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Q: glycolysis and respiration?

Andreas Brune Andreas.Brune at uni-konstanz.de
Wed May 10 09:14:21 EST 1995


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 ;-)

Regards,

Andreas


Dr. Andreas Brune         	Phone: 	+49-7531-883282
Mikrobielle Oekologie     	Fax:   	+49-7531-882966
Universitaet Konstanz     	E-mail:	Andreas.Brune at uni-konstanz.de



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