Phosphoenolpyruvate Carboxykinase Information

Enzyme Name	Phosphoenolpyruvate Carboxykinase

Reaction Catalyzed	Addition of phosphate to pyruvate with comcomitant aldol cleavage of CO₂ from Oxaloacetate
Reaction Type	Concomitant Group Transfer and Aldol Reactions
Rationale	This is the second half of the two reactions that are required to bypass the pyruvate kinase reaction of glycolysis. This one takes the smae carboxyl group that was just put onto pyruvate by the pyruvate carbolylase enzyme. This uses the Free Energy of generating a CO₂ from and Aldol cleavage to drive the "enol" formation and then the the "enol" attacks GTP for a group transfer reaction. while the ΔG^o' is not very far from 0, this reaction is virtually irreversible toward product phosphoenolpyruvate. The reason is that CO₂ concentrations are very low in the tissues. We go to great lengths to keep our CO₂ concentration low. There are two mechanisms: In the tissues, where CO₂ is generated and therefore in relatively high concentration, an enzyme called Carbonic Anhydrase (CA) rapdidly converts CO₂ to HCO₃^- + H⁺ by reaction with water in a reversible reaction. In the lungs, where CO₂ concentration is low (atmosphereic concetrations), the same reaction catalyzed by the same enzyme runs the other way to generate CO₂ so that we can exhale it. A metal ion help the aldol reaction get started by providing a strong positive charge to start the electron pulling.
Pathway Involvement	Gluconeogenesis ONLY
Cofactors/Cosubstrates	a metal ion usually Mn²⁺ is required as a cofactor

DG^o'	-2.2 kJ/M	Starting from standard state and allowing the reaction to come to equilibrium the PEP concentration would end up ~2 times higher than the concetration of oxaloacetate. The Standard Free Energy slightly favors PEP production.
K_eq
Comments

Mechanism for Chemistry
Mechanism for Enzyme	PEP Carboxykinase. Animation of the PEPcarboxykinase reaction Blue: represents the enzyme. The Mn²⁺ represents the crucial enzyme active site metal ion. The GTP overlays the Mn because this is a 2D representation of a 3D map. "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" Notice: how the "positive charge" on the metal ion begins the process of the electron pulling cascade. In a normal aldol cleavage reaction the electron cascade would collapse back towrd the original because of the instability of the enol. In this case, however, the proximity og the GTP allows a different track. The enol actuially attacks the phosphate and a group transfer occurs... generating the final product. 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
	Ribbonsphosphoenolpyruvate carboxykinase protein structure Substrate analog A substrate analog is shown as spheres C=Gray; O=red; S=Orange. The sulfate is in the place where the CO₂ would be ultimately removed. GTP added GTP is added as well as spheres. note the proximity of the triphsophate to the substrate analog Mn added Two Manganese ions are added in bright yellow... they are somewhat hidden from view at the moment substrate and all nearby AAAll amino acid sidechains near the substrate anaolog are turned on Remove ribbons and rotate The protein structure is removed from the picture and then the active site is rotated to view the meatl ions more clearly Substrate arrangement only Last view from the rotation above. active site AA removed Note the proximity of the Mn near the between the substrate and the GTP

	Phosphofructokinase PFK-1 CHIME representation Initial Picture Substrate Analog On/Off GTP On/Off Mn ions On/Off nearby AA in active site On/Off Protein Ribbons Off/On
	Atoms Clicked on in Chime window mouse methods

*= These are concentrations obtained for one set of conditions. These will change as physiology and activity change.