4.A.6 The PTS Mannose-Fructose-Sorbose (Man) Family
The Man (PTS splinter group) family is unique in several respects among PTS porter families. (1) It is the only PTS family in which members possess a IID protein; (2) It is the only PTS family in which the IIB constituent is phosphorylated on a histidyl rather than a cysteyl residue. (3) Its porter members usually exhibit broad specificity for a range of sugars, rather than being specific for just one or a few sugars. The mannose porter of E. coli, for example, can transport and phosphorylate glucose, mannose, fructose, glucosamine, N-acetylglucosamine, and N-acteylmannosamine (Plumbridge and Vimr, 1999).
The structure of the E. coli IIAMan domain has been shown to exhibit an α/β doubly wound superfold (Hu et al. 2008). The IIB domain also exhibits an α/β doubly wound superfold, but it is very dissimilar from that of the IIA domain (Orriss et al. 2003). Instead, it has the same topology as phosphoglyceromutase. Since both proteins (IIBMan and PGM) catalyze phosphoryl transfer with a phosphohistidine intermediate, both proteins show a similar distribution of active site residues, and both exhibit similar structures, they are probably homologous.
Solution structures of complexes between the isolated IIAMan and IIBMan domains of the E. coli mannose EII complex have been solved by NMR (Hu et al. 2008). The complex of wild-type IIAMan and IIBMan is a mixture of two species comprising a productive, phosphoryl transfer competent complex and a non-productive complex with the two active site histidines, His-10 of IIAMan and His-175 of IIBMan, separated by approximately 25Å. Mutation of His-10 to a glutamate to mimic phosphorylation, results in the formation of a single productive complex. The apparent equilibrium dissociation constants for the binding of both wild-type and H10E IIAMan to IIBMan are approximately the same (KD ~0.5 mM). The productive complex can readily accommodate a transition state involving a pentacoordinate phosphoryl group with trigonal bipyramidal geometry bonded to the N-ε2 atom of His-10 and the N-δ1 atom of His-175 with negligible (<0.2 Å) local backbone conformational changes in the immediate vicinity of the active site. The non-productive complex is related to the productive one by an approximately 90 degree rotation and an approximately 37 Å translation of IIBMan relative to IIAMan, leaving the active site His-175 fully exposed to solvent in the non-productive complex (Hu et al. 2008).
IICMan of E. coli has six established transmembrane α-helical spanners while IIDMan has only one with most of the polypeptide chain localized to the periplasm. These two proteins together are required for transport although IICMan is presumed to comprise all or most of the sugar transporting channel.
The generalized reaction catalyzed by members of the Man Family is:
Sugar (out) + PEP (in) → Sugar-P (in) + pyruvate (in).