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We know that Phosphatase is a type of enzymes that removes a phosphate group.

Why can't phosphatases add a phosphate group if we know that all enzymes reactions are reversible ?

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  • $\begingroup$ @TheLastWord I think the question was quite clear - maybe you can explain better what about it you find unclear? $\endgroup$ – Bryan Krause Aug 29 '18 at 17:17
  • $\begingroup$ @BryanKrause I am just wondering if he meant to ask if Phosphatase enzyme can in itself add the phosphate group of if we can add it externally thorough some means or he is commenting on the reversibility of the reaction in general. $\endgroup$ – The Last Word Aug 29 '18 at 17:20
  • $\begingroup$ @TheLastWord Does my edit help? Maybe the 'it' in the second sentence was unclear, but I feel like this question addresses a fairly common misconception to biochemistry novices so I'm fairly confident I've extracted the correct intended meaning. $\endgroup$ – Bryan Krause Aug 29 '18 at 17:23
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    $\begingroup$ @BryanKrause That should be sufficient. $\endgroup$ – The Last Word Aug 29 '18 at 17:36
  • $\begingroup$ Enzymes make reactions happen faster in both directions. If the chemistry favours removing a phosphate an enzyme cannot change that. Only get there faster (or maybe slower too I guess.) $\endgroup$ – Cell Aug 29 '18 at 18:26
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I think the description of enzymatic reactions as 'reversible' is potentially a bit misleading. It might be better to think of enzymes as agnostic to reaction direction.

The big missing piece is the difference in free energy of the reaction itself. Reactions that remove a phosphate group are energetically favorable in biological conditions. A phosphatase will effectively never add a phosphate group back because one would have to supply additional energy to make the reaction proceed in that direction.

Phosphatases catalyze a reaction that results in a free phosphate ion. However, kinases that add phosphate groups are catalyzing quite a different reaction, where a phosphate group is lost from a molecule of ATP which becomes ADP. This reaction is also energetically favorable, and the reverse reaction does not occur.

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  • $\begingroup$ The feasibility of an enzymatic reaction depends on the activation rather than reaction free energy. It is the climbing up the hill that requires effort. The effort depends on the difference in elevation between the valley and the top of the hill, not on the difference in the elevation between the two valleys on the opposite sides of the hill. $\endgroup$ – user37894 Aug 30 '18 at 16:47
  • $\begingroup$ @MartinKlvana True, but the reversibility of the reaction depends on the free energy. The question is more about reactions proceeding in both directions rather than the reaction being feasible in at least one of the directions. If there is little difference in elevation between the two valleys, the reaction will go in both directions (or neither at all if the activation energy is too high); if there is a huge difference, one direction may be feasible and the other is not. $\endgroup$ – Bryan Krause Aug 30 '18 at 16:50
  • $\begingroup$ The reversibility depends on the feasibility of the reverse reaction. And the feasibility of the reverse reaction depends on the activation free energy barrier of the reverse reaction, and it is the barrier for the reverse reaction that determines the rate of the reverse reaction, not the reaction free energy of the reverse reaction. And it is the rate of the reverse reaction that determines the possibility of the reverse reaction occurring in biologically meaningful time. $\endgroup$ – user37894 Aug 30 '18 at 16:55
  • $\begingroup$ @MartinKlvana Right, but the difference in the reaction directions is critically dependent on the difference in energy between the products on each side of the reaction: that is what determines the asymmetry in reaction rates. The energy of the intermediate state is the same in either direction. $\endgroup$ – Bryan Krause Aug 30 '18 at 17:02
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    $\begingroup$ @MartinKlvana I know we are really talking about the same thing here and going in circles around each other. I think it's maybe important that I clarify where I am coming from here...for someone having trouble understanding these concepts, I want to avoid talking about differences in the activation energy because people are taught to think about the purpose of enzymes as reducing the activation energy, which they do, but that gets confusing then when you are talking about the two directions. I want to emphasize that the reaction won't go in a direction where the products are too high energy. $\endgroup$ – Bryan Krause Aug 30 '18 at 18:47
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Not all enzyme catysed reaction are reversible reactions .reversible reactions are those which have free energy change between product and reactant sholud approximately or exactly equal to zero. Phosphatase canot do the reverse of the reaction because if both the reactions where done by the same enzyme in both the direction then a cell will not be having the ability to decide weather forward or backward reaction that should actually take place at any given time.

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