Invariance in the packing of amino acid residues of Cythochrome C and Myoglobin during biological evolution

In spite of the overall irregularity of its tertiary structure, a globular protein may be regarded as a crystal mol. on the basis of its packing which is very similar to that of mol. crystals.. This concept has suggested a quant. anal. of the possible invariance of the packing of homologous proteins, e.g., cytochrome c and myoglobin sequences. The total van der Waals vols. and the total effective vols. were calcd., as well as the packing densities of a large no. of the sequences examd. Clearly, apart from invariants in the primary structure, the van der Waals vol. of a protein remains const., to a very good approxn., during evolution. The same constancy holds for the packing d. Thus, the tertiary structure of homologous proteins tends to be conserved during biol. evolution, in spite of even remarkable changes in the primary structure, and its packing is typical of mol. crystals even if thermal motions are conceivably much higher.