PTS_IIB_galactitol: subunit IIB of enzyme II (EII) of the galactitol-specific phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). In this system, EII is a galactitol-specific permease with two cytoplasmic domains (IIA and IIB) and a transmembrane channel IIC domain that are expressed on three distinct polypeptide chains, in contrast to other PTS sugar transporters. The three genes encoding these subunits (gatA, gatB, and gatC) comprise the gatCBA operon. Galactitol PTS permease takes up exogenous galactitol, releasing the phosphate ester into the cytoplasm in preparation for oxidation and further metabolism via a modified glycolytic pathway called the tagatose-6-phosphate glycolytic pathway. The IIB domain fold includes a central four-stranded parallel open twisted beta-sheet flanked by alpha-helices on both sides. The seven major PTS systems with this IIB fold include galactitol, chitobiose/lichenan, ascorbate, lactose, mannitol, fructose, and a sensory system with similarity to the bacterial bgl system.
Comment:most of the active site residues are contained within a P-loop which forms part of a binding pocket for the phosphoryl group
Comment:for the E coli chitobiose transporter IIB subunit, in addition to the P-loop residues, two conserved residues are likely to be important in catalysis: a tyrosine whose side-chain points towards the phosphorylation site and a Gln residue whose side-chain lies close to the sulfur atom of the catalytic cysteine.