STRUCTURAL MODEL OF A SYNTHETIC CA2+ CHANNEL WITH BOUND CA2+ IONS ANDDIHYDROPYRIDINE LIGAND

Grove et al. have demonstrated L-type Ca2+ channel activity of a synth etic channel peptide (SCP) composed of four helices (sequence: DPWNVFD FLI(10)VIGSIIDVIL(20)SE) tethered by their C-termini to a nanopeptide template. We sought to obtain the optimal conformations of SCP and loc ate the binding sites for Ca2+ and for the dihydropyridine ligand nife dipine. Eight Ca2+ ions were added to neutralize the 16 acidic residue s in the helices. Eight patterns of the salt bridges between Ca2+ ions and pairs of the acidic residues were calculated by the Monte Carlo-w ith-energy-minimization (MCM) protocol. In the energetically optimal c onformation, two Ca2+ ions were bound to Asp-l residues at the intrace llular side of SCP, and six Ca2+ ions were arrayed in two files at the diametrically opposite sides of the pore, implying a Ca2+ relay mecha nism. Nine modes of nifedipine binding to SCP were simulated by the MC M calculations. In the energetically optimal mode, the ligand fits snu gly in the pore. The complex is stabilized by Ca2+ bound between two A sp-17 residues and hydrophilic groups of the ligand. The latter substi tute water molecules adjacent to Ca2+ in the ligand-free pore and thus do not obstruct Ca2+ relay. The ligand-binding site is proximal to a hydrophobic bracelet of lle-10 residues whose rotation is sterically h indered, In some conformations, the bracelet is narrow enough to block the permeation of the hydrated Ca2+ ions. The bracelet may thus act a s a ''gate'' in SCP, Nifedipine and (R)-Bay K 8644, which act as block ers of the SCP, extend a side-chain hydrophobic moiety toward the lle- 10 residues, This would stabilize the pore-closing conformation of the gate, In contrast, the channel activator (S)-Bay K 8644 exposes a hyd rophilic moiety toward the lle-10 residues, thus destabilizing the por e-closing conformation of the gate.