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.
Ray Manzarek, who has died aged 74, was the keyboardist with The Doors, doing every bit as much to shape the bandâs dark, psychedelic sound as its charismatic, Dionysian lead singer, Jim Morrison. The Doors (from left) John Densmore, Robbie Krieger, Ray Manzarek and Jim Morrison Photo: AP. 5:37PM BST 21 May 2013.