The fourth ligand on the Zn(II) ion is a water molecule. The zinc coordination is tetrahedral. The three histidine residues are sometimes referred to as first coordination sphere ligands. This set of ligands is appropriate to give the water ligand the pKa of 6.5, a value where catalysis is most efficient.
Behind Closed Doors  is a country song written by Kenny O'Dell. It was first recorded by Charlie Rich for his 1973 album Behind Closed Doors. The single was Rich's first number-one hit on the country charts, spent 20 weeks on this chart, and was also a crossover hit on the pop charts. Background vocals were provided by The Nashville Edition.
Not to be confused with Behind Hazel Eyes. Behind These Hazel Eyes is a song recorded by American singer Kelly Clarkson for her second studio album, Breakaway (2004). It was written by Clarkson with the song's producers Max Martin and Dr. Luke. The song was released on April 12, 2005 as the second single from the album.