Reversible inhibition of CO 2 fixation by ribulose 1,5‐bisphosphate carboxylase/oxygenase through the synergic effect of arsenite and a monothiol

• Published in: Plant, cell and environment cell and environment, 2013-06, Vol.36 (6), p.1160-1170
• Main Authors: SUDHANI, HEMANTH P. K., GARCíA‐MURRIA, MARíA‐JESúS, MORENO, JOAQUíN
• Format: Article
• Language: English
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.12050

The photosynthetic carbon‐fixing enzyme, ribulose 1,5‐bisphosphate carboxylase/oxygenase ( R ubisco), is reversibly inhibited by micromolar arsenite in the presence of an enhancer monothiol such as cysteine, cysteamine, 2‐mercaptoethanol or N ‐acetylcysteine, but not glutathione. Arsenite reacts specifically with the vicinal C ys172‐ C ys192 from the large subunit of Rubisco and with the monothiol to establish a ternary complex. The combination of arsenite and monothiol stops also the carbon fixation in illuminated cultures of C hlamydomonas reinhardtii but the in vivo arrest appears to be driven by an undetermined effector that is even more sensitive to the arsenite‐thiol synergism than R ubisco. The activity of the photosynthetic carbon‐fixing enzyme, ribulose 1,5‐bisphosphate carboxylase/oxygenase ( R ubisco), is partially inhibited by arsenite in the millimolar concentration range. However, micromolar arsenite can fully inhibit R ubisco in the presence of a potentiating monothiol such as cysteine, cysteamine, 2‐mercaptoethanol or N ‐acetylcysteine, but not glutathione. Arsenite reacts specifically with the vicinal C ys172‐ C ys192 from the large subunit of R ubisco and with the monothiol to establish a ternary complex, which is suggested to be a trithioarsenical. The stability of the complex is strongly dependent on the nature of the monothiol. Enzyme activity is fully recovered through the disassembly of the complex after eliminating arsenite and/or the thiol from the medium. The synergic combination of arsenite and a monothiol acts also in vivo stopping carbon dioxide fixation in illuminated cultures of C hlamydomonas reinhardtii . Again, this effect may be reverted by washing the cells. However, in vivo inhibition does not result from the blocking of R ubisco since mutant strains carrying R ubiscos with C ys172 and/or C ys192 substitutions (which are insensitive to arsenite in vitro ) are also arrested. This suggests the existence of a specific sensor controlling carbon fixation that is even more sensitive than R ubisco to the arsenite–thiol synergism.