Pyruvate Kinase Information

Enzyme Name

Pyruvate Kinase

Reaction Catalyzed

Transfer of phosphate group from phosphenolpyruvate to ADP to make ATP and pyruvate

Reaction Type

Group Transfer Reaction

Rationale

Elmination Reaction. The phosphate has already been placed on C₂ of glycerate from the previous enzyme.

The Enol functional group.  
Just prior to this in the pathway an elimination reaction (hydroxyl -OH group from C₃ and a hydrogen from C₂) generated a bond between C₂ and C₃. Compounds that contain a carbon - carbon double bond (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 tautomerization and can occur extremely quickly. Thermodynamically, the ketone form is greatly favored - therefore 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.
The extra energy from this can be used to transfer the phosphate from PEP to ADP to make ATP.

How much energy is in the Ketone <-> enol conversion. The ΔG^o' as written is +45 kJ/M which meand that the ketone form is favored by a factor of ~65,000,000 over that of the enol form in pyruvate.

Pathway Involvement

Glycolysis ONLY

The favorable thermodynamics of the enol -> ketone tautomerization virtually precludes this reaction from running in the gluconeogenesis direction. Rather TWO enzymes that EACH use an ATP (or equivalent in GTP) are required to get from pyruvate to PEP in the gluconeogenesis direction

1. Pyruvate Carboxylase which makes oxaloacatate (a 4 Carbon compound) using ATP and CO₂.
2. Phosphoenolpyruvate CarboxyKinase which takes a CO₂ off of oxaloacetate and puts a phophate on the enol using GTP (for our purposes GTP is an ATP equivalent since it serves the same function).

Cofactors/Cosubstrates

None

DG^o'

-31.7 kJ/M

Starting from standard state and allowing the reaction to come to equilibrium the Pyruvate and ATP concentration would end up ~20,000 times higher than the product of the concetrations of ADP and PEP.

The Standard Free Energy strongly favors ATP production.

K_eq

Comments

Note: the Standard Free Energy grealy favors production of ATP.

This is the last of the ATPs generated in the glycolysis pathway.

1. it took two ATP (hexokinase and phosphofructokinase-1) to the the pathway started.
2. Two ATP were recovered in the glycerate kinase reaction
3. two more ATP are generated in pyruvate kinase.

"In cell" Substrate Concentrations^*

S₁ =

S₂ =

ADP

0.14 mM

P₁ =

P₂ =

ATP

1.85 mM

DG for these conditions

-23.0 kJ/M

Mechanism for Chemistry

Mechanism for Enzyme

Picture of Enzyme with substrate

1. RibbonsPyruvate Kinase contains 4 identical subunits
2. one subunit Three of the subunits deleted from the picture
3. substrate analog substrate analog added.
4. Mg ion addedMg ion added
5. ATP addedATP added to the picture - notice proximity of the substrates and Mg this is key to catalysis in this enzyme
6. Nearby AAamino acid sidechains near the substrates have been added
7. Remove Ribbon and rotate The ribbon is removed from the picture and then the active site is rotated for a clear picture of the orientation of the key players.