(thing) by Ravensong Thu Dec 06 2001 at 18:21:47
Also called the Embden-Meyerhof pathway (for some reason leaving out the third guy, Parnas, who was involved in figuring it out), glycolysis is possibly the single most important metabolic pathway. It involves the breaking down of glucose and other simple sugars for energy. It is also explained in a song called In Praise of E.M.P.

This pathway is common to all forms of life on this planet. Individual forms of the enzymes involved are different, but what they do is essentially the same.

The net result of glycolysis is the breaking down of glucose into two molecules of pyruvate and the formation of two molecules of ATP. What happens to the pyruvate next depends on the organism and whether free oxygen is available.

If there no oxygen, pyruvate will be reduced into either lactic acid or ethanol, depending on the organism. This is why yeast makes alcohol. If there is (and the organism can use it), the pyruvate is turned into acetyl CoA and goes on into the TCA cycle (also called the citric acid cycle or the Krebs cycle).

So here's what happens in glycolysis. Further details of regulatory and catalytic mechanisms will be given in the enzyme write-ups.

  1. Glucose to glucose-6-phosphate (G6P)
  2. G6P to fructose-6-phosphate (F6P)
  3. F6P to fructose-1,6-bisphosphate (F-1,6-BP or FBP, previously called fructose diphosphate, so also FDP)
    • catalysed by phosphofructokinase
    • ATP to ADP
    • regulation: complex and very important. This is the step that really controls the rate of glycolysis.
      • inhibited by citrate
      • inhibited by high concentrations of ATP
      • ATP's inhibition is countered by high concentrations of AMP
      • fructose-2,6-bisphosphate (F-2,6-BP) both counters this inhibition and enhances the reaction on its own
  4. FBP to dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P)
  5. DHAP to G3P
  6. G3P to 1,3-bisphosphoglycerate (BPG)
  7. BPG to 3-phosphoglycerate (3-PG)
  8. 3-PG to 2-phosphoglycerate (2-PG)
  9. 2-PG to phosphoenolpyruvate (PEP)
    • catalysed by enolase
    • water released
    • reversible reaction
  10. PEP to pyruvate

Other sugars can also be used in glycolysis. Mannose, which is very similar to glucose, can be phosphorylated and then converted into F6P.

Fructose, however, is not directly made into F6P. Instead it is made into fructose-1-phosphate, which is then broken into DHAP and glyceraldehyde, both of which can be turned into G3P.

Galactose is a more unusual case. It must be phosphorylated at the first carbon (making gal-1-P) and then replace glucose-1-P on uridine diphosphate (UDP). The G1P can be turned into G6P, and the galactose on UDP is coverted to glucose, so it can be freed when the next galactose comes along.

I like it!