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 truth behind The Green Door. Still knocking on the Green Door ... Shakin' Stevens performing in 2005. The truth behind The Green Door. In his compilation of gay pop anthems, From the Closet to the Charts: Queer Noises 1961-1978, Jon Savage missed a trick: he never told us what was going on behind The Green Door.