Isomerases are a broad class of enzymes which catalyze structural change in a single molecule in biological systems. These structural changes can be extremely various, from very simple modifications such as the reorientation from levo to dextro alignment in the following amino acid:

     COO-       alanine          COO-  
      |         racemase          |
H3N+--C--H      ------>         H--C--N+H3
      |                           |
     CH3                         CH3
  L-alanine                    D-alanine

Up to the fantastically complex topology topological modifications in DNA Topoisomerases, which uncoil and recoil long strands of DNA to allow reading and copying.