Imperfect symmetry facilitated the evolution of specificity and high-order stoichiometry in vertebrate hemoglobin.

Many molecular complexes are made up of proteins related by gene duplication, but how these assemblies evolve is poorly understood. Using ancestral protein reconstruction and biochemical experiments, we dissected how vertebrate hemoglobin, which comprises two copies each of two related proteins, acquired this architecture from a homodimeric ancestor. Each aspect of this transition - from dimer to tetramer and homomer to heteromer - had a simple genetic basis: a single-site mutation in each protein drove the changes in size and specificity. These rapid transitions were possible because hemoglobin's architecture is symmetric, which amplified the effect of small biochemical changes on the assembly of the complex. Many protein complexes are symmetrical, suggesting that they too may have evolved via simple genetic mechanisms.