Enolase Information

Enzyme Name

Enolase

Reaction Type

Elimination Reaction

Cofactors/Cosubstrates

a metal ion usually Mg²⁺ is required as a cofactor

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.

The conversion is allowed to occur only IF the the enol has an -OH on it like compound "A" ... 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"which 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.

ΔG°'

+1.8 kJ/M

K_eq

Comments

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.

"In cell" Substrate Concentrations^*

S₁ =

S₂ =

P₁ =

P₂ =

ΔG for these conditions

+1.1 kJ/M

Mechanism for Chemistry

Mechanism for Enzyme

Hover your mouse over a step to reveal a description

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 elimination 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