Maltose Transport in Escherichia coli K12: A Comparison of Transport Kinetics in Wild‐Type and λ‐Resistant Mutants with the Dissociation Constants of the Maltose‐Binding Protein as Measured by Fluorescence Quenching

The kinetic parameters for the maltose transport system in Escherichia coli K12 were determined with maltose and maltotriose as substrates. The system exhibits an apparent K_m of 1 μM for maltose and 2 μM for maltotriose. The V of entry was determined as 2.0 and 1.1 nmol substrate/min per 10⁸ cells. Mutations in lamB, the structural gene for the receptor protein of phage λ, increased the K_m for maltose transport by a factor of 100–500 without influencing the maximal rate of transport. Maltotriose is no longer transported in these lamB mutants. The maltose‐binding protein, an essential component of the maltose transport system, was found to exhibit substrate‐dependent fluorescence quenching. This phenomenon was used to determine dissociation constants and to estimate the rate of ligand dissociation. A K_d of 1 μM for maltose and of 0.16 μM for maltotriose was found. From the comparison of the kinetic parameters of transport of maltose and maltotriose in wild‐type and λ‐resistant mutants with the binding constants for both sugars to purified maltose‐binding protein, we conclude that the λ receptor facilitates the diffusion of maltose and maltodextrins through the outer membrane.