Life - DNA, Proteins, physiology, behavior, and all - has managed to weather extreme environments - almost every hole we've poked a stick into contains thriving living communities. Proteins are a necessity for living, and therefore tuning protein structures to an extreme environment is of paramount value to an evolving organism seeking an extraordinary niche. In this article we'll present the biophysical strategies apparent from some extreme protein structures.
Extreme myoglobin allows whales and dolphins to dive and stay submerged for long periods of time
The classic myoglobin structure (default scene) was solved by John Kendrew in the mid-1900s,and continues to be a classic in protein structure research. Myoglobin is a relatively small protein at 153 (sometimes 154) amino acids. The polypeptide chain simply folds over the heme ligand, cradling it between halves of the protein chain. But myoglobin research has revealed that proteins are dynamic: myoglobin the protein "breaths" in molecular imitation of our lungs movement, as it changes conformations to take up oxygen and release it. This is one example among many, of the contributions myoglobin has made to the structural biology field of research.
Over half a century after the myoglobin structure was solved, in a fascinating article[1], a team of researchers illuminate how a behavior of animals across evolutionary time has been influenced by this classic protein. The researchers demonstrate that across the animal kingdom, aquatic animals have myoglobin protein with a greater net positive charge than terrestrial animals. They calculate that for every increase in one positive net charge, the animal can accumulate a incredible additional ten times the amount of myoglobin in its muscle cells, and for two more positive amino acids, the animal can actually accumulate 100 times more myoglobin. More myoglobin translates to more oxygen, which allows aquatic animals to hold their breath for long periods during dives underwater. While the exact mechanism is a fascinating area of ongoing research, it is apparent that myoglobin protein with a greater net positive charge remain soluble at much higher concentrations. The higher net charge prevent myoglobin from aggregating at high concentrations.
Here we see the whale and elephant myoglobin proteins aligned, with the elephant's amino acids in yellow halos. Without these divergent amino acids, the whole protein has a net charge. Following along the protein chain from end to end, and summing the divergent amino acids in whales and elephant, where positive amino acids in blue have a charge value of +1, negative in red of -1, and histidine of +1/2, we see that whales overall have a net charge of +3.5, while elephants have only +1.
Whether this effect is do to the overall charge of the protein, in repelling two strongly positive proteins; or, whether it is a more local effect, where two proteins cannot interact without unfavorably burying the positively charged amino acids; or whether the interactions between myoglobin and the other molecules in the cell, somehow affects its potential to bind to other myoglobins, again, awaits theoretical and experimental insight.
excellent myoglobin tutorial to complement proteopedia articles
- ↑ Mirceta S, Signore AV, Burns JM, Cossins AR, Campbell KL, Berenbrink M. Evolution of mammalian diving capacity traced by myoglobin net surface charge. Science. 2013 Jun 14;340(6138):1234192. doi: 10.1126/science.1234192. PMID:23766330 doi:https://dx.doi.org/10.1126/science.1234192
