Cartoon representation of hen eggwhite lysozyme, the first enzyme structure solved:
http://en.wikipedia.org/wiki/Lysozyme

The catalytic residues are represented as spheres.

The model was created using Protein Data Bank entry 2LYZ:
http://www.rcsb.org/pdb/explore.do?structureId=2lyz

The model was created with VMD:
http://www.ks.uiuc.edu/Research/vmd/