PT - JOURNAL ARTICLE AU - Liisa Laakkonen AU - Moshe Finel TI - A molecular model of the human UGT1A1, its membrane orientation and the interactions between different parts of the enzyme AID - 10.1124/mol.109.063289 DP - 2010 Jan 01 TA - Molecular Pharmacology PG - mol.109.063289 4099 - http://molpharm.aspetjournals.org/content/early/2010/03/09/mol.109.063289.short 4100 - http://molpharm.aspetjournals.org/content/early/2010/03/09/mol.109.063289.full AB - The vertebrate UDP-glucuronosyltransferases (UGTs) are membrane bound enzymes of the endoplasmic reticulum that process both endogenous and exogenous substrates. The human UGTs are well known biologically but biophysical understanding is scarce, largely due to problems in purification. The one resolved crystal structure covers the C-terminal domain of the human UGT2B7. Here we present a homology model of the complete monomeric human UGT1A1, the enzyme that catalyzes bilirubin glucuronidation. The enzyme can be seen as composed of 4 different domains, two large ones, the N- and C-terminal domains, and two small ones, the "envelope" helices and the transmembrane segment that includes the cytoplasmic tail. The hydrophobic core of the N-terminal domain and the two envelope helices that connect the large domains are shown to be structurally well conserved even among distant homologs, and can thus be modeled with good certainty according to plant and bacterial structures. We consider alternative solutions for the highly variable N-terminal regions that probably contribute to substrate binding. The bilirubin binding site, known pathological mutations in UGT1A1 and other specific residues have been examined in the context of the model with regard to available experimental data. A putative orientation of the protein relative to the membrane has been derived from the location of predicted N-glycosylation sites. The model presents extensive interactions between the N-terminal and C-terminal domains, the two envelope helices and the membrane. Together, these interactions could allow for a concerted large-scale conformational change during catalysis.The American Society for Pharmacology and Experimental Therapeutics