PTS_IIB_lactose: subunit IIB of enzyme II (EII) of the lactose-specific phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) found in Firmicutes as well as Actinobacteria. In this system, EII is a lactose-specific permease with two cytoplasmic domains (IIA and IIB) and a transmembrane channel IIC domain. The IIC and IIB domains are expressed as a single protein from the lac operon. 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 lactose, chitobiose/lichenan, ascorbate, galactitol, 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,(not included in this IIB subgroup) 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.