Enolase Information


 

Enzyme Name

Enolase

Reaction Catalyzed

Elimination of water from 2-Phosphoglycerate

Reaction Type

Elimination Reaction

Rationale

Elmination Reaction. The phosphate has already been placed on C2 of glycerate from the previous enzyme.

The Enol functional group.  
Now an elimination reaction will remove a water (hydroxyl -OH group for C3aand a hydrogen from C2) The result is a double bond between C2 and C3. Compounds that contain a carbon - carbon double bonds (C=C) are always named with an ending of -ene. For example Propane is a three carbon compound with only single bonds between carbons and all other bonds are filled by hydrogen. Propene is the same except that it contains one C=C. In the case of phosphoenolpyruvate, C2 has this double bond to C3 AND it also has a hydroxy group on it as well. Therefore C2 is now named an "enol" ("en" = ene for the C=C and "ol" is for alcohol).

The significance of the Enol group
Enols are not very stable in water. Generally there is a rapid, facile, nonezymatic conversion between an enol and a ketone.This type of conversion is called tautomerization and occurs extremely quickly. Thermodynamically, the ketone form is greatly favored - therefore the ketone is in much high concentration than the enol fom.ene enolconversion


The conversion is allowed to occur only IF the the enol has an -OH on it like compound "A" ...enol because the terminal -H must be able to dissociate as H+ for the conversion.

In phosphoENOLpyruvate the enol does not terminate with -H but with -PO3  like compound "B"phosphoenolwhich cannot dissociate therefore the molecule is trapped in this less favorable enol form.

Thermodynamically speaking hydrolysis of phosphate from phosphoenolpyruvate has a very high favorable Standard Free Energy because two things happen... we get the hydrolysis AND then the enol converts mostly to the much more stable ketone.

Pathway Involvement

Glycolysis AND gluconeogenesis


Cofactors/Cosubstrates

a metal ion usually Mg2+ is required as a cofactor

DGo'

+1.8 kJ/M

Starting from standard state and allowing the reaction to come to equilibrium the 2-phosphoglycerate concentration would end up ~2 times higher than that of phosphoenolpyruvate (PEP)

The Standard Free Energy favors 2-phosphglycerate.

Keq

Comments

"In cell" Substrate Concentrations*

 

 

S1 =

2-PhosphoGlycerate

0.030 mM

S2 =



P1 =

Phosphoenolpyruvate(PEP)

0.023 mM

P2 =

Glyceraldehyde-3-Phosphate (G-3-P)

0.019 mM

DG for these conditions

 

+1.1 kJ/M

 


Mechanism for Chemistry

Mechanism for Enzyme

Enolase. Animation of the Enolase reaction Blue: represents the enzyme. The E-NH2 represents the crucial enzyme active site amino Lysine in their basic (deprotonated). "Start" begins an animation of the group transfer reaction. It proceeds through the reaction in the "forward" direction and then "backwards" again. Note how the enzyme is involved. "+" increases speed while "-" decreases the animation speed. You may also step through the reaction using "next" or "previous"

This reaction is a simple elimination reaction catalyzed by the presence of the metal ion which provides a strong "pulling" on the hydroxyl group of C3. effectively pulling off water and generating the C=C double bond. Perhaps it is easier to see in the other direction. In which case the metal ion is used to aid in dissociating water to H+ and HO- as in the metal catalyzed hydrolysis reaction. Here though... instead of attacking a carbonyl, it will attack a C=C double bond and add water across it. The animation shows both directionss.

Compare the animated reaction to the "arrow pushing" scheme at the right. See if you can correlate the electron movement in the animation to the arrows in the static picture above.

Picture of Enzyme with substrate


  1. RibbonsEnolase in ribbons diagram
  2. Ribbons + Mg The Mg2+ ion is displayerd as a green sphere
  3. Ribbons + Mg + 2-P-Glycerrate2-Phosphoglycerate is added
  4. Nearby AAThe amino acid sidechains near the Mg ion are added.
  5. remove ribbonsSame as above with protein ribbons removed
  6. rotation to demonstate orientationThe active site amino acids and substrates are rotated to demonstate their orientation
Enolase CHIME representation
  Initial Picture
  Mg ion On/Off
  2-PhosphoGlycerate On/Off
  Active side AA On/Off
  Protein Ribbons Off/On
Atoms Clicked on in Chime window

mouse methods

*= These are concentrations obtained for one set of conditions. These will change as physiology and activity change.