Question

When E. coli is growing aerobically and is fed glucose that contains 13C (a stable isotope of the normal 12C) at the C3 and C4 positions, half the pyruvate generated has the 13C in the carboxylic acid portion of the molecule with the other half having the 13C in the methyl group.  When the bacterium is switched to growing anaerobically and is fed the same 13C-labeled glucose, all of the pyruvate has the 13C as the carboxylic acid portion of the molecule.  

-0 -0 H.CO C3H-C-0-H H-O-C-H H-C-0-H C4H-C-0-H H-C-0-H -0 -0-

What explains this difference? As part of your answer, draw the structures of the metabolites that connect the 13C glucose to labeled pyruvate.  

Answer 1

Answer:

Stable isotopes have the same number of protons as common elements, and consequently share the same physicochemical properties, but they differ in mass due to a difference in the number of neutrons. Among biochemically relevant elements, carbon, hydrogen, nitrogen, oxygen and sulfur all have two or more stable isotopes with measurable abundance in nature. The natural abundance of stable isotopes is often exploited in label-free metabolomics studies to assist metabolite identification, in combination with accurate mass, to determine the molecular formula. For example, carbon is formed predominantly as the light isotope, ¹²C (98.89% abundance), but also in the form of a heavy stable isotope, ¹³C (1.11%), with an additional neutron, in addition to trace amounts of a radioactive heavy isotope, ¹⁴C.

Isotopes, that is metabolites containing stable isotopes and their unlabeled counterparts, have the same chemical formula and structure and hence generally behave identically during chromatographic separation ( an exception being deuterated compounds that can differ from their common hydrogen containing counterparts in chromatographic properties. However, in a mass spectrometer isotopologs can be readily differentiated by mass (m/z). Therefore, the mass spectrum for an unlabeled metabolite contains the major monoisotopic peak, in addition to low abundance peaks representing all combinations of the naturally abundant isotopes. For example, a metabolite with four carbons (e.g., aspartate; C₄H₇NO₄ ) naturally possesses a ¹³C peak 1.00335 Da higher in mass, and at roughly 4% abundance ( %), compared with the monoisotopic ( ¹²C ) peak.

4x 1.11