Glyceraldehyde-3-Phosphate Dehydrogenase

Enzyme Name	Glyceraldehyde-3-Phosphate Dehydrogenase

Reaction Catalyzed	two step reaction: Written in the direction of glycolysis Oxidation of Glyceraldehyde-3-phosphate to glycerate-3-phosphate addtion of phosphate to C₁ This order reverses for gluconeogenesis, of course
Reaction Type	Two Step Reaction Oxidation/Redxution (REDOX) Hydrolysis type reaction (with a twist here since phosphate (instead of water) is used to attack it is called phosphorolysis)
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

Rationale

this enzyme demonstrates how acid, base and covalent calatysis ALL work together - not in isolation but in concert - to affect the reaction. (See the mechanism tab)

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 because of a covalently bound intermediate 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 hydrolysis of phosphate from an organic acid (phosphoanhydride) is MUCH higher than that for hydrolysis of a phosphate from an alcohol. This extra energy is indeed used in the next enzyme 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

ΔG^o'	+6.3 kJ/M
K_eq		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 equilibrium.
Comments	Starting from standard state (all concentrations 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 concentrations of 1,3 bisphosphoglycerate NADH. The Standard Free Energy favors Gluconeogenesis direction. This reaction is made possible because of two factors: Coupling of the phosphate addition to the NAD⁺ linked redox 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 concentrations 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
Complete Mechanism for Enzyme	Hover your mouse over a step to reveal a description
	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 redox/phosphorolysis 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


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Formation of Covalent Intermediate

Oxidation of glyceceraldehyde

Stable intermediate

Phosphate Attacks instead ot water

Done

Anaerobic Glucose Metabolism

Glyceraldehyde-3-Phosphate Dehydrogenase Information

Glyceraldehyde-3-Phosphate Dehydrogenase