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 guarding - or more accurately put, "tuning" - their 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.
The green alga Dunaliella salina lives in an environment where salt levels change swiftly and dramatically from low to high salt concentrations (see an interesting Scientific American article about life in the Dead Sea of Israel [1]). The problem for its extra-cellular proteins is staying soluble in both solvents. Professors Sussman and Zamir from the Weizmann Institute report the first such protein crystal structure and suggest that the protein's relative increase of negative surface charge density turns the protein into a anion-like molecule capable of dissolving in high salt. However, unlike the halophilic enzyme from Haloarcula marismortui which Profs. Sussman and Maverach (Tel Aviv University) crystallized earlier, the negative surface charge is not so high that the protein becomes insoluble in lower salt concentrations. The three-way comparison between the salt-adapting properties of a mesophilic, halotolerant, and halophilic enzyme illuminates a biophysical strategy for tuning protein structures to extreme salt conditions.
In the list below, the increasing negative charge density on the surface is apparent. Notice also that while the halotolerant enzyme (middle) switches positive amino acids to neutral, the halophilic enzyme (last), also switches neutral amino acids to become negative.
In a nutshell, the problem of salty condition includes the problem of how to remain soluble; the general solution is to become anion-like through increasing the negative charge surface density; and the molecular implementation is through decreasing the relative amount of positively charged amino acids and/or increasing the relative amount of negatively charged amino acids.
High temperatures encourage using proline to lower entropy loss and between-chain ion-network bonding to increase enthalpy gain
Some bacteria and even animals can survive great temperatures. Studying a thermophilic enzyme (1ykf), Prof. Burstein noticed two special features that appear to explain this enzymes ability to maintain its structure in over 83℃ (!) For comparison, you could fry an egg at 65℃, which mean the protein in an egg denature at significantly less than 83℃. To demonstrate the special structural properties of the thermophilic enzyme underlies its thermophilic prowess, Prof Burstein selectively altered normal enzymes to have the two structural features, and indeed found that the normal enzymes had become thermophilic. The two properties relate to ∆H and to ∆S. Firstly, he found the thermophilic enzyme had a unique four amino acid binding-network that encompassed two monomers of the tetrameric enzyme, repeating between each monomer and its two partner monomers. This network apparently makes the oligomer more stable, or ∆H more negative. Secondly, the thermophilic enzyme was enriched for proline. Because proline's side chain has minimal degree of freedom, proline's, unlike other amino acids, are minimally restricted by folding. Therefore, ∆S is less negative.
