Molecular model of CCK bound to the type A CCK receptor. Shown are lateral (left) and top (right) views of the lowest energy molecular model of CCK bound to the CCK receptor. The receptor amino-terminal domain is illustrated in brown, while the rest of the receptor is green. The peptide ligand, CCK-8, is shown as a stick model with transparent surface skin in white. Key residues of interest have been highlighted. These include receptor residues Asp5 and Lys187 that are involved in charge-pairing, Cys pairs that are involved in disulfide bonds (Cys18-Cys29 and Cys114-Cys196), and three sites of N-linked glycosylation (Asn10, Asn13, and Asn24). The TM helices have been identified in the top view.