The Role of Proton Transport in Gating Current in a Voltage Gated Ion Channel, as Shown by Quantum Calculations

Alisher M Kariev; Michael E Green

A second line of evidence that has been interpreted in terms of the standard models comes from fluorescence quenching. FRET (fluorescence resonance energy transfer) or LRET (lanthanide resonance energy transfer-often Tb 3+ is used as the lanthanide) measurements give an indication of the distance between residues, which appears to change with applied voltage (27) (28) (29) (30) (31) (32) (33) (34) . Again, there are difficulties with this type of measurement. These have been discussed in the literature, and major efforts to overcome these difficulties have been made. However, side chain rotations, loss of salt bridges, and, for that matter, proton transfers, would affect the results, so that the error limits on these measurements must be considered to be comparable to the distances measured. As these errors are well known, we . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/371914 doi: bioRxiv preprint will not discuss them. LRET is somewhat better than standard FRET, but does not overcome the local structural effects of mutation. Undoubtedly there is value to these measurements, but they do not necessarily lead to high resolution results. The measurements are made on a mutant channel, which may be a few Angstroms different in spacing from the wild type, a problem for a measurement of distances in which 3 Ã… is a major difference. This said there is evidence from these experiments that does not necessarily fit with the standard models. Posson et al found a vertical movement of only 2 Ã… for S4 (35) ; they proposed a model in which the electric field was refocused in such a way as to produce a gating charge of 13e, in accord with the measured value for the Shaker channel. Part of the reason this could be done was that it was shown, by use of a solvatochromic dye, that the field was very large at the location of a single arginine residue (36) . Related data showing that the motion of S4 was very limited was added by a series of LRET measurements by Chanda et al (32) . Slightly earlier work by Cha et al also led to the same conclusion (37) . There are additional problems with assigning gating current, as its exact value is something of a problem. Ishida et al measured the gating current for a channel that was supposed to be very similar, K v 1.2, and found a gating charge of only 10e (38) ; based on mutation experiments, they concluded that the gating current was produced by a different mechanism from that of Shaker. There are multiple states through which the channel passes on its way to opening (21) ; essentially all models require multiple states, and this is consistent with the measured kinetics, although not all results lead to exactly the same number of states In addition to experiments, molecular dynamics (MD) calculations have been used as further evidence for the standard models. MD calculations use classical force fields; these force fields include electrostatic terms, as well as kinetic energy, and empirical force terms that should, in principle, account for all other energy terms (van der Waals, torsion, etc.) However, quantum calculations seem to be required because the exchange and correlation energy terms, which have no classical analogues, have differences between similar positions of protons or side chains of the order of 10k B T, so cannot be ignored. Proton shifts are not possible with MD