Static and dynamic disorder in protein folding: experiments with single maltoporin channels

The reversible binding of sugar to a single maltoporin channel allows us to study time and ensemble variations in the channel functional properties and interpret them using the language of static and dynamic disorder in protein folding. The channel is a trimer that is characterized by two primary parameters: the rate of sugar binding and the ion conductance. Time-resolved binding of maltohexasose molecules shows that whereas dynamic disorder -- the fluctuations in binding rate or in ionic conductance of a single trimer channel with time -- is relatively small, static disorder -- the heterogeneity of reaction rates or conductances among different trimers -- is highly pronounced. This heterogeneity suggests variations in maltoporin folding. The disorder in conductance shows no measurable correlation with the disorder in binding strength; variations in protein folding that are responsible for variations in protein folding that are responsible for variations in ionic conductance do not seem to affect sugar binding. We find 'cooperativity' in static disroder: conductances of monomers in the same trimer are closely similar compared to the range of possible conductances seen over an ensemble of trimers.