Abstract
β-carbonic anhydrases (βCA) accelerate the equilibrium formation between CO2 and carbonate. Two plant βCA isoforms are targeted to the chloroplast and represent abundant proteins in the range of >1% of chloroplast protein. While their function in gas exchange and photosynthesis is well-characterized in carbon concentrating mechanisms of cyanobacteria and plants with C4-photosynthesis, their function in plants with C3-photosynthesis is less clear. The presence of conserved and surface-exposed cysteinyl residues in the βCA-structure urged to the question whether βCA is subject to redox regulation. Activity measurements revealed reductive activation of βCA1, whereas oxidized βCA1 was inactive. Mutation of cysteinyl residues decreased βCA1 activity, in particular C280S, C167S, C230S, and C257S. High concentrations of dithiothreitol or low amounts of reduced thioredoxins (TRXs) activated oxidized βCA1. TRX-y1 and TRX-y2 most efficiently activated βCA1, followed by TRX-f1 and f2 and NADPH-dependent TRX reductase C (NTRC). High light irradiation did not enhance βCA activity in wildtype Arabidopsis, but surprisingly in βca1 knockout plants, indicating light-dependent regulation. The results assign a role of βCA within the thiol redox regulatory network of the chloroplast.
Highlights
IntroductionSince the beginning of industrialization in the 18th century, the CO2 concentration is exponentially increasing and has reached 417 ppm in May 2020 (Mouna Loa, update 5th of June 2020)
The atmospheric CO2 concentration in the preindustrial era was close to 280 ppm
In contrast to βCA1 from A. thaliana, the αCA from Sulfurihydrogenibium azorense, the carbonic anhydrase (CA) known for its high catalytic efficiency [32], remained active upon oxidation (Figure S1), which could be due to the fact that no cysteine is involved in the zinc binding [33]
Summary
Since the beginning of industrialization in the 18th century, the CO2 concentration is exponentially increasing and has reached 417 ppm in May 2020 (Mouna Loa, update 5th of June 2020). Despite this increase, the current CO2 concentration is still limiting for C3 photosynthesis where the lyase ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO) carboxylates ribose-1,5-bisphosphate. The current CO2 concentration is still limiting for C3 photosynthesis where the lyase ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO) carboxylates ribose-1,5-bisphosphate This reaction is limited by the low KM(CO2) of RubisCO in the range of 10–12 μM, and the competing reaction of RuBP oxygenation with a KM(O2) of about 600 μM [1]. Shell breakage may cause carboxysome loss in cyanobacteria [4]
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