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Why_Do_Proteins_Fold - Why Do Proteins Fold Proteins are...

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1 Why Do Proteins Fold? Proteins are usually only marginally stable Protein stability is intimately connected with protein folding - proteins have to be folded into their final active state (and maintain it) to be stable. N unique 3D structure U ensemble of non-structured, solvent-accessible peptide chains (random coils) D ensemble of conformations present under elevated T, pH extremes, denaturants D F or U F (Denatured or Unfolded state to folded state; D is not the same as U, and there can be a variety of different D or U states). D can often be different states depending on the denaturing condition; D phys is often a denatured physiological state of a protein (usually a molten globule state, with extensive residual secondary structure). U is sometimes defined as the protein at a very high chemical denaturant concentration. Folding intermediates: promise to help understand the folding pathway G = H - T S Free energy of folding is the sum of enthalpic and entropic forces. These are large forces, and the resulting G is often a small number (-5 to -15 kcal/mol is typical for many proteins). G is comparable to the energy of a few hydrogen bonds, even though a protein may have hundreds of H bonds. Proteins are thus only marginally stable. The favorable enthalpic gains (H bonds, salt bridges, electrostatics, etc.) during the folding of a protein are offset by the substantial loss of entropy as the protein goes from an extended chain into a compact structure.
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