Molecular docking of bilirubin into the human UGT1A1 protein. (A) Diagram of the bilirubin glucuronidation reaction catalyzed by UGT1A1. (B) Stereo diagram showing positions of the modeled substrates. The UGT1A1 is shown in a ribbon diagram with the secondary structure highlighted. The donor UDP glucuronic acid (UDPGA) is shown as blue stick. The two docked conformations of bilirubin are shown as red and green sticks. (C) Stereo diagram showing bilirubin (red conformation) docked into the acceptor-binding pocket of UGT1A1. UDPGA (blue) and bilirubin (red) are shown as lines. Some residues in the acceptor-binding pocket are labeled and shown in cyan. Distances between the OH group of the bilirubin porphyrin C propionate and the C1' atom of the UDPGA or the NE2 atom of the residue histidine 39 (H39) (shown in pink), or between the NE2 atom of the residue H39 and the OD2 atom of the residue aspartic acid 151 (D151) (shown in pink) are indicated with dashed lines.