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.
Jenkins had written the song when working with the Isham Jones orchestra, and Jones allegedly rejected it as it was too sad. Music critic Alec Wilder described Goodbye as as sad a song I know and Leonard Feather called it among his top ten songs it would be hardest to tire of hearing.he song was used as the closing theme for radio broadcasts by the Benny Goodman orchestra, and was recorded several times by Goodman in the mid 1930s. Upon Goodman's death in 1986, Richard Stoltzman commissioned an arrangement of Goodbye by Bill Jenkins for clarinet and strings.