Probing the mechanism of a cyanobacterial Delta 9 fatty acid desaturase from Spirulina platensis C1 (Arthrospira sp PCC 9438)

The initial and rate determining step in the mechanism of fatty acid desatu rases has been proposed to be breakage of one of the C-H bonds at the site of the incipient double bond. This has been investigated and supported for a number of eukaryotic fatty acid desaturases through the use of kinetic is otope effect experiments with deuterated substrates, In order to probe the reaction catalyzed by the cyanobacterial Delta9 desaturase and compare it t o the eukaryotic desaturases, the desC gene of Spirulina platensis, strain C1 (Arthrospira sp. PCC 9438) was expressed in a desaturase mutant of baker 's yeast. Kinetic isotope effects were performed by culturing yeast transfo rmants with deuterated thia-substituted stearic acids. A large kinetic isot ope effect was found for the 9 position, in qualitative agreement with resu lts from eukaryotic desaturases. (C) 2001 Elsevier Science Inc. All rights reserved.