[Polleke]

Dear all,

We all know that enzymes catalyse a reaction.
For example
A +B  C with the help of enzyme X
and C  A +B with the help of enzyme Y

This is how I learned it, but: are there enzymes that can catalyse both reactions? (in stead of having two seperate, X and Y, enzymes?)

[Yggdrasil]

All catalysts (including enzymes) will speed up both the forward and reverse directions of a chemical reaction.  So, if an enzyme speeds up the transformation A + B --> C, then it will also speed up the transformation C --> A + B.  The direction the reaction proceeds is generally determined by the thermodynamics of the reaction (in which direction is the free energy change more favorable) and the concentrations of the reactants and products.

Often, in biology, there will be pairs of enzymes that seemingly catalyze opposing reactions.  For example, kinases catalyze the phosphorylation of proteins whereas phosphatases remove the phosphate groups from the proteins.  While these reactions seem to be the forward and reverse of the same reaction, they are, in fact, different chemical reactions.  In the example of kinases and phosphatases, kinases catalyze the reaction:

Protein-OH + ATP --> Protein-PO₄ + ADP

whereas phosphatases catalyze

Protein-PO₄ + H₂O --> Protein-OH + PO₄^3-

As you can see, kinases and phosphatases catalyze two distinct chemical reactions (one is a transesterification, the other is a hydrolysis reaction).

[Polleke]

All catalysts (including enzymes) will speed up both the forward and reverse directions of a chemical reaction.  So, if an enzyme speeds up the transformation A + B --> C, then it will also speed up the transformation C --> A + B.  The direction the reaction proceeds is generally determined by the thermodynamics of the reaction (in which direction is the free energy change more favorable) and the concentrations of the reactants and products.

Often, in biology, there will be pairs of enzymes that seemingly catalyze opposing reactions.  For example, kinases catalyze the phosphorylation of proteins whereas phosphatases remove the phosphate groups from the proteins.  While these reactions seem to be the forward and reverse of the same reaction, they are, in fact, different chemical reactions.  In the example of kinases and phosphatases, kinases catalyze the reaction:

Protein-OH + ATP --> Protein-PO₄ + ADP

whereas phosphatases catalyze

Protein-PO₄ + H₂O --> Protein-OH + PO₄^3-

As you can see, kinases and phosphatases catalyze two distinct chemical reactions (one is a transesterification, the other is a hydrolysis reaction).

I understand the second part of your post, this is what I know (2 different enzymes/reactions), but can you then give an example of an enzyme that speeds up A+B :rarrow:C and at the same time can (under different conditions) speed up C  :rarrow:A+B .

[Yggdrasil]

Many enzymes involved in glycolysis (which breaks down glucose into pyruvate) can also aid in gluconeogenesis (the production of glucose from pyruvate).  See for example the following diagram: http://rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/images/glneosum2.gif

In cells performing glycolysis, phosphoglycerate kinase will primarily catalyze the conversion of 1,3-bisphosphoglycerate into 3-phosphoglycerate.  In cells performing gluconeogenesis, the same enzyme will primarily catalyze the reverse reaction.

[Polleke]

Many enzymes involved in glycolysis (which breaks down glucose into pyruvate) can also aid in gluconeogenesis (the production of glucose from pyruvate).  See for example the following diagram: http://rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/images/glneosum2.gif

In cells performing glycolysis, phosphoglycerate kinase will primarily catalyze the conversion of 1,3-bisphosphoglycerate into 3-phosphoglycerate.  In cells performing gluconeogenesis, the same enzyme will primarily catalyze the reverse reaction.

Ok thanks, just one more question.

If you take for example your link/image I see 2 kinds of arrow, the  ones and the  ones..
Now does the  mean that that particular enzyme only catalyse this reaction and never the opposite one? ANd why is this? Is this because the reactions kinetics are not good for the opposite one or is it becaue that enzyme just cant do it ? (which seems weird becaus in theory all catalysts should be able to catalyse both reactions).

And a  then means that the enzyme can catalyse both reactions? Or does it mean that perhaps the enzyme catalyses 1 reaction and that the other (backwards) reaction can just happen spontanously?

[Yggdrasil]

The direction of a chemical reaction is determined by two factors: 1) the thermodynamics of the reaction, and 2) the availability of products and reactants.

In the pyruvate carboxylase reaction, the enzyme works only in the pyruvate-to-oxaloacetate direction because the thermodynamics of the reaction greatly favors that direction.  You'll note that the reaction is coupled to the hydrolysis of ATP, which provides the large decrease in free energy (ΔG << 0).  Running the reaction in the oxaloacetate-to-pyruvate direction involves the very thermodynamically unfavorable process of synthesizing ATP from ADP and PO₄^3-.

Reaction represented by the reversible arrows indicate reactions where the change in free energy of the reactions is close to zero (ΔG ~ 0).  In these cases, the relative concentrations of products and reactants will determine the direction of the reaction.  For example, if 1,3-bisphosphoglycerate accumulates, phosphoglycerate kinase will facilitate its conversion into 3-phosphoglycerate.  Alternatively, if the 1,3-bisphosphoglycerate gets depleted, the enzyme will work to restore the equilibrium between the two species by converting 3-phosphoglycerate into 1,3-bisphosphoglycerate.

In essence, enzymes speed up the progress of a reaction towards an equilibrium state.  If the equilibrium greatly favors products over reactants, the enzyme will appear to catalyze only the forward reaction.  Even though it is fully capable of catalyzing the reverse reaction, it will very do so because of the thermodynamic unfavorability of the reverse reaction.  If the equilibrium constant is such that products and reactants co-exist in equilibrium, the enzyme will catalyze whichever direction brings the concentration of products and reactants to their equilibrium values.

[Polleke]

Ok, I see.
Thank you very much Yggdrasil, you made it easier for me to understand it.
Thanks a lot.

[Nescafe]

In essence, enzymes speed up the progress of a reaction towards an equilibrium state.  If the equilibrium greatly favors products over reactants, the enzyme will appear to catalyze only the forward reaction.  Even though it is fully capable of catalyzing the reverse reaction, it will very do so because of the thermodynamic unfavorability of the reverse reaction.  If the equilibrium constant is such that products and reactants co-exist in equilibrium, the enzyme will catalyze whichever direction brings the concentration of products and reactants to their equilibrium values.

Does not "equilibrium" mean that the forward rate equals the reverse rate? How can you say "if the equilibrium greatly favors the product"? Wouldn't this mean that the reaction is no longer in equilibrium as one direction is so much more favoured than the reverse?

Nescafe.

[Yggdrasil]

Yes, at equilibrium the forward rate of reaction will equal the reverse rate.  However, this says nothing about the quantity of products and reactants.  When I say the equilibrium favors products, I mean that the concentration of products is much higher than the concentration of reactants at equilibrium.

[Nescafe]

Understood thank you sir.