Glycolysis

Glycolysis is responsible for the production of
ATP (adenosine triphosphate) through the degradation of glucose.
It is is a fundamental reaction performed by all
organisms where glucose is turned into pyruvate. 

There are ten steps to glycolysis and each step is facilitated by a different enzyme.  
All reactions occur in the cytoplasm and can take place with or without oxygen. 

The general reaction for glycolysis is as follows:
glucose + 2Pi + 2ADP + 2NAD arrow.gif (125 bytes)2 pyruvate + 2ATP + 2NADH + 2H+ +2H2O

The net energy gain for this reaction is 2ATP's and 2NADH's
(glycolysis is not very efficient).

The first five steps of glycolysis require free energy, they are endergonic.

Step One:

Glucose  to  G6P
This step is mediated by
 the enzyme hexokinase

ATP molecule is used:
gives a P to glucose to get G6P.

*kinase refers to "the addition of a phosphate"
 

Step Two:

G6P  to  F6P
This step is mediated by the
enzyme phosphoglucomutase

Glucose ring changes its shape
to form a fructose ring.

 

Step Three:

F6P  to  FBP
This step is mediated by
the enzyme phosphofructokinase

ATP molecule is used. 
P is transferred to create FBP.

 

Step Four:

FBP to DAP + G3P
This step is mediated by the enzyme aldolase

The fructose ring opens and breaks into
two different sugar phosphates.

 

Step Five:

DAP and G3P  to  two G3P's
This steps is mediated by the enzyme isomerase

DAP rearranges to form G3P

 

Why phosphorylate to get glycolysis going?  

1)  Phosphate can be used later to convert ADP to ATP
2)  The addition of phosphate changes the 3D shape of the
glucose molecule so that G6P can be recognized by enzymes.
3)  Phosphate helps to trap glucose within the cell.
G6P is unrecognizable to the transporter.

Step Six:

2 G3P  to  2 BPG
This step is facilitated by the
enzyme phosphate dehydrogenase

G3P's are oxidized. 
Each gain a P group to yield BPG. 
2NAD+ is reduced to form 2NADH + 2H+
There is a huge drop in free energy here.

 

Step Seven:

2 BPG  to  2 3PG
This step is facilitated by the
enzyme phosphoglycerate kinase

BPG gives P's to ADP. 
Two ATP molecules are made.

 

Step Eight:

2 3PG  to  2 2PG
This step is mediated by the
 enzyme phosphoglyceromutase

The P groups on the two 3PG's move. 
2 2PG's are formed.

 

Step Nine:

2 2PG  >>>  2PEP
This step is mediated by the enzyme enolase

The 2PG's lose H2O and become PEP's.

 

Step Ten:

2PEP to  2 Pyruvates
This step is facilitated by
the enzyme pyruvate kinase

Two PEP's transfer their P's to ADP. 
Two ATP molecules are made.

 

Without O2:  NADH + H+ is used in fermentation.
With O2:  More NADH (in addition to that formed in glycolysis)
is produced in pyruvate oxidation and in the citric acid cycle.

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