[schafer]

Hoping someone could help me understand why a certain reaction is reversible.
I was told that if the ΔG of a reaction is relatively high, then the reaction is irreversible, and the direction of the reaction can be told by whether ΔG is positive or negative.
In glycolysis, fructose 1,6-biphosphate forms glyceraldehyde 3-P and dihydroxiacetone 3-P, with Aldolase enzyme. The ΔG is ~24 kJ/mol.
 
How is it that the the enzyme can cause this reaction without any ATP, if the free energy difference is so high?
How come the two products do not form F16BP again in a spontaneous reaction?

[Irlanur]

I guess you forgot a minus? Fructose 1,6-BP is a very energy rich compound. Enzymes (or catalysts in general) do not change  ΔG. it's just faster. The high energy difference is also the reason why the reaction is practically irreversible. You could also ask why water and carbon dioxide do not spontaneously form methane and oxygen.

[Yggdrasil]

You are correct that ΔG^o for the aldolase reaction is +24 kJ/mol.  However, remember that the ΔG^o refers to the free energy change under standard conditions (all reactants and products at 1M concentration).  The true free energy change associated with a reaction depends on the relative concentrations of products and reactants: ΔG = ΔG^o + RT ln Q, where Q is the reaction quotient (in this case [DHAP][G3P]/[FBP]).  Thus, a reaction with a positive ΔG^o can still occur spontaneously if you have a large excess of reactant and a small amount of product (i.e. Q << 1). 

Indeed, in glycolysis, the aldolase reaction happens after a particularly favorable reaction (the phosphofructokinase reaction) which leads to the build up of FBP. In this way, ATP hydrolysis in the PFK reaction is being indirectly coupled to the aldolase reaction in order to help drive the reaction forward. Similarly, products get depleted by thermodynamically favorable reactions later on in glycolysis (in particular the 3-phosphoglycerate kinase reaction). 

However, you are correct, that aldolase should be able to run in reverse.  In cells undergoing gluconeogenesis, where the favorable PFK reaction is inhibited, aldolase does run in reverse, producing FBP from DHAP and G3P.

For more information see http://onlinelibrary.wiley.com/doi/10.1016/0307-4412%2881%2990102-3/abstract

[schafer]

Thank you very much, that really cleared this up!

[Irlanur]

Have a snack Yggdrasil