A Sub-Angstrom Geometric Constant in the RuBisCO Large Subunit Reveals a Deeply Conserved Structural Anchor

The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is a cornerstone of global carbon fixation, yet the fundamental physical principles governing its structural conservation remain elusive. While traditionally studied through the lens of sequence-based evolution, we hypothesized that its functional resilience might be dictated by a non-obvious, deeply conserved geometric constant at the atomic level. To test this, we performed a systematic, non-homologous superposition screening between the large catalytic subunits of RuBisCO from two deeply divergent phototrophs, Chlamydomonas reinhardtii (PDB: 1gk8) and Thalassiosira hyalina (PDB: 5n9z). Our analysis identified a previously unreported structural anchor within the catalytically crucial Loop 6. A 21-atom backbone constellation (residues 255-260 in 1gk8 and 191-196 in 5n9z Both alpha-helical) exhibits an extraordinary geometric identity, with a Root-Mean-Square Deviation (RMSD) of only 0.128 Å. The preservation of a multi-residue architecture with sub-angstrom precision across a three-billion-year evolutionary divide challenges conventional models of structural drift. This observation suggests that functional convergence may be constrained by geometric factors to a greater extent than previously appreciated, warranting further investigation into the role of structural homology, independent of sequence, in molecular evolution.