CARBON ISOTOPE EFFECTS (13C/12C) IN BIOLOGICAL SYSTEMS

Abstract

Carbon isotope (13C/12C) fractionation (CIF) occurring in cells of organisms of different types is considered. Three metabolic points are shown to have an exceptional significance for the observed carbon isotope distribution in a living matter: 1) ribulose bisphosphate carboxylation in CO2 photoassimilation; 2) glycine decarboxylation in photorespiration; and 3) pyruvate decarboxylation in respiration metabolism. Carbon isotope effects (CIE) in ribulose bisphosphate (RuBP) carboxylation is characteristic of photosynthesizing organisms. It is responsible for the observed 12C enrichment of photosynthesizing organism biomass relative to environmental CO2. According to proposed mechanism of CIF, CO2 assimilation is a discrete process, which can be described as a flow of CO2 batches into a cell. Discreteness of the flow arises due to ability of Rubisco (the key enzyme of photosynthesis) periodically operate both as carboxylase and oxygenase depending on CO2/O2 ratio in a cell. It provides periodic filling/depletion of cytoplasmic pool of CO2 in the cell and determines the appearance of isotope Releigh effect following the pool depletion. The degree of pool depletion determines the extent of 12C enrichment of biomass. A full consumption of CO2 batches entering the cell means the complete depletion of CO2 cytoplasmic pool and is considered to be the main reason for the observed 13C enrichment of C4 plants compared with C3 and CAM plants. CIE in glycine decarboxylation is the key element of CIF in photorespiration. The effect of the reaction is also amplified by the Releigh effect of photorespiratory pool depletion. The removal of CO2 enriched in 12C results in 13C enrichment of photorespiratory substrates and biomass itself. Thus the sign of CIE of photorespiration is opposite that of CO2 photoassimilation. Some factors inducing intense photorespiration can even cause 13C enrichment of biomass in respect to ambient CO2 when CIE of photorespiration prevails that of CO2 photoassimilation. Due to the coupling of photorespiration and CO2 photoassimilation via double function of Rubisco both CIE are in reciprocal relations to each other and generate two isotopically different carbon flows in photosynthesizing cells. CIE effect in pyruvate decarboxylation is responsible for the appearance of carbon isotope heterogeneity in respiration metabolism (isotopic discrepancies of fractions, metabolites and uneven intramolecular 13C distribution of biomolecules). The reaction is a crosspoint of the central metabolic pathways. It is typical of the overwhelming majority of the organisms regardless of their position on the evolutionary stairs. The mechanism of CIE is the combination of the effect in the enzymatic reaction with Releigh effect of pyruvate pool depletion. The reaction provides C2 and C3 structural units for biosyntheses of most of the cell components whose carbon isotope ratio changes in parallel with the pool depletion determining the differences in carbon isotope composition of metabolites. At a certain energy state of the organism and the corresponding energy states of the cells, the synthesis of each component in cell cycles occurs synphasically, i.e., within the same interval of pyruvate pool depletion. In such a way a certain isotopic pattern of metabolites and 13C distribution in biomass are provided. Some applications of the technique to study plants, highly organized animals and humans are shown.