Molecular modelling investigation of wild-type and the R528H mutated segment IIS4 of human L-type voltage-gated calcium channels

A molecular modelling study was performed in order to investigate the patho logically modified properties of L-type voltage-gated calcium channels caus ed by the arginine-to-histidine mutation at position 528 (R528H) in segment IIS4, For an appropriate consideration of the ionization state the finite difference Poisson-Boltzmann method was applied to compute the apparent pK( a) values of all titratable residues using standard conditions and an expli cit lipophilic environment, respectively. Restrained molecular dynamics sim ulations were carried out for the alpha-helical transmembrane segments of t he wild-type and the R528H mutant to explore their conformational behaviour , While both structures showed almost the same side-chain flexibility aroun d the conserved residues, only the mutant partially formed a hydrogen bond from H528 to R531 during dynamics simulations. This local interaction not o nly causes a lower mobility of the directly involved residues but also lead s to a global distortion of all positively charged amino acids of the mutan t. Mostly affected is the side-chain of R534 that is shifted about 61 degre es closer to R531 and about 5 Angstrom in the direction of the cytoplasm. S ubsequent examination of the molecular characteristics of this putative vol tage sensor of the channel revealed considerable variations with regard to hydrogen bonding and electronic properties. Most obvious are the dramatic l oss of a strong positive molecular electrostatic potential and the reduced hydrogen donor activities around position 528 of the mutant. How these resu lts may be interpreted in relation to an enhanced inactivation rate is disc ussed, considering earlier findings at homologous voltage-gated potassium a nd sodium channels.