THE FUNCTIONAL IMPORTANCE OF STRUCTURAL DIFFERENCES BETWEEN THE MANNITOL-SPECIFIC IIA(MANNITOL) AND THE REGULATORY IIA(NITROGEN)

The three-dimensional structures of the IIA domain of the mannitol-spe cific phosphoenol-pyruvate dependent phosphotransferase system (PTS) o f Escherichia coli and the regulatory IIA(ntr) enzyme have been compar ed. The enzymes are 20% identical in sequence and contain the sequence motif specific for IIA proteins belonging to the mannitol-fructose fa mily of the PTS. The fold of the enzymes is nearly identical, which co nfirms their evolution from a common ancestor. Moreover, the phosphory lation site of IIA(mtl) (His65) and one of the observed conformations of the active site Arg49 are extremely similar to their equivalents (H is73 and Arg57) in IIA(ntr). In contrast, His120, the equivalent of th e second active site His111 of IIA(mtl), is not located in the active site of IIA(ntr) but points into the solvent on the other side of the molecule. The different position of His120 makes it unlikely that this residue assists in phosphoryl transfer in the regulatory IIA(ntr)s. A s His120 is conserved in all IIA(ntr) enzymes, it could have an essent ial role in the recognition of the target protein of IIA(ntr).