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If i'd have to pick one i'd pick conformation, if two... i'd add size aswell.
Do you understand what i'am saying or should i explain it further?
The one word answer is "structure" or, synonymously, "conformation." How structure determines catalytic ability is more complicated. The binding energy of the enzyme - what enables the enzyme to bind to its reactants - depends on energetically favourable interactions between the enzyme and the reactant and (to an underappreciated extent) on entropic (i.e., structural) changes to the enzyme induced by binding. That is, the reactant has to fit appropriately into a binding site in the enzyme and the enzyme's structure has to react to binding in certain, energetically favourable ways. This helps the enzyme by providing specificity: binding the reactant it should is energetically favourable compared to binding other possible reactants it shouldn't. Once bound, the properties of the residues which interact with the reactant determine the mechanism or how the reaction actually procedes. These catalytic mechanisms vary: many active sites, like those in kinases, have a metal cofactor which may or may not bond transiently with the reactant; others involve covalent bonds and non bonded interactions between the enzyme and the reactant. All of these interactions force the reactant through a high energy transition state and convert it into product. And all these interactions are driven by structural rearrangements or conformational changes of the enzyme itself. Clear?
For a tl;dr version, (too long; didn't read) in intro classes they say "structure begets function" But for a very literal answer, it's the active site of the enzyme that does the chemical processes, and then it's the sequence of amino acids in the active site (if the enzyme is catalyzed by a protein-based enzyme) that determine the behavior of the active site.