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.
Originally, Danoff and Nivert had planned to sell the song to popular country singer Johnny Cash, but when Denver heard the song and decided he had to have it, the duo who wrote the original lyrics decided not to make the sale. Country music artist Daryle Singletary covered the song on his 2009 album  Rockin' in the Country 2  . In 2010 Swedish singer Meja covered the Studio Ghibli version of the song on her album AniMeja.