Transition State

∞ generated and posted on 2016.12.16 ∞

Unstable chemical structure found approximately halfway between substrate or reactant and product.

When converting from one state to another, there has to be a transition in the middle, and the Transition State is part of that transition, particularly such that the transitioning entity is just at the point where it can 'fall', energetically, towards becoming the ending or product state.

Enzymes function in part by stabilizing the transition state, though keep in mind that all chemical reactions pass through transition states even if not catalyzed. The difference is that enzymes as well as catalysts in general increase rates of chemical reactions by stabilizing the transition state.

Note that use of the word "approximately" is to denote somewhere between the structure of reactant and product rather than in any way literally halfway.

Figure legend: Transition states occupy a structural and chemical 'space' partway between reactants/substrates and products, and this is so regardless of whether the chemical reaction is or is not catalyzed and is or is not catalyzed by an enzyme. Literally this is the somewhat unstable, and therefore ephemeral, transitional structure found between what you are starting with and what you are ending up with in a chemical reaction.

Stabilization of the transition state occurs by bringing substrates into the same location, properly orienting substrates, providing specific chemical microenvironment that are conducive to the progress of specific chemical reactions, straining the shape of substrates so that they "look" more like products, etc.

Importantly, these efforts, precisely directed, result in much less expenditure of energy than when transition states are randomly arrived at, such as via an application simply of large amounts of heat to systems.

Why is the transition state an unable structure? Generally transitions between states are unstable. If they weren't unstable then they would not be a transition but instead an end point! Basically, things hang around in whatever state is the most stable for them, and to nudge them away from that stability you have to put in energy of some sort. This can create a state that is partway between two stable states, and this is exactly what a transition state is. Think of a ball in a valley. If you wanted to get the ball over to the next valley you would have to push that>ball up to the top of whatever pass separates the valleys. But at the top of the pass the position of the ball is unstable because then it could roll downhill, including back down to the original state at the bottom of the valley. Indeed, at the very middle of the pass between the two valleys, it may be able to fall in either direction, and thus the mountain pass, at that point, is the transition state.

What happens if you stop putting energy into a catalyzed reaction? You fail to reach the transition state, and thus you fail to achieve catalysis.