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Carbonyl sulphide (COS) is an important trace gas of the atmosphere. Considerable uncertainties remain concerning the global sinks of COS. Vegetation is believed to be an unquantified sink in the global cycle of COS. We investigated whether there is an enzymological background for the consumption of COS by higher plants in analogy to COz. Photometric measurements demonstrated that all enzymes involved in COz assimilation by higher plants can also metabolise COS. The key enzyme for COS metabolism in higher plants is carbonic anhydrase. an enzyme which probably directly splits COS into COB and H2S. Such a pathway would explain the observed deposition of COS to vegetation. Kev wordsAtmospheric trace gases, carbonic anhydrase, CA, carbonyl sulphide, COS. COS metabolism of higher plants. phosphoenolpyruvate carboxylase. PEP-Co, ribulose-1.5-bisphosphate carboxylase, Rubisco, vegetation. References sulphur gas emissions from soils in eastern and south eastern United States. J. Air. Pollut. Control Ass. -Direct measurements of emission rates of some atmospheric biogenic sulphur compounds. Tellus 31 (1979a). 174-178. Atmospheric OCS: Evidence for a contribution of anthropogenic sources? J. Geophys. Res. 95/12 (1990),20617-20622. Brown, K. A. and Bell, J. N. B. -Vegetation -The missing sink in the global cycle of carbonyl sulphide (COS). Atmos. Environ. 20/ 3 Brown, K. A.. Kluczewski, S. M.. and Bell. J. N. B. -Metabolism of (35S)-carbonyl sulphide in perennial ryegrass (Loliurn perenne L.) and radish (Raphanus satiuus L,). Environ. and Exp. Bot. 26/ 4 Carroll, M. A,. Heidt. L. E., Cicerone, R. J., and Prinn. R. G. -OCS. H,S and CS2 fluxes from a salt water marsh. J. Atmos. Chem. 4 (1986). 375-395. Chengelis. C. P. and Neal, R. A. -Hepatic carbonyl sulphide metabolism. Biochem. Biophys. Res. Comm. 9013 (1979). 993-999. Chengelis. C. P. and Neal, R. A. -Studies of carbonyl sulphide toxicity: Metabolism by carbonic anhydrase. Toxicology and Applied Pharmacology 55 (1980).

Carbonic anhydrase (CA) is known to react with carbonyl sulfide, an atmospheric trace gas, whereby H(2)S 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(3)ZnOH+COS-->His(3)ZnSH+CO(2). In this study, we elucidate the mechanism of reactivation of carbonic anhydrase (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+H(2)O right harpoon over left harpoon His(3)ZnOH+H(2)S. The initial step is a protonation equilibrium at the zinc-bound hydrosulfide. The hydrogen sulfide ligand thus formed is then replaced by a water molecule, which is subsequently deprotonated to yield the reactivated catalytic centre of CA. Such a mechanism is thought to enable a plant cell to expel H(2)S or rapidly metabolise it to cysteine via the cysteine synthase complex. The proposed mechanism of desulfuration of the hydrosulfide analogue of CA can thus be regarded as the missing link between COS consumption of plants and their sulfur metabolism.