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RE‐EXAMINATION OF THE DOUBLE BOND POSITIONS IN ALKENONES AND DERIVATIVES: BIOSYNTHETIC IMPLICATIONS 1
Author(s) -
Rontani JeanFrançois,
Prahl Fredrick G.,
Volkman John K.
Publication year - 2006
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/j.1529-8817.2006.00251.x
Subject(s) - double bond , chemistry , biosynthesis , emiliania huxleyi , stereochemistry , methylene , elongation , strain (injury) , medicinal chemistry , organic chemistry , biology , phytoplankton , materials science , anatomy , nutrient , metallurgy , ultimate tensile strength , enzyme
By derivatizing with OsO 4 , we determined the double bond positions of all the diunsaturated alkenones present in different haptophytes and demonstrated that the positions in C 37 –C 40 homologs occur at a fixed carbon number from the carbonyl group, a finding contrary to earlier speculation. These data have allowed us to recognize three “families” of alkenones, for which we propose different biosynthetic pathways. The proposed biosynthesis of classical C 37 –C 40 alkenones starts with acetyl‐ or propionyl‐SCoA and involves classical chain elongation steps with malonyl‐ and methylmalonyl‐SCoA affording alkanoyl‐ACP intermediates, which undergo subsequent Δ 14, 21 ‐desaturation and decarboxylation reactions. Unusual diunsaturated shorter‐chain (C 35 and C 36 ) ketones now being produced for yet unknown physiological reasons by our specimen of Emiliania huxleyi (Lohm). Hay et Mohler strain CCMP1742. These compounds exhibit ω15,22 and ω16,23 double bonds, which are at a shorter distance (two carbon atoms) from the carbonyl group than the higher homologs, implying that their biosynthesis must involve an additional chain‐shortening step after Δ 14, 21 ‐desaturation of alkanoyl‐ACP. Alkenones exhibiting a very unusual double bond spacing (three methylene groups instead of five) were detected in Holocene Black Sea sediments, in particulate matter collected in the Ligurian Sea and in Gephyrocapsa oceanica strain JB‐02. The formation of these compounds seems to be linked to the biosynthesis of monounsaturated alkenones previously detected in several haptophytes. Our work demonstrates the value of determining the double bond locations of alkenones whenever possible.