CRYSTAL-STRUCTURE OF DELTA(9) STEAROYL-ACYL CARRIER PROTEIN DESATURASE FROM CASTOR SEED AND ITS RELATIONSHIP TO OTHER DIIRON PROTEINS

The three-dimensional structure of recombinant homodimeric Delta(9) st earoyl-acyl carrier protein desaturase, the archetype of the soluble p lant fatty acid desaturases that convert saturated to unsaturated fatt y acids, has been determined by protein crystallographic methods to a resolution of 2.4 Angstrom. The structure was solved by a combination of single isomorphous replacement, anomalous contribution from the iro n atoms to the native diffraction data and 6-fold non-crystallographic symmetry averaging, The 363 amino acid monomer consists of a single d omain of 11 alpha-helices, Nine of these form an antiparallel helix bu ndle, 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 inte ractions with the side chains of E196 and H232 and the second iron wit h the side chains of E105 and H146. Two additional glutamic acid side chains, from E143 and E229, are within coordination distance to both i ron ions. A water molecule is found within the second coordination sph ere from the iron atoms, The lack of electron density corresponding to a mu-oxo bridge, and the long (4.2 Angstrom) distance between the iro n ions suggests that this probably represents the diferrous form of th e enzyme, A deep channel which probably binds the fatty acid extends f rom the surface into the interior of the enzyme, Modelling of the subs trate, stearic acid, into this channel places the Delta(9) carbon atom in the vicinity of one of the iron ions.