Everything I read says enzymes are organic catalysts and do not supply energy for chemical reactions. My understanding is they speed up reactions by providing alternate pathways that have a lower activation energy. A quiz I took asked "What type of energy do enzymes contribute to chemical reactions?" I answered that enzymes do not contribute energy. The answer I was given was "kinetic energy." In class, the professor said kinetic energy does not affect the thermodynamics or the free energy change in a reaction. He said it was obvious enzymes were contributing kinetic energy because they were bouncing into the substrate. My question is, how is that a contribution? If you contribute energy, you are giving it up, and you are supplying it to the reaction. Yes, the enzymes are of course moving around, and yes, they depend on kinetic stability to hold substrates in place. But are they giving kinetic energy to the reaction, or is there just some interchange going on? I know some enzymes actually operate by placing the substrate under physical stress, but not all enzymes. Otherwise, for every instance where some part of an enzyme imparts momentum to some part of a substrate, there are collisions where the opposite is occuring. I mean, if I gave you 20 bucks and took it back an hour later, you wouldn't say I "contributed" it to you. And if they are giving kinetic energy, how does that not change the energy of the participants in the reaction? If an enzyme imparts kinetic energy to a substrate, I feel like that is definitely if not useful work at least something that changes the free energy in the system. This seems to kind of violate the central dogma I keep hearing about catalysts. What am I getting wrong?