We analyzed the nonlinear current-voltage relationships of the early conducting state of channelrhodopsin-2 expressed in Xenopus oocytes and human embryonic kidney cells with respect to changes of the electrochemical gradients of H(+), Na(+)/K(+), and Ca(2+)/Mg(2+). Several models were tested for wild-type ChR2 and mutations at positions E90, E123, H134, and T159. Voltage-gating was excluded as cause for the nonlinearity. However, a general enzyme kinetic model with one predominant binding site yielded good fits throughout. The empty site with an apparent charge number of about -0.3 and strong external cation binding causes some inward rectification of the uniport function. Additional inward rectification is due to asymmetric competition from outside between the transported ion species. Significant improvement of the fits was achieved by introducing an elastic voltage-divider formed by the voltage-sensitive barriers.
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