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.
The version of Behind Blue Eyes on the original Who's Next album was the second version the band recorded; the first was recorded at the Record Plant in New York on 18 March 1971 and features Al Kooper on Hammond organ. The first version was released as a bonus track on the 1995 CD reissue of Who's Next.