Lactose permease

Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as lactose in the same direction into the cell.

LacY proton/sugar symporter
Crystal Structure of Lactose Permease in Complex with an Affinity Inactivator. PDB 2y5y[1]
Identifiers
Symbol	LacY_symp
Pfam	PF01306
Pfam clan	CL0015
InterPro	IPR022814
PROSITE	PDOC00698
TCDB	2.A.1
OPM superfamily	15
OPM protein	2cfq
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

The protein has twelve transmembrane alpha-helices and its molecular weight is 45,000 Daltons.[2] It exhibits an internal two-fold symmetry, relating the N-terminal six helices onto the C-terminal helices. It is encoded by the lacY gene in the lac operon. The LacY gene is a component of the lac operon that encodes lactose permease, a protein responsible for breaking down lactose into glucose and galactose, alongside transacetylase and beta galactosidase. The absence of lactose permease leads to the inability of lactose to enter the cell for further metabolic processes. Therefore, lactose permease plays a crucial role in the utilization of lactose as a source of energy.

The sugar lies in the hydrophilic core of the protein which is accessible from the periplasm.[2] On binding, a large conformational change takes place which makes the sugar binding site accessible from the cytoplasm.

Mechanism: hydronium ions from the outside of the cell binds to a carboxyl group on the enzyme that allows it to undergo a conformational change. This form of lactose permease can bind lactose from outside the cell. The enzyme then everts and lactose is transported inward.

The X-ray crystal structure was first solved in 2003 by J. Abramson et al.[3]