Quantification of tertiary structural conservation despite primary sequence drift in the globin fold

The globin family of protein structures was the first for which it was recognized that tertiary structure can be highly conserved even when primary sequences have diverged to a virtually undetectable level of similarity. This principle of structural inertia in molecular evolution is now evident for many other protein families. We have performed a systematic comparison of the sequences and structures of 6 representative hemoglobin subunits as diverse in origin as plants, clams, and humans. Our analysis is based on a 97-residue helical core in common to all 6 structures. Amino acid sequence identities range from 12.4% to 42.3% in pairwise comparisons, and, despite these variations, the maximal RMS deviation in α-carbon positions is 3.02 Å. Overall, sequence similarity and structural deviation are significantly anticorrelated, with a correlation coefficient of —0.71, but for a set of structures having under 20% pairwise identity, this anticorrelation falls to —0.38, which emphasizes the weak connection between a specific sequence and the tertiary fold. There is substantial variability in structure outside the helical core, and functional characteristics of these globins also differ appreciably. Nevertheless, despite variations in detail that the sequence dissimilarities and functional differences imply, the core structures of these globins remain remarkably preserved.