Effects of Carbohydrates on the Ion Conductance of the Hemocyanin Channel

The effects of glucose and sucrose on the ionic conductance properties of the channel formed by Megathura crenulata hemocyanin in planar lipid bilayers have been studied using membranes of different compositions. It was found that glucose at high concentrations strongly affects the time constants of the current relaxations observed in membranes containing many channels after a step in the voltage clamp from ground to a positive value. At much lower concentrations both sucrose and glucose strengthened the binding of Ba2+ to the protein, what in turn has the effect to shift the conductance voltage curve of the pore towards negative potentials. The possible mechanism underlying these effects and the analogies with other studies on the interaction of sugars and alcohols with proteins have been discussed.

A three-barrier model for the hemocyanin channel.

Keyhole limpet hemocyanin forms ion-conducting channels in planar lipid bilayer membranes. Ionic current through the open hemocyanin channel presents the following characteristics: (a) it is carried mainly by cations; (b) it is a nonlinear function of membrane potential; (c) channel conductance is a saturating function of ion activity; (d) it shows ionic competition. A model for the open hemocyanin channel is developed from absolute reaction rate theory. The model calls for three energy barriers in the channel. Two energy barriers represent the entrance and exit of the ion into and out of the channel. The third barrier separates two energy minima that represent two binding sites. Furthermore, only one ion is allowed inside the channel at a given time. This model is able to recreate all the hemocyanin characteristics found experimentally in negatively charged and neutral membranes.