Extremophile

Extraordinary Proteins

Where there is no man, be a bacteria

Where no man or plant could survive, bacteria have been eking out a living, and some even thriving. From the Dead Sea which has 10 times the concentration of salt in salt sea water to the hot springs heated by the molten center of the earth, that pour forth through vents deep under the sea (see this fantastic Thermophile Video from BBC Wildlife) - in all these hostile environments, life has found footing. To make the question stronger, realize that many things can go wrong, cells could burst or shrivel, DNA can become undone and tattered, protein can unfold into a jumbled mass of amino acids, and membranes made of fat molecules can rip and melt. Environment stress usually achieves all or many of these deadly process to organisms - yet some bacteria survive. To study how the extremophiles (extreme-loving bacteria) survive involves explaining how each of the above processes that should kill the bacteria, in fact do not occur. To understand all of these is a tall order, but to start, in this Proteopedia article, we'll tackle the protein survival under extreme stress problem.

Extremophiles talk in thermodynamics terms

All stresses must be interpreted into the language of thermodynamics, since that is the most faithful representation of the problems the environment brings to proteins, and the way the structures have solved this problem, and found a way to maintain their stability.

The principle equation is:

∆G = ∆H - T∆S.

Where ∆G is negative, the movement to products in the reaction is spontaneous. This means, for the case of going from unfolded protein to folded protein as the product, a negative ∆G wold correspond to a stable protein structure. The other three terms: ∆H, T, and ∆S correspond to the change in enthalpy, the temperature (in Kalvins), and the change in entropy. Where the product is more stable than the reactants, ∆H will be negative, and the products are more ordered than the reactant, ∆S will be negative. It can be seen from the equation that ∆G becomes more negative for a more negative ∆H or for a less negative ∆S.

Negative surface charge and solubility

In salty water, most proteins aggregate. That proteins on the outside of some archea in the dead sea manage to remain soluble in solutions entering up to one salt molecule for every two H20 molecules is quite stunning. Over a decade's work, Joel Sussman, Ada Zamir, and others at Weizmann Institute and Tel Aviv University have shown that the negative density on the surface of proteins turn them into anion-like, hence soluble in salt-containing solutions. The more recent research involved halotolerant organisms's halotolerant enzymes, and showing that their intermediate negative surface charge enables them to walk the tightrope between little salt and salf-saturating conditions, repectively. Still mysterious, though, is why all halophilic proteins aren't for the same price halotolerant - what use is all that extra negative surface charge?

We next look at a thermophilic enzyme, also solved by Weizmann Institute lab - Professor Yigal Burstein and his team of scientists.

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Proline in Entropy and between-chain ion-network bonding

Some bacteria and even animals can survive great temperatures. Studying (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 all 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 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 . 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. These two structural properties are labelled by the corresponding term in the thermodynamics equation:

= - T.