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 sugar
s 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 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.
- Glucose to glucose-6-phosphate (G6P)
- G6P to fructose-6-phosphate (F6P)
- 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
- FBP to dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P)
- DHAP to G3P
- G3P to 1,3-bisphosphoglycerate (BPG)
- BPG to 3-phosphoglycerate (3-PG)
- 3-PG to 2-phosphoglycerate (2-PG)
- 2-PG to phosphoenolpyruvate (PEP)
- catalysed by enolase
- water released
- reversible reaction
- 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.