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IV - Hydration-Assisted Coil Formation

Although linear structuring of hydration on all sides of a polypeptide may transiently stabilize the chain in a linear conformation, neither water nor the chain are thermodynamically stable and the chain will spontaneously alter its shape to decrease linearity and increase hydration entropy. By rotating the alpha carbon of peptide 3 by 30o, as shown below in B, a water trimer can bridge around the hydrogen and produce a transiently stabilized intermediate.     

As rotation continues through hydration-stabilized conformations C and D, peptides 4, 5 and 6 will rotate 180o into a tight bend between peptides 3 and 4. Counter rotation around the alpha carbon of peptide 4 permits the loss of additional bridging water molecule with the carbonyl oxygen of peptide 5 now hydrogen-bonded by a single water molecule to the carbonyl of peptide 2 in E. Continued rotation around alpha carbons of 5 and 6 permits a final dehydration to coil H with the carbonyl oxygen of peptide 6 bonded to the amide hydrogen of peptide 3.

As rotations continue around the alpha carbons of successive hydrophobic peptides, coils will continue to grow in length until glycines, serines or highly-charged peptides are reached in the chain which directly hydrogen-bond with surface water and halt covalent-bond formation. Prolines, which are unable to hydrogen bond with other peptides, are particularly important in altering directions of linear chains and coils. In fact, if coupled with a serine or a glycine, a proline will often initiate a broad hydrated loop, a beta-sheet or a beta turn but, if bordered by lipid peptides, it may simply bend a coil. Since polypeptide chains containing a series of hydrophobic peptides induce the hydration layering of water on all sides, coils form extremely rapidly.39 As pointed out in the introduction, in order to avoid erroneous assembly, long coils may begin forming within the ribosomal tunnel before being released into water.25 Although the sequence of hydration steps described above satisfies mean angles and distances of covalent hydrogen-bonding, other sequences may be equally probable. The purpose of the above presentation is to emphasize the importance of surface water in all stages of folding.

Download PDF of this page   Download PDF Book  Beta-Sheet Formation Covalent surface water around regions of polypeptides as they are released from ribosomes drives them into coils.