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V – Hydration-Assisted Beta-Sheet Formation

As mentioned before, linear chains with hydration-ordering peptides on one side and intermittent hydration-disordering peptides on the other side have sufficient stability to remain as linear segments but the ordering side is still unstable.

As illustrated schematically in A, two linear chains are moved spontaneously toward each other by the dynamics of water on opposite sides until linear ordering begins to intersect, as in B. As approach continues, ordered water is released and peptide chains begin to overlap. If sufficient water is released to achieve an initial state of thermodynamic stability, as in D, the chains then rotate, bridging water is released and an initial beta-sheet is formed, as in F. However, if sufficient water is not released, kinetic energy of the retained water will drive the chains apart to continue the search for a thermodynamically-stable union. If the continuing polypeptide contains complimentary sequences of polar, non-polar and ionic peptides, the beta-sheet will widen and assembly will continue until almost all hydration-ordering peptides are bound.

Order-disrupting peptides, like glutamine, aspartame and highly-charged arginine, usually are on the outer surfaces of finished proteins, binding water and increasing solubility while a limited number of ordering peptides will be on the surface providing binding between proteins and reversible binding sites for substrate and regulator molecules. In early phases of molecular evolution when polypeptides most likely were produced at random, those with sequences which could wrap spontaneously to produce thermodynamically-stable assemblies yielded proteins which could perform a variety of cooperative functions – those with sequences which did not yield stable forms, were chewed up by lytic enzymes and ribozymes. It was surface water which defined which structures would be stable and survive and which ones would provide for spontaneous assembly and function. For more information on the possible role of water in the evolution of natural molecules, check out www.molecularcreation.com.

Download PDF of this page   Download PDF Book  Beta-Turn Formation Covalent linear elements of hydration on one side of polypeptides directs and drives them into beta-sheets.