Carbonyl sulfide hydrolase (; abbreviated as COSase) is an enzyme that degrades carbonyl sulfide (COS) to

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

(H₂S) and

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

(CO₂). Isolated from ''

Thiobacillus thioparus ''Thiobacillus'' is a genus of Gram-negative Betaproteobacteria. '' Thiobacillus thioparus'' is the type species of the genus, and the type strain thereof is the StarkeyT strain, isolated by Robert Starkey in the 1930s from a field at Rutgers U ...

'' bacterium, the potential of COSase would reduce the high global warming effect of COS and change the ozone chemistry, because COS is the source of sulfur in the troposphere.

Etymology

Being that it is a hydrolase, which is an enzyme that uses water to break chemical bonds, the name suggests that within the mechanism are water molecules that are involved in disseminating molecules within the reaction. The very name when broken down means that it is an enzyme that breaks down carbonyl sulfide.

History

COSase was isolated, characterized and structure was determined from ''

'' bacterium. In search for a chemical method to break down COS more efficiently than the biologically established methods that employ the soil environment for degradation enzymes. These enzymes are

carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) () form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site ...

, carbonic disulfide hydrolase, nitrogenase,

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

, and RuBisCO. The enzymes listed are limited in their use due to specificities and optimal environments, which is why chemical development of an enzyme unique to catalyzing the degradation of COS is researched. - ''Thiobacillus thioparus'' is a bacterium found both in soil and freshwater and is known for its sulfur-oxidizing properties. The strain used to create COSase is THI11, which was originally isolated as a thiocyanate degrading microorganism. The enzyme was found by putting the extract of ''T. thioparus'' strain THI115 through column chromatography to purify it and ICP-MS to deduce the structure.

Structure

Using sodium dodecyl sulfate–polyacrylamide gel electrophoresis, a subunit molecular mass of 27 kDa was found. After testing for expression in

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

the true molecular mass of ~94 kDa was found by SEC-MALS. ICP-MS shows that there is one zinc ion per sub unit. 35 amino acid sequence found on the N-terminal: MEKSNTDALLENNRLYAGGQATHRPGHPGMQPIQP. There are five strands (β1−β5) that make up

β-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...

core and four α-helices (α1, α2, α3, and α6) in its flank, with two additional helices (α4 and α5) that protrude from its core. They arrange in homodimer pairs to form ten-stranded β-sheets. Between two subunits of a homodimer is the catalytic site. Cys44, His97, Cys 100, and a water molecule coordinate with a zinc ion, with a thiocyanate molecule in the catalytic site pocket.

Function

COSase is responsible for the degradation of COS to H₂S and CO₂ in the second step of SCN₋ assimilation. It hydrolyzes COS with a certain specificity over a wide range of concentrations both ''in vivo'' and ''in vitro''.

Mechanism

Thiocyanate hydrolase (SCNase) found in THI115 initiates enzymatic formation of thiocyanate (SCN⁻). SCNase hydrolyzes SCN⁻ to

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

and COS. The COS that results from the hydrolysis is metabolized to form hydrogen sulfide (H₂S) which is oxidized to sulfate to produce energy. Hydroxide and zinc io ns perform a nucleophilic attack on the carbon in the COS molecule, which creates an intermediate with zinc bound to hydroxide oxygen and sulfur of the COS molecule. Oxygen is then released from zinc and forms CO₂. Water from the solvent interacts with the su lfur-zinc ion and regenerates the active site and releases H₂S.

Carbonyl sulfide hydrolase inhibitor

COSase is weakly inhibited by SCN⁻.