming the basis for co-substrate recognition and uncoupling are not independently variable. By postulating that translocation involves transformation of the stable intermediate of the co-transport cycle to unstable transition state conformations of the carrier, it is possible to consider the consequences of mutagenesis in terms of transition state theory. Consistent with several experimental observations, the analysis predicts in each mutant the occurrence of more than one abnormality in the transport cycle (such as changes in sugar recognition, cation recognition or the coupling reaction). We have called the general phenomenon a 'mutational double-effect' because any mutation which alters the Gibbs free energy change of one reaction in the transport cycle must affect the free energy change of at least one other reaction in this cycle.