Investigation of Fatty Acid Elongation and Desaturation Steps in Fusarium lateritium by Quantitative Two-dimensional Deuterium NMR Spectroscopy in Chiral Oriented Media
Résumé
The origin of hydrogen atoms during fatty acid biosynthesis in Fusarium lateritium has been quantified by isotope tracking close to natural abundance. Methyl linoleate was isolated from F. lateritium grown in natural abundance medium or in medium slightly enriched with labeled water, glucose, or acetate, and the H-2 incorporation was determined by quantitative H-2-\H-1\ NMR in isotropic and chiral oriented solvents. Thus, the individual (H-2/H-1)(i) ratio at each pro-R and pro-S hydrogen position of the CH2 groups along the chain can be analyzed. These values allow the isotope redistribution coefficients (a(ij)) that characterize the specific source of each hydrogen atom to be related to the nonexchangeable hydrogen atoms in glucose and to the medium water. In turn, these can be related to the stereoselectivity that operates during the introduction or removal of hydrogens along the fatty acid chain. First, at even CH2 the pro-S hydrogen comes only from water by protonation, whereas the pro-R hydrogen is introduced partly via acetate but principally from water. Second, the nonexchangeable hydrogens of glucose ( positions H-6,6 and H-1) are shown to be introduced to the odd CH2 via the NAD(P) H pool used by both reductases involved in the elongation steps of the fatty acid chain. Third, it is proved that hydrogens removed at sites 9,10 and 12,13 during desaturation by Delta(9)- and Delta(12)-desaturases are pro-R, and that during these desaturation steps alpha-secondary kinetic isotope effects occur at the 9 and 12 positions and not at the 10 and 13 positions.