After carbon assimilation, isotope effects during biosynthesis further affect the 513C values of specific lipids within an organism. This can cause differ ences in 513C values among compound classes (e.g. lipids vs. carbohydrates as described above) and also between 513C values of individual lipids within a structural class. The traditional paradigm regarding biosynthetic isotope effects is that fractionation occurs during formation of acetyl-CoA with the strongest depletion occurring at the carboxyl carbon ; presumably this fractionation is associated with either the oxidation of pyruvate to acetyl-CoA by pyruvate dehydrogenase  or conversion of acetyl phosphate to acetyl-CoA by phosphotransacetylase . Monson and Hayes  determined that the kinetic isotope effect associated with pyruvate dehydrogenase in E. coli was 23% (spdh), but this full effect is rarely expressed because of Rayleigh distillations (such that the residual pyruvate carbon pool becomes progressively enriched in 13C as the reaction progresses). Instead, the fractionation expressed during decarboxylation is (1 - f )spdh , where f is the fraction of pyruvate flowing to acetyl-CoA.
Because these reactions occur during both photoautotrophic and het-erotrophic lipid synthesis, acetogenic lipids in most organisms are expected to be ca. 4% depleted relative to biomass . Isoprenoids synthesized by the mevalonic acid pathway, which results in a 3 : 2 ratio of methyl to carb-oxyl atoms, are expected to be depleted relative to biomass by only ca. 2%. However, it is now recognized that isoprenoids are not synthesized via the mevalonic acid pathway in many or most organisms; instead, the C5 isoprene unit can be formed by condensation of a C2 subunit derived from pyruvate decarboxylation and a C3 subunit derived from triose phosphate [158-161]. This pathway actually appears to be predominant in bacteria and has also been identified in higher plants , unicellular algae, and cyanobacte-ria . The effect of the glyceraldehyde phosphate/pyruvate pathway on the carbon isotopic composition of isoprenoids remains unclear. It is possible that isoprenoids derived from this pathway could be enriched in 13C relative to mevalonate-derived isoprenoids, because the former contain only one 13C-depleted carboxyl carbon.
Generally, biomarker lipid 513C values tend to match these general considerations, but significant exceptions are common. In the following sections, we discuss algal, higher plant and prokaryotic biomarker 513C values in the context of the above considerations.
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