The “Activatory Model” of Plant Response to Moderate Cadmium Stress - Relationship Between Carbonic Anhydrase and Rubisco

Abstract

Rubisco is the key enzyme of the Calvin cycle in C3 plants chloroplasts. It constitutes as much as 60% of total chloroplast protein. Native Rubisco is double octamer, consisting of 8 nucleus-encoded small subunits and 8 chloroplast-encoded large subunits. Its activity is maintained and regulated by sophisticated system including specific regulatory enzyme — Rubisco activase — and a set of internal and external factors (Fig. 1), [1,2,3]. The Rubisco activase is ATP-dependent enzyme which maintains correct structure of Rubisco and its active centres, regulates carbamylation of lysine residue and Mg²⁺ binding, as well as removal of natural Rubisco inhibitors: 5C sugars — metabolites of the regeneration phase of the Calvin cycle and the nocturnal one — CA1P (carboxyarabinitol 1-phosphate) [1,2,3,4,5]. CO₂ is both activator and substrate for Rubisco. Even in optimal conditions, when leaf gas exchange is undisturbed, diffusion processes may be not sufficient for good CO₂ supply, therefore, carbonic anhydrase (CA), enzyme carrying out reversible conversion CO₂↔HCO₃⁻, can be also considered as an important element of this “activation system”, particularly in stress conditions [3,6,7].