JMS/sandbox15

Extraordinary ProteinsExtraordinary 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 divesPositively 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. A group of researchers contacted museums and zoos around the world.

They collected muscle tissue from hundreds of aquatic and terrestrial mammalian species. myoglobin stores oxygen in muscle tissue, and the researchers thought they might find differences relating to this protein in terrestrial versus aquatic animals. They measured the cellular concentration and net charge of myoglobin from each species muscle tissue sample. To measure the net charge, they sequenced each specie's myoglobin gene and computationally modeled the net charge. for a representative sample of species they additionally directly measured the net charge of the different myoglobin protein via native gel electrophoresis.

Their hypothesis was that whales and other aquatic animals can hold their breath for so long because they can store more oxygen in their muscles by increasing the concentration of myoglobin in each muscle cells. Specifically, they predicted that species could increase the concentration of myoglobin by increasing its solubility through increasing the net charge, so that there would be repulsion between the myoglobin protein even at high concentrations, which would prevent aggregation and precipitation.

Amazingly, they found an association between an animals' ability to hold its breath, high concentrations of myoglobin in muscle tissue, and a larger positive 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.

Molecular Tour The ability of increasing net charge to enable higher solubility is a known phenomenon, and this study is consistent with previous reports. The aquatic animals have increased their net charge in a variety of ways - different combinations of amino acids switches. We present one such manifestation of this overall trend, by comparing the elephant and whale myoglobin structures. It comes down to that affect that charge - out of a total of 27 divergent amino acids. Without these eight differently charged amino acids, myoglobin in both whale and elephants has a charge of +1. With them, whale myoglobin has a net charge of +4 and elephants of +2. Importantly, all eight of these divergent amino acids are . It is interesting to note that in this comparison (between whale and elephant) that there is no difference in net charge in the region in the vicinity of the heme group. This may reflect the key role of the heme group and residues near it, which can not be easily changed without a drastic affect on function. Calculate along the chain, how just several amino acid switches bring the positive net charge of whale myoglobin up to +4 and elephants to +2: (summing up the total charge of the protein) (glu in elephents versus gln in whales), (lys vs. his), (thr vs. asp), (thr vs. lys ), (gln vs. lys ), (gln vs. his), (lys vs. asn), (asn vs. lys ). note about this scene: Asp and Glu have a charge of -1, Arg and Lys have a charge of +1, His in the positions shown here - 12 and 116 (Table S2) - have a charge of about +0.5. However, when viewing not , but all charged amino acids, it become apparent that some of the ; and though in some cases the divergent charges share their partner with a different amino acid, as in the case of the , according to the superposition principle, the two electrostatic bonds are in fact independent, and do not impact or diminish from the other's attractive force.

spinqualitylabelsall models myoglobin (PDB entry 1mbn) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

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.