Extremophile

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Extraordinary Proteins

Life 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. In this article, we present the biophysical modifications present in extreme protein structures.

Positively charged myoglobin allows whales to hold their breath during long dives

Elephants can hold their breath for 2 minutes, but whales can hold their breath for 60 minutes - and they do, migrating underwater around the world. To get a clue as to why whales can hold their breath for so long, several researchers gathered tissue samples from hundreds of aquatic and terrestrial mammalian species (mainly from museum collections)[1]. They measured the concentration of myoglobin, the protein that stores oxygen in muscle tissue, which is used for muscle activity, and also sequenced each specie's myoglobin gene, and used this sequence - as well as the protein's mobility on a native gel (which depends soley on the 3D structure and charge - with myoglobin from different species all having the same overall 3D structure), when possible - to calculate the net charge of each myoglobin protein.

Amazingly, they found that aquatic mammals, across the mammalian phylogeny, independently had acquired the ability to hold their breath, by increasing the concentration of myoglobin, via increasing the net charge of myoglobin. Typically, purified terrestrial mammal's myoglobin has a solubility of 20 mg/g in an aqueous solution at neutral pH ([Sigma Aldrich]) which turns out to be the maximum level of myoglobin found in most terrestrial mammal's tissue. But whales and other aquatic mammals far exceed this solubility limit, e.g., whales have 70 mg/g. The way that they overcome the solubility constraint may be traced back to a modest increase in the net charge of myoglobin - from around +2 in terrestrial animals to around +4 in aquatic animals.

However, a 3-fold increase in concentration of myoglobin ought to result in a similar fold increase in max time of breath holding, and the researchers show that body mass also makes a critical contribution to an animal's ability to hold its breath, with the overall equation for the contribution of body mass and myoglobin net charge as follows:

log (maximum time underwater) = 0.223*log(body mass) + 0.972*log(myoglobin net charge) + 0.891

As Asian elephant's weight is ~3K Kg, and a sperm whale's weight is ~50K Kg, it is clear that the modest increase in net charge contributes about the same as the enormous difference in body mass to the maximum time underwater.

myoglobin (PDB entry 1mbn)

  1. 1.0 1.1 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:http://dx.doi.org/10.1126/science.1234192
  2. Brocchieri L. Environmental signatures in proteome properties. Proc Natl Acad Sci U S A. 2004 Jun 1;101(22):8257-8. Epub 2004 May 24. PMID:15159533 doi:http://dx.doi.org/10.1073/pnas.0402797101
  3. Goh CS, Lan N, Douglas SM, Wu B, Echols N, Smith A, Milburn D, Montelione GT, Zhao H, Gerstein M. Mining the structural genomics pipeline: identification of protein properties that affect high-throughput experimental analysis. J Mol Biol. 2004 Feb 6;336(1):115-30. PMID:14741208 doi:http://dx.doi.org/10.1016/S0022283603014748

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