Protein families arise when a protein sequence that generates a stable fold diverges over many generations and acquires new functions. One example of this can be seen in the globin family. Myoglobin is a stable monomeric protein that can help carry oxygen using a heme molecule. Hemoglobin is stable as a tetramer [Quarternary structure comprising of 4 units]. It also carries oxygen through the use of heme groups, but it is useful over a much more dynamic range of oxygen than myoglobin (because each monomer can carry oxygen).
The “globin fold” is structurally conserved across these proteins, but the ability to tetramerize arose through genetic drift and natural selection.
Provide an explanation for how the globin protein sequence can change and still produce the same overall fold. Support your explanation by
– Suggesting the location and type of sequence alterations that might have little effect on the overall protein fold. Which amino acids (in the context of where they are in the structure are the best candidates for mutations?
– Which kinds of mutations would not alter the fold but then may favor the formation of multi- subunit protein?