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Systemic mutation of histidine residues at the SDHC and SDHD subunits of Succinate:ubiquinone oxidoreductase or the cytochrome b556 complex II was undertaken to determine the residues that function as axial ligands for the heme moiety. Results reveal that the likely axial ligands are the histidine residues at position 84 of SDHC and position 71 of SDHD. Mutation of the histidine residue at position 30 of SDHD prevented the assembly of the complex.

Previous studies have reported that a tryptophan substitution on the alphaC418 position at the lipid-exposed transmembrane segment M4 of Torpedo californica acetylcholine receptor (AChR) resulted in a 23-fold decrease in the closing rate constant of the (AChR). Herein, measurements of the open and closed rate constant in three multiple mutants for tryptophan revealed that tryptophan substitutions produced significant effects on the channel gating kinetics. Data indicated that the number of substitutions was directly related to the open probability of the channel. Kinetic equations that describe the channel gating mechanisms are presented.