Insights into molecular mechanism underlying carbon fixation inhibition of rice induced by cadmium

Abstract

Cadmium (Cd) contamination poses a significant threat to the carbon fixation potential of farmland ecosystems, yet the molecular mechanisms underlying its inhibitory effects remain poorly understood. This study reveals that Cd competitively binds to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the key enzyme in photosynthetic carbon fixation, by displacing its native co-factor, magnesium (Mg). Both Cd²⁺ and Mg²⁺ bind to identical sites on Rubisco, forming a hexacoordinated complex with the oxygen atoms of ribulose-1,5-bisphosphate (RuBP) and key residues in Rubisco, including Asp203, His294, Glu204, and Lys201. While the binding affinity and stability of the Cd²⁺-Rubisco-RuBP complex are comparable to those of the Mg²⁺-Rubisco-RuBP complex, Cd²⁺ markedly shifts the catalytic activity of Rubisco from carboxylation to oxygenation. This shift results in the accumulation of 2-phosphoglycolate (2-PG), a photorespiration byproduct, by up to 11.57-fold. Consequently, the enhanced photorespiration pathway increases CO₂ release, leading to a significant reduction in net CO₂ fixation and ultimately inhibiting rice growth under hydroponic conditions. By elucidating the molecular mechanism through which Cd disrupts Rubisco’s dual catalytic activity, this study advances our understanding of how heavy metals impair carbon metabolism and carbon sequestration in plants, offering critical insights for mitigating Cd-induced carbon sink losses in cropland.

Graphical abstract

Keywords

Cadmium / Rice / Carbon fixation / Photorespiration / Metal-protein interaction

Highlight

	● Cd2+ can competitively bind to the same active site of Rubisco as Mg2+.
	● Cd2+ equipped Rubisco with high binding capacity to its substrate RuBP.
	● Cd2+ shifted the catalytic activity of Rubisco from carboxylation to oxygenation.
	● The accumulation of oxidation products significantly promoted photorespiration.
	● Rice growth was inhibited by the decrease in the net CO2 fixation.

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