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.
Don't Close Your Eyes (Keith Whitley song) Don't Close Your Eyes is a song written by Bob McDill, and recorded by American country music artist Keith Whitley. It was released in March 1988 as third single from his album of the same name. It peaked at number-one in the United States, and number 2 in Canada.