Lactose permease is a membrane protein which is a member of the

major facilitator superfamily The major facilitator superfamily (MFS) is a superfamily of membrane transport proteins that facilitate movement of small solutes across cell membranes in response to chemiosmotic gradients. Function The major facilitator superfamily (MFS) are ...

. Lactose permease can be classified as a

symporter A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...

, which uses the

proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and th ...

towards the cell to transport β-galactosides such as

lactose Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from ' (gen. '), the Latin word for milk, plus the suffix '' - ...

in the same direction into the cell. The protein has twelve transmembrane alpha-helices and its molecular weight is 45,000 Daltons. 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 sugar lies in the hydrophilic core of the protein which is accessible from the periplasm. On binding, a large conformational change takes place which makes the sugar binding site accessible from the cytoplasm. Mechanism:

hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is d ...

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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

was first solved in 2003 by J. Abramson ''et al.''

References

{{Reflist Transport proteins Transmembrane proteins