The catalytic core of an archaeal 2‐oxoacid dehydrogenase multienzyme complex is a 42‐mer protein assembly

The dihydrolipoyl acyl‐transferase (E2) enzyme forms the structural and catalytic core of the tripartite 2‐oxoacid dehydrogenase multienzyme complexes of the central metabolic pathways. Although this family of multienzyme complexes shares a common architecture, their E2 cores form homo‐trimers that, depending on the source, further associate into either octahedral (24‐mer) or icosahedral (60‐mer) assemblies, as predicted by the principles of quasi‐equivalence. In the crystal structure of the E2 core from Thermoplasma acidophilum, a thermophilic archaeon, the homo‐trimers assemble into a unique 42‐mer oblate spheroid. Analytical equilibrium centrifugation and small‐angle X‐ray scattering analyses confirm that this catalytically active 1.08 MDa assembly exists as a single species in solution, forming a hollow spheroid with a maximum diameter of 220 Å. In this paper we show that a monodisperse macromolecular assembly, built from identical subunits in non‐identical environments, forms an irregular protein shell via non‐equivalent interactions. This unusually irregular protein shell, combining cubic and dodecahedral geometrical elements, expands on the concept of quasi‐equivalence as a basis for understanding macromolecular assemblies by showing that cubic point group symmetry is not a physical requirement in multienzyme assembly. These results extend our basic knowledge of protein assembly and greatly expand the number of possibilities to manipulate self‐assembling biological complexes to be utilized in innovative nanotechnology applications. Database The final coordinates of the E2 structure have been deposited in the Protein Data Bank (PDB accession code 3RQC) Structured digital • E2 and E2 bind by x‐ray crystallography (View interaction) • E2 and E2 bind by x ray scattering (View interaction) The family of 2‐oxoacid dehydrogenase multienzyme complexes associate into either octahedral (24‐mer) or icosahedral (60‐mer) assemblies, as predicted by the principles of quasi‐equivalence. Surprisingly, the E2 core from Thermoplasma acidophilum assembles into a unique 42‐mer oblate spheroid showing that a macromolecular assembly of identical subunits can form an irregular protein shell via non‐equivalent interactions. These results extend our basic knowledge of protein assembly.