Proline polypeptides

Elastin-like polypeptides (ELPs) have been extensively studied due to the fact that they combine similar stimulus response properties to other artificial polymers such as poly(iV-isopropylacrylamide) (pNIPAM) with the advantages of a biologically derived material, that is, it is biocompatible, modular in its composition, and can be obtained by biological processes. ELPs are polypeptides that contain a short, repetitive peptide sequence, most commonly (VPGXG) that is derived from tropoelastin, the precursor of elastin. In this sequence, X represents any amino acid sequence except proline. Polypeptides composed of the pentapeptide repeat unit VPGXG possess a reversible lower critical solution temperature (LCST). Below the LCST, the peptide is soluble... 

Figure 14.1 Each polypeptide chain in the collagen molecule folds into an extended polyproline type II helix with a rise per turn along the helix of 9.6 A comprising 3.3 residues. In the collagen molecule three such chains are supercoiled about a common axis to form a 3000-A-long rod-like molecule. The amino acid sequence contains repeats of -Gly-X-Y- where X is often proline and Y is often hydroxyproline. (a) Ball and stick model of two turns of one polypeptide chain.

Synthetic experiments have established the constitution of the polypeptide and the succession of the amino-acids, proline being always at the distal end and the second amino-acid in the middle. 

ENZYMATIC ANALYSIS WITH CARBOXYPEPTIDASES. Carboxypeptidases are enzymes that cleave amino acid residues from the C-termini of polypeptides in a successive fashion. Four carboxypeptidases are in general use A, B, C, and Y. Carboxypeptidase A (from bovine pancreas) works well in hydrolyzing the C-terminal peptide bond of all residues except proline, arginine, and lysine. The analogous enzyme from hog pancreas, carboxypeptidase B, is effective only when Arg or Lys are the C-terminal residues. Thus, a mixture of carboxypeptidases A and B liberates any C-terminal amino acid except proline. Carboxypeptidase C from citrus leaves and carboxypeptidase Y from yeast act on any C-terminal residue. Because the nature of the amino acid residue at the end often determines the rate at which it is cleaved and because these enzymes remove residues successively, care must be taken in interpreting results. Carboxypeptidase Y cleavage has been adapted to an automated protocol analogous to that used in Edman sequenators. 

Hydrophobic-tailed tetramers Abundant form in the mammalian CNS. Anchored to plasma membranes by a hydrophobic, 20 kDalton length polypeptide subunit named PRiMA (Proline-Rich Membrane Anchor). 

Fig. 10. Possible collagen-linke structure depending on the hydrogen-bonding capacity of certain polypeptides based on their proline content...

The Pn conformation of poly-L-proline (PP) or collagen in the solid state could be identified from X-ray fiber diffraction results (Cowan and McGavin, 1955). Persistence of this basic structure in solution was inferred from the resemblance between the CD spectra of solutions and films of the polypeptide. The CD spectra of the charged forms of PGA and PL closely resemble that of Pn (compare Fig. IB, 1C, and ID) however, these spectra differ significantly from those of PP peptides at high temperature or in the presence of high concentration of salts... 

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