Enzymatic Pathways for the Consumption of Carbonyl Sulphide (COS) by Higher Plants*

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 unqualified 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 CO₂. Photometric measurements demonstrated that all enzymes involved in C0₂ 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 C0₂ and H₂S. Such a pathway would explain the observed deposition of COS to vegetation.     

Enzymatic consumption of carbonyl sulfide (COS) by marine algae

We show that the marine algae Mantoniella squamata, Prymnesium parvum, and Amphidinium klebsii take up carbonyl sulfide (COS) from their surrounding medium. Inhibitor studies confirm that this COS uptake is catalyzed by the enzyme carbonic anhydrase, which was not detectable with conventional methods. As shown for M. squamata, the COS uptake can be dependent on the growth conditions. Furthermore, COS uptake shows a clear positive correlation with the COS concentration in the growth medium. The value of K_1/2 for the COS uptake was estimated to be around 222 mol/m³. The COS consumption by the marine algae species investigated was estimated to be negligible compared to the photoproduction and hydrolysis of COS in seawater.     

Feedforward non-Michaelis-Menten mechanism for CO(2) uptake by Rubisco: contribution of carbonic anhydrases and photorespiration to optimization of photosynthetic carbon assimilation

Rubisco, the most abundant protein serving as the primary engine generating organic biomass on Earth, is characterized by a low catalytic constant (in higher plants approx. 3s(-1)) and low specificity for CO(2) leading to photorespiration. We analyze here why this enzyme evolved as the main carbon fixation engine. The high concentration of Rubisco exceeding the concentration of its substrate CO(2) by 2-3 orders of magnitude makes application of Michaelis-Menten kinetics invalid and requires alternative kinetic approaches to describe photosynthetic CO(2) assimilation. Efficient operation of Rubisco is supported by a strong flux of CO(2) to the chloroplast stroma provided by fast equilibration of bicarbonate and CO(2) and forwarding the latter to Rubisco reaction centers. The main part of this feedforward mechanism is a thylakoidal carbonic anhydrase associated with photosystem II and pumping CO(2) from the thylakoid lumen in coordination with the rate of electron transport, water splitting and proton gradient across the thylakoid membrane. This steady flux of CO(2) limits photosynthesis at saturating CO(2) concentrations. At low ambient CO(2) and correspondingly limited capacity of the bicarbonate pool in the stroma, its depletion at the sites of Rubisco is relieved by utilizing O(2) instead of CO(2), i.e. by photorespiration, a process which supplies CO(2) back to Rubisco and buffers the redox state and energy level in the chloroplast. Thus, the regulation of Rubisco function aims to keep steady non-equilibrium levels of CO(2), NADPH/NADP and ATP/ADP in the chloroplast stroma and to optimize the condition of homeostatic photosynthetic flux of matter and energy.