I'm wondering if it is possible, theoretically, to create compounds which perform the opposite function of a catalyst (thus an anticatalyst). That is to say, could a compound be made which raises the activation barrier of a reaction rather than lowers it?
I know enzyme inhibitors exist, but I would argue these merely disable or slow the catalyzed reaction rather than slowing the reaction down relative to its baseline speed. (If I'm mistaken and there are enzyme inhibitors that actually lead to a slower reaction than if the enzyme weren't there at all, I would also consider that an answer).
I would think an anticatalyst could be made by having an enzyme which binds a molecule in a way that reduces the favorability of the transition state (or alternatively, stabilizes the reactants, as mentioned by Nicolau Saker Neto below). The binding to the anticatalyst enzyme would also have to be favorable enough where the reactants would not just react and ignore the slower anticatalyzed pathway (one could also assume, for the sake of argument, that enough of the anticatalyst is added where the majority of the reactant molecules are bound).