Site-specific isotope fractionation in the characterization of biochemical mechanisms. The glycolytic pathway

For a given biochemical transformation, such as the fermentation reaction, the redistribution coefficients, which relate the natural site-specific isotope contents in end products to those of their precursors, are a source of mechanistic information. These coefficients characterize the traceability of specific hydrogens in the products (ethanol and water) to their parent hydrogens in the starting materials (glucose and water). In conditions of complete transformation, they also enable intermolecular exchanges with the water medium to be estimated. Thus it is directly confirmed that hydrogens 1, 2, 6, and 6' of glucose are strongly connected to the methyl site I of ethanol obtained by fermentation by Saccharomyces cerevisiae. However, whereas hydrogens 6 and 6' are transferred to a great extent, transfer is only partial for hydrogen 2, and it is even less for hydrogen 1. Because the two moieties of glucose corresponding to carbons 1-2-3 and 4-5-6 are scrambled by the aldolase and triosephosphate isomerase reactions, additional exchange of hydrogens at positions 1 and 2 must have occurred before these steps. The value of the coefficient that relates site 2 of glucose to site I of ethanol in particular can be used to quantify the contribution of intermolecular exchange occurring in the course of the transfer from site 2 of glucose 6-phosphate to site 1 of fructose 6-phosphate mediated by phosphoglucoisomerase. The average hydrogen isotope effects associated with the transfer of hydrogen from the water pool to the methyl or methylene site of ethanol are estimated. In contrast to conventional experiments carried out in strongly deuterium-enriched media where metabolic switching may occur, the NMR investigation of site-specific natural isotope fractionation, which operates at tracer isotopic abundance, faithfully describes the unperturbed metabolic pathways.