GLYOXYLATE PATHWAY

 

·        Certain plants contain PEROXISOMES (~ 0.5 mm dia.) known as GLYOXYSOMES.

 

·        Text Box: Glyoxysomes contain enzymes for glyoxylate pathway which allows:

     acetyl-CoA ®  ® glyoxylate                           ®  ® OAA ®  ® glucose

 

 

·        Most of the enzymes of the glyoxylate pathway are the same as those in TCA cycle.

 

·        See Fig. 16-17

 

·        Glyoxysome contains no aconitase.  Presumably reaction 2 takes place in cytosol.

 

·        Involves the transport of succinate from glyoxysome to cytosol and, in turn, to mitochondrion.

 

·        The malate formed inside of mitochondrion has two metabolite fates: (a) à OAA à TCA, or (b) transported to cytosol à OAA à gluconeogenesis.

 

·        The pathway (a) allows glyoxylate cycle to replenish the TCA intermediates.  The replenishing reactions are called anaplerotic reactions.

 

Overall:

     2 acetyl-CoA + 2 NAD+ + FAD ® OAA + 2 CoA + 2 NADH + FADH2 + 2 H+

 

·        Net conversion of 2 acetyl-CoA to OAA (or succinate) instead of 4 CO2 as in TCA.

 

·        The transport of a succinate from glyoxysome to mitochondrion depletes 4 carbon atoms from glyoxysome.  The source of these 4 carbon atoms are 2 acetyl-CoA.