Chymotrypsin

Not only is Chymotrypsin a pretty looking enzyme, it also has an elegant reaction mechanism that should satifsy any organic chemist's need.

Chymotrpysin is a protease, an enzyme used to digest proteins into amino acids. To do that it has to cleave peptide bonds. This reaction is thermodynamically favourable in aqueous solution but it has a high activation energy.

According to the XRD-structure chymotrypsin is a homodimer. Its two units can be seen to the left and right. They can be transformed into each other by a 180° rotation (the corresponding axes points away from the screen).

This is one unit. At the bottom a ball and stick model of the active site is included.

The active site consists of serine, histidine and asparte in close proximity. With that system peptide cleavage can be performed at very moderate conditions (pH 8 and body temperature). The mechanism is close to a typical nucleophilic acyl substituition (or Sn_2t as my professor calls it, but I don't think that's an official term). The difference is that you don't have random solvent molecules accepting and donating protons.

I am not going to say any more about the mechanism so you can think about it yourself. I'll post it if I find a good way to draw or model it.