How can amino acid sequences that are very different for proteins be very similar in their 3D structure?

Question

In particular the distinction of myoglobin and hemoglobin. The additional part to this question is described as "Consider the location and type of sequence alterations that might have little effect on the overall protein fold (i.e. alterations that conserve protein structure) but may favor the formation of a multisubunit protein (i.e. alterations that alter protein interactions)". Thank you in advance!

Answer 1

Hi Lola...the first thing to keep in mind is that proteins are three-dimensional critters. They are in many cases very long chains of monomers called amino acids. The second thing thing is that these amino acids have several different chemical "personalities" - that means some are nonpolar (they like oil and hate water), polar (they hate oil and love water), acidic, and basic. All 20 amino acids fall into one of these categories. Now, when you have a particular chain of amino acids like globin it is composed of about 141 or 145 amino acids, each of a particular chemical personality, the order and identity of each of those amino acids will determine which may interact with others in same chain and hence the "chain" becomes a folded "ball" of sorts that is very predictable in how it folds and thus its 3-dimensional shape. Now, to answer (finally!) your question about alterations - sometimes a different amino acid is substituted because of a mutation in the DNA that coded for it. If that amino acid has a very similar chemical property to the one it replaced the effect may be minimal on the overall protein - the protein may "not notice the difference" with the new amino acid and therefore not change too awful much despite there being a switch. In a similar way a polypeptide (fancier term for "chain of amino acids") composed of many different amino acids but have similar chemical properties and are in a similar order may fold into a very similar shape. Hemoglobin is a multisubunit protein - it has four separate folded chains that interact to form a much larger entity.