Kinetics and consequences of binding of nona- and dodecapeptides to the oligopeptide binding protein (OppA) of Lactococcus lactis

The oligopeptide transport system (Opp) of Lactococcus lactis belongs to th e class of binding protein-dependent ABC-transporters. This system has the unique capacity to mediate the uptake of peptides from 4 up to at least 18 residues. Kinetic analysis of peptide binding to the binding protein, OppA* , revealed a relationship between the peptide dissociation constants and th e length of the Ligand. The dissociation constants varied from submicromola r for dodecapeptides to millimolar for pentapeptides. This implies that the residues 6-12 of the peptide contribute to the binding affinity, and, in c ontrast to the current views on peptide binding by homologous proteins, the se residues must interact with OppA*. Analysis of pre-steady-state kinetics of binding showed that the observed differences in the E-d(L)-values resul t primarily from variations in the dissociation rate constants. These resul ts are discussed in relation to the affinity constant for transport of thes e substrates. Overall, the data suggest that the slow dissociation rate con stants for the larger peptides are rate determining in the translocation of peptides across the membrane.