Rubisco from α-Carboxysomes (PDB IDs: 7ZC1, 7ZBT) — BI3323-Aug2025

From Proteopedia

Structure of Rubisco inside α-carboxysomes

Introduction

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the key CO₂-fixing enzyme in the Calvin-Benson cycle.

In many bacteria, Rubisco is packaged within α-carboxysomes, which enhance CO₂ fixation by concentrating bicarbonate and limiting oxygenation.

Although Rubisco structures have been determined in isolation, its native organization inside intact carboxysomes has remained unclear.

Cryo-electron tomography and subtomogram averaging resolved near-atomic structures of Rubisco inside α-carboxysomes from Cyanobium sp. PCC 7001 and Halothiobacillus neapolitanus.

This reveals species-specific packing arrangements and provides insight into Rubisco recruitment and stabilization during carboxysome assembly.

Significance

Carboxysomes significantly enhance carbon fixation, and their efficiency depends on how Rubisco is organized inside the compartment.

Determining Rubisco’s native packing provides insight into bacterial CO₂-concentrating mechanisms and how protein microcompartments achieve high catalytic output.

Structural highlights

Cyanobium Rubisco forms three concentric layers with consistent radial orientation producing a highly ordered internal architecture.

Halo Rubisco assembles into intertwined spiral strings rather than layers.

Neighboring enzymes interact along small-subunit interfaces () () to form continuous chains.

CsoS2 recruitment differs between species:

In Cyanobium, CsoS2 binds Rubisco across all layers, while in Halo it is restricted to shell-proximal regions, indicating distinct recruitment mechanisms.

High-resolution maps reveal active-site residues including carbamylated indicating catalytically competent enzyme assemblies.

References

Ni T, Sun Y, Burn W, Al-Hazeem MMJ, Zhu Y, Yu X, Liu L-N & Zhang P (2022). Structure and assembly of cargo Rubisco in two native α-carboxysomes. Nature Communications, 13:4299. doi:10.1038/s41467-022-32004-w.