Aldolase Information

Enzyme Name

Aldolase

Reaction Catalyzed

Aldol cleavage of Fructose-1,6-bisPhosphate AND Aldol Condensation of Dihydroxyacetonephosphate + glyceraldehyde-3-Phosphate

Reaction Type

Aldol Reaction

Rationale

Aldol Reaction. I call this an aldol reaction because it goes BOTH directions; cutting into fragments is more commonly called an Aldol Cleavage, while putting two pieces together into one is called an Aldol Condensation. For this reaction the Standard Free Energy (DGo' - see below) favors the formation of Fructose-1,6-bisphosphate. Aldol Cleavage yields two differnt products. the subsequent Isomerization of one of the three carbon fragments makes them identical. If the aldol cleavage is performed on Fructose-1,6-bisphosphate, we will get two 3 carbon pieces each with a charged group (phosphate). Are the two pieces identical? Not quite... how are they related? The two halves to DO each have a phosphate, what do you have to do to ONE of the halves to get the phosphates on top of each other (superimpose). The pieces from the aldol reaction are not identical, but are related to each other by a simple conversion. Another isomerization! On one of the halves the ketone is on C2 while on the other it is on C1. How can they be made to look identical? The glyceralhehyde does NOT NEED to do an ismomerization. ONLY dihydroxyacetone phosphate (ketone on C2) needs to be isomerized. Afterwards it is identical to the other half (glyceraldehyde-3-phosphate). FOR ALL REACTIONS FROM HERE ON, THERE ARE TWO MOLECULES GOING THROUGH EACH CONVERSION FOR EACH GLUCOSE THAT STARTED.

Pathway Involvement

Glycolysis AND gluconeogenesis

Cofactors/Cosubstrates

In mammalian enzymes no cofactor or cosubstrates are required. In some bacterial enzymes a metal ion is required.

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 the same mechanism and utilize the same intermediate... as must be the case for all enzyme 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

Aldolase. Animation of the Aldolase 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 happens in three phases: 1: Schiff's base formation; 2: Aldol Reaction; 3: Release of Schiff's base. These phases are labeled in the animation as well. The Schiff's base is formed to provide the necessary "pulling" force on the electrons to initiate the aldol reaction. Notice: how the "positive charge" on the nitrogen of the Schiff's base begins the process of teh electron pulling cascade.

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. 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 C₄ in cyan The LYS near C4 is colored cyan, C4 of mannitol is color magenta.