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 C₃ aand a hydrogen from C₂) The result is a douvble bond between C₂ and C_3. 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 phsophoenolpyruvate, 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 tautimerization and occurs extremely quickly (on the nanosecond time scale). Thermodynamically, the ketone form is greatly favored - therfore 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 the 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 favorable ketone.

Pathway Involvement

Glycolysis AND gluconeogenesis

Cofactors/Cosubstrates

a metal ion usually Mg²⁺ is required as a cofactor

DG^o'

+23.9 kJ/M

Starting from standard state and allowing the reaction to come to equilibrium the Fructose-1,6-bisphosphate concentration would end up ~15,000 times higher than the product of the concetrations of DHAP and G-3-P.

The Standard Free Energy favors Fructose-1,6-bisphosphate production.

K_eq

Comments

Note: the Standard Free Energy grealy favors production of Fructose-1,6-bisphosphate.

The mammalian forms of this enzyme go through a required covalently bound substrate for catalysis. This is true regardless direction of the reaction. Meaning, both directions go through through the same mechanism and utilize the same intermediate... as must be the case for all enzymes recations.

"In cell" Substrate Concentrations^*

S₁ =

S₂ =

P₁ =

P₂ =

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

0.019 mM

DG for these conditions

-0.23 kJ/M

Note this rather large turn around from DG^o' to DG. This reaction does not have the same issues as that of a hydrolysis (water concentration at 50M/l)or that of an ATP dependent group transfer (ATP held fairly high and constant at 5mM).

Mechanism for Chemistry

Mechanism for Enzyme

Picture of Enzyme with substrate

1. RibbonsAldolase. Here only the main chain is represented by these ribbons. There are four identical subunit
2. one subunit with substrate Three of the subunit deleted and a substrate analog (mannitol-1,6-bisphosphate) is added in the atom colored spheres. C=Gray; O=red; P=Orange. three critical AA in the active site are also added.
3. as above closeupSame picture as in "2" but zoomed in
4. substrate rotationas in "3" ribbons deleted and allowed to rotate to show the orientaion of these 3 amino acids. A.) LYS near C2 - this will form the Schiff's base; B.) GLU near C2 - will help the Schiff's base form by "attracting the water" as the Schiff's base forms; C.) LYS near C4 - will help the hydroxyl group deprotonate as the aldol reaction begins.
5. substrate and all nearby AAAll amino acid sidechains near the mannital are turn on
6. Schiff's base LYS in cyan and near GLU in yellow The LYS that will form the Schiff's base is color cyan, C2 of mannitol is colored magenta and the GLU is color yellow. The roles are highlighted in "4"
7. LYS near and near C_{4/sub> in cyan The LYS near C4 is colored cyan, C4 of mannitol is color magenta.}