Extraordinary Proteins
Organisms have managed to colonize extraordinary environments, but without proteins, living doesn't happen. "Extreme" proteins demonstrate many intriguing biophysical features neccessary for living in harsh environments.
Well-tuned negative surface charge density enables solubility in a broad range of salt conditions
The green alga Dunaliella salina lives in the Dead Sea of Israel where water currents can change its environment swiftly and dramatically from low to high salt concentrations. The problem for its proteins is staying soluble in both solvents.
In 2005, scientists from the Weizmann Institute reported the first crystal structure of a halotolerant enzyme, from D. salina, a (1y7w). In 1995, they solved (together with scientists from Tel Aviv University) the first structure of a halophilic enzyme, a (1hlp) from Haloarcula marismortui.
They conclude that a general solution for remaining soluble in salty conditions is to become "anion-like" through increasing the negative charge surface density. Too little negative charge and the enzyme can only tolerate low salt conditions, too much negative charge and the enzyme can only stand high salt conditions, but the "right" amount of negative charge enables an enzyme to remain soluble in both low and high salt conditoins.
In the list below, notice how the negative surface charge density - the ratio of negative "redish" amino acids to positive "bluish" amino acids - is low for the mesophilic enzymes, high for the halophilic enzymes, and medium for the halotolerant enzyme. The ratios are approximately 1:1 (negative to positive amino acids on the surface) for the mesophilic enzymes; 3:1 for the halophilic enzyme, and 2:1 for the halotolerant enzyme.
High temperatures encourage using proline to lower entropy loss
Some bacteria and even animals can survive great temperatures. Eggs fry - meaning their proteins denature, at 65℃. But Thermoanearobacter brockii, discovered in Yellowstone Park, continues to grow in 80℃.
Professors Yigal Burstein (Weizmann Institute) and Felix Frolow (Tel Aviv University) studied a (1ykf) from T. brockii that maintains its structure in over 83℃.
They identified that the hyperthermophilic enzyme was in position 275, as was the thermophilic enzyme in position 100. Because proline's side chain has minimal degree of freedom, proline's, unlike other amino acids, are minimally restricted by folding. There is therefore a smaller loss of entropy upon folding into the native structure.
