Molecular modeling of the rabbit colonic (HKα2a) H⁺, K⁺ ATPase

Abstract

A model of the HKα2a subunit of the rabbit colonic H⁺, K⁺ ATPase has been generated using the crystal structure of the Ca⁺² ATPase as a template. A pairwise sequence alignment of the deduced primary sequences of the two proteins demonstrated that they share 29% amino acid sequence identity and 47% similarity. Using O (version 7) the model of HKα2a was constructed by interactively mutating, deleting, and inserting the amino acids that differed between the pairwise sequence alignment of the Ca⁺² ATPase and HKα2a. Insertions and deletions in the HKα2a sequence occur in apparent extra-membraneous loop regions. The HKα2a model was energy minimized and globally refined to a level comparable to that of the Ca⁺² ATPase structure using CNS. The charge distribution over the surface of HKα2a was evaluated in GRASP and possible secondary structure elements of HKα2a were visualized in BOBSCRIPT. Conservation and placement of residues that may be involved in ouabain binding by the H⁺, K⁺ ATPase were considered and a putative location for the β subunit was postulated within the structure.

Figure Possible architecture of the HKα2a subunit. The residue in green is the lysine (position 517, Fig. 1) that lies in the nucleotide binding pocket and the residue in red is the aspartic acid at the phosphorylation site (position 385). Based on an alignment with the Ca⁺² ATPase, ten transmembrane helices were modeled into HKα2a. The ten transmembrane helices are drawn as rods and shown in different colors for clarity. From left to right, the transmembrane helix designations are M10 (blue), M7 (gray), M8 (purple), M9 (orange), M5 (pink), M6 (green), M3 (brown), M4 (cyan), M2 (teal), and M1 (almond).