Now, this could get quite interesting. A recent paper in PNAS talks about “downsizing” biologically active proteins to much shorter mimics of the alpha-helical parts of their structures. These show a good deal more stability than the parents, and show a sometimes startling amount of biological activity.
The building block for all this is the smallest helical peptide yet reported, a cyclic pentapeptide (KAAAD) curled as as a lactam between residues 1 and 5. Joining two or more of these up give you more turns, and replacing the alanines gives you plenty of possible mimics of endogenous proteins. An analog of nociceptin turned out to be the most potent agonist at ORL-1 ever described (40 picomolar), and an analog of RSV fusion protein is, in its turn, the most potent inhibitor of that viral fusion process ever found as well.
Meanwhile, the paper states that these constrained peptides were stable in human serum for over 24 hours, as very much opposed to their uncyclized counterparts, which are degraded rapidly. (Exocyclic amino acids, when present, do get degraded off in a time span of hours, though).
I’m quite amazed by all this, and I’m still processing it myself. I’ll let the authors have the last word for now:
“This work is a blueprint for design and utility of constrained α-helices that can substitute for α-helical protein sequences as short as five amino acids. . .The promising conformational and chemical stability suggests many diverse applications in biology as molecular probes, drugs, diagnostics, and possibly even vaccines. The constrained peptides herein offer similar binding affinity and/or function as the proteins from which they were derived, with the same amino acid sequences that confer specificity, while retaining stability and solubility akin to small molecule therapeutics. . .”