Active Site
The specific region of a catalyst or enzyme at which the reacting molecules bind and are transformed.
Unit
Dimensionless
In depth
On a solid catalyst active sites are typically steps, kinks and defects where surface atoms have unsatisfied bonding capacity. In enzymes the active site is a precisely shaped pocket that binds the substrate, strains it towards the transition state and often supplies acidic or basic groups.
Examples in the real world
The zinc-containing pocket of carbonic anhydrase; the under-coordinated edge atoms of a platinum nanoparticle.