PTS_IIB_ascorbate: subunit IIB of enzyme II (EII) of the L-ascorbate-specific phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). In this system, EII is an L-ascorbate-specific permease with two cytoplasmic subunits (IIA and IIB) and a transmembrane channel IIC subunit. Subunits IIA, IIB, and IIC are encoded by the sgaA, sgaB, and sgaT genes of the E. coli sgaTBA operon. In some bacteria, the IIB (SgaB) domain is fused C-terminal to the IIA (SgaT) domain. 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 ascorbate, chitobiose/lichenan, lactose, 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.