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.
Song of the Week: fun., Carry On. Close the door, hold the phone: Nate Ruess and fun. at Terminal 5. Nate Ruess, frontman of the band fun., frets about his parents. But not the way Eminem or Roger Waters did â unlike most pop stars who have sung and rapped about their moms and dads, heâs got nothing but glowing things to say about them.