Quaternary structure of α-amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase (ACMSD) controls its activity

α-Amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase (ACMSD) plays an important role in l-tryptophan degradation via the kynurenine pathway. ACMSD forms a homodimer and is functionally inactive as a monomer because its catalytic assembly requires an arginine residue from a neighboring subunit. However, how the oligomeric state and self-association of ACMSD are controlled in solution remains unexplored. Here, we demonstrate that ACMSD from Pseudomonas fluorescens can self-assemble into homodimer, tetramer, and higher-order structures. Using size-exclusion chromatography coupled with small-angle X-ray scattering (SEC-SAXS) analysis, we investigated the ACMSD tetramer structure, and fitting the SAXS data with X-ray crystal structures of the monomeric component, we could generate a pseudo-atomic structure of the tetramer. This analysis revealed a tetramer model of ACMSD as a head-on dimer of dimers. We observed that the tetramer is catalytically more active than the dimer and is in equilibrium with the monomer and dimer. Substituting a critical residue of the dimer-dimer interface, His-110, altered the tetramer dissociation profile by increasing the higher-order oligomer portion in solution without changing the X-ray crystal structure. ACMSD self-association was affected by pH, ionic strength, and other electrostatic interactions. Alignment of ACMSD sequences revealed that His-110 is highly conserved in a few bacteria that utilize nitrobenzoic acid as a sole source of carbon and energy, suggesting a dedicated functional role of ACMSD's self-assembly into the tetrameric and higher-order structures. These results indicate that the dynamic oligomerization status potentially regulates ACMSD activity and that SEC-SAXS coupled with X-ray crystallography is a powerful tool for studying protein self-association.