#lactose permease

Membrane Transportation

In secondary active transport, lactose permease in E. coli utilizes:

Secondary active lactose permease LacY does not itself hydrolyze ATP nor perform direct phosphorylation of sugar substrate like phosphotransferase systems that import glucose as glucose-6-phosphate. Instead energy source is electrochemical proton gradient across E. coli inner membrane established by respiratory chain proton pumping. Lactose entry is strictly coupled to downhill H+ flow: one proton symported per lactose disaccharide. Experimental evidence shows that abolishing proton motive force with uncouplers carbonyl cyanide m-chlorophenyl hydrazone, nigericin, or by inhibiting respiration

Ref: Kaback et al., J Gen Physiol 2008, Proton coupling LacY; Poolman et al., Mol Microbiol 2004.

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Membrane Transportation

Lactose permease functions as a:

Lactose permease LacY of Escherichia coli, characterized extensively by Kaback, is a twelve-transmembrane helix member of major facilitator superfamily and textbook exemplar of secondary active symport. LacY itself does not possess ATPase activity nor nucleoside-binding motifs; energy comes indirectly from electrochemical proton gradient across inner membrane maintained by respiratory chain H+ extrusion. In outward-open state protonation of Glu325 increases affinity for lactose at the central cavity; coupled binding induces rocker-switch movement of N and C terminal six-helix bundles to inward

Ref: Kaback et al., Nat Rev Mol Cell Biol 2001, LacY mechanism; Alberts, Chapter 11 carriers.

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Lac Operon

Function of lactose permease is

Lactose permease functions through classical alternating-access transport mechanism: outward-open conformation binds extracellular lactose plus proton on specific side chains, undergoes conformational change to occluded state, then rearranges to inward-open releasing substrates into cytoplasm. Proton coupling utilizes electrochemical gradient across inner membrane to drive accumulation to concentrations hundredfold higher than environment even when external lactose remains micromolar. Influx supplies beta-galactosidase substrate producing glucose, galactose, and allolactose inducer driving fur

Ref: Wikipedia Lactose permease – LacY proton symport transports lactose using proton gradient, increasing cell permeability.

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Lac Operon

lacY gene product is

lacY gene product is lactose permease LacY, 417-residue member of major facilitator superfamily transporter folded into twelve transmembrane alpha helices forming central hydrophilic cavity accessible alternately outward and inward during cycle. It operates as galactoside-proton symporter driven entirely by proton motive force, accumulating lactose against concentration gradient for metabolic use. Protein functionality is distinct from soluble beta-galactosidase encoded by lacZ and transacetylase LacA. By concentrating intracellular beta-galactosides, LacY enables efficient generation of allol

Ref: ScienceDirect Lactose Permease overview – LacY transmembrane symporter concentrates β-galactosides for hydrolysis and induction.

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