Denaturation electrostatic effects

In early work, it was proposed that A2 and Fx were the same chemical species detected under different experimental conditions [37]. This proposal fits most of the present data, although Fx has also been proposed to be identified with P-430. In recent work [40,41] it was shown that a mild treatment with lithium dodecyl sulfate denatures F and Fb, but not Fx- In such preparations, a flash-induced charge separation at room temperature decays with t 2 = 1.2 ms. This could be equivalent to the 250 /as decay observed earlier which occurs when F and Fg are chemically reduced, the kinetic difference originating possibly in an electrostatic effect of Fa and Fb on the lifetime of (P-700", Fx ). An important difference between the behaviour of A2 and Fx resides in the efficiency of their light-induced reduction a saturating flash reduces A2 in all the reaction centers at room temperature [37], whereas Fx is reduced in only 10-15% of the centres at low temperature [31]. A good correlation has been established between Fx and A2 by studying the effect of time in darkness after illumination at 3°C, in PS I particles [42]. 

From the foregoing discussion it can be seen that a determination of the pH dependence of Aftb or of T%t for reversible denaturation may provide information on the kinds of side-chain hydrogen bonds which play a role in the stabilization of the native protein. Such experiments should be carried out at high ionic strength to minimize electrostatic effects. 

The electrostatic effects alone are insufficient to account for the denaturation of proteins that occurs at extreme pH values. The alteration induced either by acid or by alkaline pH may be more or less drastic according to the proteins. For some proteins there are similarities between denaturation at high and low pH for others, the alkaline denaturation is different from the acid one. Generally, proteins retain less residual structures when exposed to high pH values than to acid denaturation because buried residues, such as tyrosines, tend to become accessible to the solvent. 

Results from thermal denaturation and heat capacity studies have shown that the proteins are not necessarily completely unfolded in this process. The volume observations also suggest that the denatured state is not one in which all hydrophobic groups are exposed to water. But the results can also be understood from the effect of close polar and electrostatic groups interacting with the water structure surrounding the hydrophobic groups. The volume change is heavily... 

Even extremophilic organisms and their proteins contain the same 20 amino acids with bonds similar to those in mesophiles. As the difference in free enthalpy between folded and unfolded states of globular proteins AG N >G is only about 45 15 kj mol-1 the sequence and structure of extremophilic proteins should differ from those of ordinary species. However, the main question, namely which properties cause the increase in denaturation temperature of thermostable proteins, is still debated (Rehaber, 1992). Theoretical and experimental analyses have shown that thermal stability is largely achieved by small but relevant changes at different locations in the structure involving electrostatic interactions and hydro-phobic effects (Karshikoff, 2001). There is no evidence for a common determinant or for just one effect causing thermostability. 

---