SamGunn

I just figured out why- it's because they don't catalyze the same reactions! The kinase catalyzes ATP + substrate <> ADP + phosphorylated substrate, while the phosphatase catalyzes H2O + substrate <> dephosphorylated substrate.

All enzymes are just catalysts, we have to constantly replenish them because they are constantly broken down. This reaction not being in equilibrium is not an answer to my question, because the kinase and phosphatase would still both move the reaction in the same direction- towards equilibrium. The question is, how can they go opposite directions? What is the explanation that Stryer gives? Edit: I just looked at Stryer and it does not address this. Also, feedback-regulated catalysis is not specific to phosphorylation. That's like saying, "I see your particular example is one of a transferase" and it does not actually adress the question at all.

Right, but I don't see how that works for situations in which two enzymes act antagonistically to eachother. For the kinase/phosphatase example, you would expect that activating a kinase would simply move the reaction to equilibrium so that, when the phosphatase is activated later, nothing would happen because the reaction is already in equilibrium (because of the action of the kinase). But as I understand it, this is not the case- when you activate a kinase, it phosphorylates its substrate, but when you activate its corresponding phosphatase, the concentration of the dephosphorylated species increases.

I just realized something that I do not understand, which is kind of embarassing since I have been learning things that are built on this for the past 2 years without ever actually understanding it. An enzyme is nothing more than a catalyst, and any catalyst increases the rates of the forward and reverse reactions proportionally (i.e. enzymes do not alter the equilibria of the reactions they catalyze). So how can there be kinases and phosphatases that catalyze reactions that are reverses of eachother? Wouldn't a kinase dephosphorylate its substrate just as much as it phosphorylates it, and if both a kinase and a phosphatase act by reducting the activation energy of the same reaction, then shouldn't a kinase and a phosphatase be the exact same? I have only one idea for an explanation of this as of yet, which is that the product can change conformation (after leaving the enzyme) in such a way that it no longer can act as a substrate for the enzyme. For example, some kinase may phosphorylate its substrate, which may then leave the active site and immediately change conformation so that it can no longer bind the kinase. The phosphatase, however, would bind it and convert it back to the original. This seems like a reasonable explanation to me, but I don't know that it is (1) true and (2) universally applicable, and I don't want to just go on it like it is true without knowing since its such a fundamental part of biochemistry.