Mina Ghiasi - Department of Chemistry, Faculty of Physics & Chemistry, Alzahra University,Vanak, Tehran, Iran.
Azar Larijani - Department of Chemistry, Faculty of Physics & Chemistry, Alzahra University,Vanak, Tehran, Iran.
Samira Nasiri - Department of Chemistry, Faculty of Physics & Chemistry, Alzahra University,Vanak, Tehran, Iran.

Carbonyl sulfide is the most stable reduced sulfur compound in the troposphere, which plays a key role in the global distribution of sulfur. In this context, COS can surly be regarded as a natural substrate for carbonic anhydrase (CA) enzyme that catalyzes irreversible hydration according to equation 1.COS + H2O CO2 + H2S (1)The reaction follows the same principle as the carbon dioxide reaction. However there is no experimental and theoretical studies which use the native enzyme contains histidin residues on COS fixation. In addition, despite the numerous studies in this field, many questions are still open [1-4].In this study our attention focus on several mechanistic aspects: (1) the details of nucleophilic attack of the zinc-bond hydroxide ion on COS, (2) study of different transition state trough the reaction path. According to our calculated results, the nucleophilic attack of the zinc bound hydroxide at the C=S bond and results in a four-center transition state is formed and then a zinc bound thiocarbonate is formed. Interestingly, in the course of this reaction, the active form of the catalyst [Zn(II) (his)3(OH)] is converted to its hydrosulfide form [Zn(II) (his)3(SH)] and a water molecule helps to reproduce the active form of the catalyst.