A natural protein, citrate synthase from the cyanobacterium Synechococcus elongatus, forms Sierpiński triangles through self-assembly. This discovery reveals that a metabolic enzyme can assemble into fractal geometries, a phenomenon previously observed only in synthetic systems. Cryo-electron microscopy shows that the enzyme forms regular triangular complexes, with the 18mer representing the first order of the Sierpiński triangle. The fractal assembly is regulated by different stimuli and may not serve a physiological function in vivo. Ancestral sequence reconstruction suggests that the fractal may have evolved as a harmless evolutionary accident. The fractal assembly of citrate synthase is based on a specific interface between dimers that allows the formation of regular triangular structures. The 18mer and 54mer assemblies were analyzed using cryo-EM, revealing their fractal nature through their Hausdorff dimensions. The fractal assembly is also supported by small-angle X-ray scattering data, which shows that the protein can form larger assemblies. The structural basis of the fractal assembly involves a specific interface between dimers that allows the formation of the Sierpiński triangle. The fractal assembly of citrate synthase has functional consequences for the enzyme. The enzyme's activity is regulated by the assembly into fractal complexes, with hexamers being the catalytically active stoichiometry. The fractal assembly also affects the enzyme's conformational dynamics, with the 18mer undergoing a rotation that may hinder substrate binding or catalysis. The fractal assembly is also pH-sensitive, with changes in pH affecting the stability of the fractal structure. The evolution of the Sierpiński assembly was studied using ancestral sequence reconstruction, revealing that the fractal evolved from non-fractal precursors. The fractal assembly is thought to have evolved as a result of a small number of amino acid substitutions, with the crucial substitution q18L playing a key role in the formation of the fractal. The fractal assembly may have evolved as a result of an accidental mutation, with the protein's ability to form larger assemblies being an evolutionary by-product of its unusual symmetry. The study highlights the emergence of fractal geometries in the evolution of a metabolic enzyme, demonstrating that complex and regulatable assemblies can evolve in a single substitution. The findings expand the space of possible protein complexes and suggest that fractal structures may be more common in nature than previously thought. The study also raises questions about the evolutionary significance of fractal assemblies, suggesting that they may be the result of accidental mutations rather than adaptive evolution.A natural protein, citrate synthase from the cyanobacterium Synechococcus elongatus, forms Sierpiński triangles through self-assembly. This discovery reveals that a metabolic enzyme can assemble into fractal geometries, a phenomenon previously observed only in synthetic systems. Cryo-electron microscopy shows that the enzyme forms regular triangular complexes, with the 18mer representing the first order of the Sierpiński triangle. The fractal assembly is regulated by different stimuli and may not serve a physiological function in vivo. Ancestral sequence reconstruction suggests that the fractal may have evolved as a harmless evolutionary accident. The fractal assembly of citrate synthase is based on a specific interface between dimers that allows the formation of regular triangular structures. The 18mer and 54mer assemblies were analyzed using cryo-EM, revealing their fractal nature through their Hausdorff dimensions. The fractal assembly is also supported by small-angle X-ray scattering data, which shows that the protein can form larger assemblies. The structural basis of the fractal assembly involves a specific interface between dimers that allows the formation of the Sierpiński triangle. The fractal assembly of citrate synthase has functional consequences for the enzyme. The enzyme's activity is regulated by the assembly into fractal complexes, with hexamers being the catalytically active stoichiometry. The fractal assembly also affects the enzyme's conformational dynamics, with the 18mer undergoing a rotation that may hinder substrate binding or catalysis. The fractal assembly is also pH-sensitive, with changes in pH affecting the stability of the fractal structure. The evolution of the Sierpiński assembly was studied using ancestral sequence reconstruction, revealing that the fractal evolved from non-fractal precursors. The fractal assembly is thought to have evolved as a result of a small number of amino acid substitutions, with the crucial substitution q18L playing a key role in the formation of the fractal. The fractal assembly may have evolved as a result of an accidental mutation, with the protein's ability to form larger assemblies being an evolutionary by-product of its unusual symmetry. The study highlights the emergence of fractal geometries in the evolution of a metabolic enzyme, demonstrating that complex and regulatable assemblies can evolve in a single substitution. The findings expand the space of possible protein complexes and suggest that fractal structures may be more common in nature than previously thought. The study also raises questions about the evolutionary significance of fractal assemblies, suggesting that they may be the result of accidental mutations rather than adaptive evolution.