The missing link in COS metabolism: A model study on the reactivation of carbonic anhydrase from its hydrosulfide analogue

Carbonic anhydrose (CA) is known to react with carbonyl sulfide, an atmospheric trace gas, whereby HS is formed. It has been shown that, in the course of this reaction, the active catalyst, the His(3)ZnOH structural motif is converted to its hydrosulfide form: His,His(3)ZnOH+COS -> His(3)ZnSH+CO2. In this study, we elucidate the mechanism of reactivation of carbonic anhydrose (CA) from its hydrosulfide analogue by using density functional calculations, a model reaction and in vivo experimental investigation. The desulfuration occurs according to the overall equation His(3)ZnSH + H2O reversible arrow His(3)ZnOH+H2S. The initial step is a protonation equilibrium at the zinc-bound hydrosuffide, The hydrogen sulfide ligand thus formed is then replaced by a water molecule, which is subsequently deprotonoted to yield the reactivated catalytic centre of CA. Such a mechanism is thought to enable a plant cell to expel H2S or rapidly metobolise it to cysteine via the cysteine synthase complex. The proposed mechanism of desulfurotion of the hydrosulfide analogue of CA can thus be regarded as the missing link between COS consumption of plants and their sulfur metabolism.