Equilibrium & Enzyme Reactions

[bravoghost]

So I think I've hit a roadblock. I see that in a lot of biochem enzymatic reactions, two substances are held in equilibrium (for example in glycolysis, DHAP and GAP are in equilibrium through an enzymatic reaction). However, I've also read that an enzyme will convert a substrate into a product and that the process is irreversible (E + S <---> ES ---> EP <---> E + P, where the middle step is irreversible). So my question is how can both be true? How can a substrate and product be converted back and forth by an enzyme (to establish equilibrium) while at the same time the form of an enzymatic reaction is irreversible?

[cypress]

Strictly speaking all chemical reactions are capable of achieving an equilibrium state. Some so much favor the product side of the equation they are said to be irreversible. Hope this helps.

[Bassman]

So I think I've hit a roadblock. I see that in a lot of biochem enzymatic reactions, two substances are held in equilibrium (for example in glycolysis, DHAP and GAP are in equilibrium through an enzymatic reaction). However, I've also read that an enzyme will convert a substrate into a product and that the process is irreversible (E + S <---> ES ---> EP <---> E + P, where the middle step is irreversible). So my question is how can both be true? How can a substrate and product be converted back and forth by an enzyme (to establish equilibrium) while at the same time the form of an enzymatic reaction is irreversible?

 

 

Enzymes are only catalysts and I think it would help to look at the specific chemical reaction that a given enzyme is catalyzing. Some chemical reactions are inherently reversible (e.g. hydration/dehydration), some are for all intent and purpose irreversible (e.g. if one product is gas which rapidly diffuses away).

[pioneer]

The path from reactants to products moves along a curve such as shown below;

 

An enzyme will lowers the height of the activation energy hill, but often only in one direction. If we go the other way, the original curve applies. This will shift equilibrium, away from the original curve in favor of the direction of the new lower activation energy hump.

 

For example, say the enzyme worked left to right. To use a number, the left-right activation hump is now half as tall. To go the other way, or right to left, the product has to climb the full height as shown above. That means it is now much easier to go left to right, (compared to the pure reaction), shifting the equilibrium to the right.

Edited September 5, 2010 by pioneer