LacY, a protein responsible for the transport of lactose across the membrane in Escherichia coli.

The substrate-binding site is in the N-terminal domain, with residues like Trp151 and Arg144 playing important roles in sugar binding.

Thiol cross-linking underestimates distances, especially on the cytoplasmic side, due to the molecule's fluctuations between inward- and outward-facing conformations.

Glutamine residues in positions 241 and 359 guide the sugar towards Phe 27, which brings the substrate to Gln 126 where it is firmly fixed.

Carefully positioned amino acids help the substrate to overcome the energy barrier of around 20 kJ/mol[5] 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.