Facilitated substrate transport through membrane proteins.

Substrate transport through the membrane protein maltoporin is facilitated by an affinity site in the channel. The analysis of the ion current fluctuations induced by penetration of the sugar into the channel yields the kinetic constants. Modification of the affinity site by replacing the aromatic residues suggests that nature has optimized the channel protein for maximum affinity at the extracellular side, as well as for an increased off-rate to eject a sugar trapped in the pore towards the periplasmic side.