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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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.
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.