Anaerobic Glucose Metabolism

Glyceraldehyde-3-Phosphate Dehydrogenase Information

Enzyme Name

Glyceraldehyde-3-Phosphate Dehydrogenase

Reaction Catalyzed

two step reaction:
Written in the direction of glycolysis
  1. Oxidation of Glyceraldehyde-3-phosphate to glycerate-3-phosphate
  2. addtion of phosphate to C1
This order reverses for gluconeogenesis, of course

Reaction Type

 Two Step Reaction
  1. Oxidation/Redxution (REDOX)
  2. Hydrolysis type reaction (with a twist here since phosphate (instead of water) is used to attack it is called phosphorolysis)

Rationale

Oxidation of an aldehyde to an acid (combined with a one slight trick). The core of this reaction is the NAD+ dependent oxidation of the aldehyde in glyceraldehyde to an organic acid (glycerate).

The slight trick here is that it is actually converted to the phosphoanhydride rather than a free organic acid. This can be accomplished becuase of a covalently bound intermedicate in the enzyme. This is described in much more detail in the mechanism section below.

The justification for the twist lies in thermodynamics of the next enzyme catalyzed step of glycolysis. Remember, in one of the quiz questions, two modules ago, I indicated that the amount of energy released from phosphoanhydride hydrolysis is MUCH higher than that for hydrolysis of a phosphate from an hydroxyl group. This extra energy is used in the next step to MAKE an ATP! (more on energy coupling later in this module).

Summing up - TWO ATP were needed to start the glycolysis pathway (hexokinase and phosphofructokinase-1) . NOW two ATP (remember there are two 3-phosphoglycerate for every glucose) are made in the following reaction... we will be back where we started in number of ATP

Pathway Involvement

Glycolysis AND gluconeogenesis
This reaction easily goes both directions depending on the concentrations of the reactants. (see discussion of ΔG below)

Cofactors/Cosubstrates

 Glycolysis cosubstrates: NAD+ and PO4= are both required. NADH is a coproduct
Gluconeogenesis cosubstrate: NADH; NAD+and PO4= are coproducts

ΔGo'

+6.3 kJ/M Starting from standard state (all concntrations at 1 M/l) and allowing the reaction to come to equilibrium the substrates of glycolysis (G-3-P, PO4 and NAD+) concentration would end up ~12 times higher than the product of the concetrations of 1,3 bisphosphoglycerate NADH.

The Standard Free Energy favors Gluconeogenesis direction.

Keq

 Since this fraction is less than 1, then the concentrations of the substances on top must be larger than those on the bottom WHEN the reaction comes to eqauilibrium.

Comments

This reaction is made possible because of two factors:
  1. Coupling of the phosphate addition to the NAD+ linked redox
  2. Formation of a required covalent enzyme bound intermediate (through a cysteine)

"In cell" Substrate Concentrations*

S1 =

Glyceraldehyde-3-Phosphate 0.019 mM

S2 =

NAD+ 0.0003 mM

S3=

 PO4= 1.0 mM

P1 =

1,3 bisphosphoglycerate 0.001 mM

P2 =

NADH 0.000003 mM

ΔG for these conditions

-1.3 kJ/M
Note: The relatively high concentration of phosphate (1.0 mM) and the very low concentration of NADH (0.000003 mM) change the direction that this reaction goes (recall that these concetrations represent an isolated time in the cell and are subject to change as the physiological state changes) from that it would prefer to go if all things were equal.

Mechanism for Chemistry step one

NAD dependent redox reaction

Complete Mechanism for Enzyme


Glyceraldehyde-3-P Dehydrogenase.Animation of the G-3-P dehydrogenase reaction Blue:represents the enzyme. The E-S-H represents the crucial enzyme active site amino Cysteine in its protonated state. "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"

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

Pictures of Enzyme with substrate

  1. Ribbons Only the main chain is represented by these ribbons. There are four identical subunit
  2. one subunit Three of the subunits are deleted.
  3. as above with substrate Same picture as in "2" but and a substrate is added in. The atoms colored spheres. C=Gray; O=red; P=Orange.
  4. nearby AA AA near the subsrate are added as sticks
  5. NAD+ is added.
  6. Ribbons removed
  7. as above different orientation Active site AA + substarte + NAD but slightly different orientation to highlight the spatial orientation of the critical cysteine, NAD substrate and HIS
  8. C2 of NAD=magenta HIS=cyan exactly as above but the C4 of NAD is colored magenta (this is the carbon that picks up the hydride) and the HIS is colored cyan.
Click on an atom to diplay identity here
Messages about the currently highlighted features