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Beta-Sheet Formation Beta-Turn Formation Carboxypeptidase A Assembly References

SURFACE HYDRATION OF POLYPEPTIDES AND PROTEINS

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VI – Hydration-Assisted Beta-Turn Formation

Linear segments, coils and sheets are the building blocks of proteins but beta-turns and loops, composed of water-bonding, covalent linear hydration-disrupting peptides, provide vital turning points in natural polypeptide chains to permit controlled and spontaneous assembly.




























As pointed out earlier, glycine is unique in that the hydrogens on its alpha carbon are so small that water can bridge around them as shown in B and rotate the bond, as shown in C and D. Continued clockwise rotation around the alpha carbons of adjacent peptides produces intermediate F with a dimer bridge between the amide nitrogen of peptide 1 and the carbonyl oxygen of peptide 4. Loss of water in two quantized steps permits the carbonyl of peptide 4 to hydrogen-bond directly with the amide nitrogen of peptide 1 to yield the beta-turn. As you can see, formation of the turn dramatically changes the relative positions of side chains and permits continuing segments of the polypeptide to form beta-sheets, coils and loops.  


If glycine peptides are flanked by large hydrophobic peptides, they may remain within linear segments or coils but, often, they will initiate critical turns. Prolines and serines also initiate turns, particularly when they are adjacent to each other, and, as you shall soon see, they, like glycines, are in strategic locations in natural polypeptide chains to permit complimentary assembly of hydrophobic surfaces to release ordered water.

Download PDF of this page  Download PDF of All Pages Carboxypeptidase A Glycine, serine and proline peptides, when adjacent to each other, promote the formation of beta-turns.