Studies of Enzyme Mechanism Transition State

All chemical transformations pass through an unstable structure called the transition state, which is poised between the chemical structures of the substfates and products, whereas the reaction intermediate is a transient stable structure that is formed along the reaction path to or from the transition state. The transition state structure of an enzymatic reaction cannot be observed directly, whereas the reaction intermediate can be observed or can often be isolated/trapped for characterization. 

For situations where the active site is relatively rigid, all chemical species along the reaction coordinate must bear a close resemblance to one another, so the enzyme has no difficnlty in binding all of than. The strain applied by the active site due to its complementarity to one of the transition states forces the substrate into a conformation toward that transition state where all favorable interactions are finally maximized. 

Some enzymes are known to nndergo large conformation changes during catalysis. It is likely that the flexibility allows the active site to adapt to each species along the reaction coordinate. 

The transition state structures of enzyme-catalyzed reactions provide fundamental information about the interactions used to catalyze biological reactions. All chemical transformations pass through the transition states, which are proposed to have lifetimes near 10 s (equivalent to the time for a single bond vibration). Enzymes typically increase the rate over the noncatalyzed reaction by factors of 10 °-10 l The enzyme-substrate complexes often have dissociation constants in the range of 10 -10 M and it has been proposed that transition state complexes are bound with dissociation constants in the range of (Schramm, 1998). 

No physical or spectroscopic methods are available for the direct observation of the transition state structure for enzymatic reactions. Yet transition state structure is central to understanding catalysis, because enzymes function by lowering activation energy. Experimental approaches to investigate the transition state structure of enzymatic reactions include  

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