How Does the Exchange of One Oxygen Atom with Sulfur Affect the Catalytic Cycle of Carbonic Anhydrase?

Abstract

We have extended our investigations of the carbonic anhydrase (CA) cycle with the model system [(H(3)N)(3)ZnOH](+) and CO(2) by studying further heterocumulenes and catalysts. We investigated the hydration of COS, an atmospheric trace gas. This reaction plays an important role in the global COS cycle since biological consumption, that is, uptake by higher plants, algae, lichens, and soil, represents the dominant terrestrial sink for this gas. In this context, CA has been identified by a member of our group as the key enzyme for the consumption of COS by conversion into CO(2) and H(2)S. We investigated the hydration mechanism of COS by using density functional theory to elucidate the details of the catalytic cycle. Calculations were first performed for the uncatalyzed gas phase reaction. The rate-determining step for direct reaction of COS with H(2)O has an energy barrier of deltaG=53.2 kcal mol(-1). We then employed the CA model system [(H(3)N)(3)ZnOH](+) (1) and studied the effect on the catalytic hydration mechanism of replacing an oxygen atom with sulfur. When COS enters the carbonic anhydrase cycle, the sulfur atom is incorporated into the catalyst to yield [(H(3)N)(3)ZnSH](+) (27) and CO(2). The activation energy of the nucleophilic attack on COS, which is the rate-determining step, is somewhat higher (20.1 kcal mol(-1) in the gas phase) than that previously reported for CO(2). The sulfur-containing model 27 is also capable of catalyzing the reaction of CO(2) to produce thiocarbonic acid. A larger barrier has to be overcome for the reaction of 27 with CO(2) compared to that for the reaction of 1 with CO(2). At a well-defined stage of this cycle, a different reaction path can emerge: a water molecule helps to regenerate the original catalyst 1 from 27, a process accompanied by the formation of thiocarbonic acid. We finally demonstrate that nature selected a surprisingly elegant and efficient group of reactants, the [L(3)ZnOH](+)/CO(2)/H(2)O system, that helps to overcome any deactivation of the ubiquitous enzyme CA in nature.

Observations of the uptake of carbonyl sulfide (COS) by trees under elevated atmospheric carbon dioxide concentrations

“…We came to similar conclusions investigating the uptake of COS by decaying leaf litter (no active stomata) with decreasing uptake of COS under high respiration rates (Kesselmeier and Hubert, 2002). Furthermore, studies modeling the consumption of COS by carbonic anhydrase (Schenk et al, 2004;Notni et al, 2007) demonstrate the similarity of the enzymatic handling of COS as compared to CO 2 . If we have to assume that CO 2 and COS compete for the same binding site, we cannot exclude competitive inhibition, especially as we measured under an 800 ppm growth regime.…”

“…The biological background for the uptake of COS by vegetation is understood to be the combined action of the carboxylation enzymes Ribulose-1,5-bisphosphate carboxylaseoxygenase (Rubisco; EC 4.1.1.39), Phosphoenolpyruvate Carboxylase (PEP-Co; EC 4.1.1.31) and the key enzyme carbonic anhydrase (CA; EC 4.2.1.1), which were previously reported to be involved in the exchange of carbon dioxide (CO 2 ) and carbonyl sulfide (COS) (Protoschill-Krebs and Protoschill-Krebs et al, 1995Schenk et al, 2004;Yonemura et al, 2005;Notni et al, 2007). This enzymatic model consisting of three enzymes assigns a key role for CA and has been confirmed very recently by Stimler et al (2011).…”

Bacterial CS2 Hydrolases from Acidithiobacillus thiooxidans Strains Are Homologous to the Archaeal Catenane CS2 Hydrolase

“…As the CS 2 hydrolase enzymes are highly homologous to the ␤-CAs, it is possible that ␤-CAs may be able to hydrate CS 2 as well as CO 2 . Although the possibility of this conversion was predicted quantum chemically (24)(25)(26), it has never been experimentally confirmed. In addition, we tested potential CS 2 hydrolase activity of bovine erythrocyte ␣-CA, the ␥-CAs Cam from Methanosarcina thermophila (38), Cab from Methanobacterium thermoautotrophicum (39), and Cam from Pelobacter carbinolicus (kindly made available by R. Siva Sai Kumar and James G. Ferry) and the ␤-CA from Streptococcus pneumoniae (40,41), but none of these could convert CS 2 .…”

“…However, in the case of Acidianus CS 2 hydrolase, two of these rings interlock, forming a highly unusual hexadecameric catenane structure, both in the crystal form and in solution (21,23). Intriguingly, despite the high homology with CAs, the Acidianus CS 2 hydrolase could not use CO 2 as a substrate, and CAs have not been found to use CS 2 as a substrate, although the conversion is theoretically possible (24)(25)(26). Recently, a COS hydrolase enzyme was purified from Thiobacillus thioparus strain THI15, which is also a ␤-CA homologue.…”

Reviews and Syntheses: Carbonyl Sulfide as a Multi-scale Tracer for Carbon and Water Cycles

“…Some of the enzymes involved in CO 2 assimilation by leaves 5 (mainly carbonic anhydrase, CA) also efficiently destroy OCS, so that leaves consume OCS whenever they are assimilating CO 2 , (Protoschill-Krebs and Kesselmeier, 1992;Schenk et al, 2004;Notni et al, 2007). Moreover, because the two molecules diffuse from the atmosphere to the enzymes along a shared pathway, the rates of OCS and CO 2 uptake tend to be closely related (Seibt et al, 2010) -although the rate of OCS is about 1 million times lower than that of CO 2 , owing to the ratio of their natural 10 abundances.…”