title: Crystal structure of delta9 stearoyl-acyl carrier protein desaturase from castor seed and its relationship to other di-iron proteins.
abstract: The three-dimensional structure of recombinant homodimeric delta9 stearoyl-acyl carrier protein desaturase, the archetype of the soluble plant fatty acid desaturases that convert saturated to unsaturated fatty acids, has been determined by protein crystallographic methods to a resolution of 2.4 angstroms. The structure was solved by a combination of single isomorphous replacement, anomalous contribution from the iron atoms to the native diffraction data and 6-fold non-crystallographic symmetry averaging. The 363 amino acid monomer consists of a single domain of 11 alpha-helices. Nine of these form an antiparallel helix bundle. The enzyme subunit contains a di-iron centre, with ligands from four of the alpha-helices in the helix bundle. The iron ions are bound in a highly symmetric environment, with one of the irons forming interactions with the side chains of E196 and H232 and the second iron with the side chains of E105 and H146. Two additional glutamic acid side chains, from E143 and E229, are within coordination distance to both iron ions. A water molecule is found within the second coordination sphere from the iron atoms. The lack of electron density corresponding to a mu-oxo bridge, and the long (4.2 angstroms) distance between the iron ions suggests that this probably represents the diferrous form of the enzyme. A deep channel which probably binds the fatty acid extends from the surface into the interior of the enzyme. Modelling of the substrate, stearic acid, into this channel places the delta9 carbon atom in the vicinity of one of the iron ions.
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institute, Stockholm, Sweden.
authors.forename: Y
authors.initials: Y
authors.lastname: Lindqvist
authors.forename: W
authors.initials: W
authors.lastname: Huang
authors.forename: G
authors.initials: G
authors.lastname: Schneider
authors.forename: J
authors.initials: J
authors.lastname: Shanklin
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry; D001665:Binding Sites; D002367:Ricinus communis / Q000201:enzymology*; D018360:Crystallography, X-Ray; D044943:Fatty Acid Desaturases / Q000737:chemistry*; D007501:Iron / Q000737:chemistry; D006899:Mixed Function Oxygenases / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010105:Oxygenases / Q000737:chemistry; D010940:Plant Proteins / Q000737:chemistry*; D010947:Plants, Toxic; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry; D012264:Ribonucleotide Reductases / Q000737:chemistry; D016415:Sequence Alignment; D013230:Stearoyl-CoA Desaturase
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D011994:Recombinant Proteins; D007501:Iron; D006899:Mixed Function Oxygenases; D010105:Oxygenases; C021555:methane monooxygenase; D044943:Fatty Acid Desaturases; D013230:Stearoyl-CoA Desaturase; C018747:acyl-(acyl-carrier-protein)desaturase; C029627:delta-9 fatty acid desaturase; D012264:Ribonucleotide Reductases