In article <3ojn6t$leg at decaxp.harvard.edu>,
Zorro <berriz at husc.harvard.edu> wrote:
>>I recently read in a popular book on exercising that muscles could not
>burn fatty acids without first burning glucose.
>I haven't been able to find confirmation to any of the above in my
>biochemistry textbooks. As far as I can tell, fatty acid oxidation,
>the citric acid cycle, and oxidative phosphorylation can all take
>place *without* any prior or concomitant glycolysis. Is this true?
>Or I'm I missing something?
On a piece of paper you can indeed oxidize acetyl-CoA to 2 CO2 via the
TCA cycle (and reoxidize the coenzymes via the respiratory chain...).
However, you should consider that the TCA cycle is the metabolic turntable
of the cell -- besides its catabolic function it serves as a continous
source for building blocks in anabolism (C4 and C5 compounds, and C3 via
decarboxylation of C4 compounds).
This depletes the cycle of C4 compounds which have to be replenished by
so-called anaplerotic reactions. In humans, this is done by carboxylation
of a C3 to a C4 compound (by pyruvate carboxylase). Since humans are
unable to go from C2 to C3 directly (i.e., by carboxylation of acetyl-CoA)
and since they also can't combine 2 acetyl-CoA to a C4 compound (like
plants do in the glyoxylic acid cycle), the TCA cycle will run out of
intermediates if no additional C3 compounds are supplied by glycolysis.
This is a quite simplified explanation why "fats burn in the fire of
carbohydrates", a phenomenon already observed in the last century. I
didn't bother to mention the names for all the compounds with 2, 3 or
4 carbon atoms since they're unnecessary details (unnecessary for a
brief answer to this question, i mean ;-)
Dr. Andreas Brune Phone: +49-7531-883282
Mikrobielle Oekologie Fax: +49-7531-882966
Universitaet Konstanz E-mail: Andreas.Brune at uni-konstanz.de