text: AC-IV was first reported in the bacterium Aeromonas hydrophila, and the structure of the AC-IV from Yersinia pestis has been reported. These are the smallest of the AC enzyme classes; the AC-IV (CyaB) from Yersinia is a dimer of 19 kDa subunits with no known regulatory components (). AC-IV forms a superfamily with mammalian thiamine-triphosphatase called CYTH (CyaB, thiamine triphosphatase).