In <9401132045.AA17957 at net.bio.net>
mroussel at alchemy.chem.utoronto.ca (Marc Roussel) writes:
>In article <2h110b$rmv at mserv1.dl.ac.uk> SCHLOSSER at ciit.org writes:
[snip]
>>For example I performed a series of
>>experiments with yeast in which the the yeast was harvested and placed in
>>an *anaerobic* medium containing glucose, but no nitrogen source. The
--------- ------------------ -------------------
(in a tartrate buffer w/ H2KPO4, HK2PO4, MgSO4, and NaCl)
>>glucose was completely consumed (to the limit of detection) being converted
>>primarily to ethanol and glycerol. I didn't do the calculations, but this
>>state is probably far from thermodynamic equilibrium.
>>I'm not so sure. I would like to do the calculations but I don't know
>how the glycerol is generated. (It would be sufficient for me to know
>the reactants and products of the reaction.) I take it that the ethanol
>is generated by anaerobic fermentation.
Based on the measured fluxes, the stoichiometry is approximately:
glucose -> 1.41(ethanol + CO2) + 0.24glycerol + "polysaccharides"
the coefficients are the ratio of the steady state fluxes which occur thru
most of the fermentation. (The actual, "over-all" coefficients may differ
somewhat from these.) The carbon not accounted for as ethanol/CO2 or
glycerol was assumed to go to disaccharide and polysaccharide storage,
as no other pathways could operate under these conditions (see above; the
fermentation was done at 20C). Note that the ethanol/CO2 and glycerol account
for 82.5% of the carbon. The yeast use the excess energy generated to create
a pH differential, which collapses on glucose exhaustion. In any case the
concentration of glucose drops from ~ 300 mM to essentially zero (below the
limit of detection).
I would be interested to see the equilibrium calculation. Another aspect
that we had noted, and attributed to metabolic regulation, is that with
mixtures of normal glucose and either 2-deoxy-glucose or 6-deoxy-glucose,
the fraction going to ethanol and glycerol increased, *with the relative
amount of ethanol/glycerol remaining virtually constant*. (2-d-glucose
can be phosphorylated once, and converted to polysaccharides, but otherwise
can't be metabolized. 6-d-glucose can be taken up, but can't be
phosphorylated). E.g., with 20% 6-d-glucose / 80% glucose, we had
glucose -> 1.66(ethanol + CO2) + 0.31glycerol + "polysaccharides"
(98% of carbon to ethanol, CO2 and glycerol).
Paul
schlosser at beta.ciit.org
