Hydroxyproline polypeptide

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.

Collagen is the most abundant extracellular matrix protein family in vertebrates. Proteins in the collagen superfamily all have three polypeptide chains with the required -Gly-Xaa-Yaa- repeated sequence, where Xaa and Yaa are frequently proline and 4-hydroxyproline, respectively. At present, more than 30 molecular species of vertebrate proteins called collagen are classified into 28 types as type I, II, III,..., XXVIII. They are typically called type N collagen , or collagen N . In addition, there are many more collagen-like proteins that... 

Values are expressed as the 3-hydroxyproline oontent per ahalyzed oollagenous polypeptides. CB meahs the CNBr fragments. Numbering of the CB peptides is written in Henkel and Dreisewerd , and Seyer ef... 

Most of the fluoro derivatives of proline described in the literature are fluorinated in the 4 position. 4-Fluoroprolines are able to mimic 4-hydroxyproline, present in some proteins and polypeptides. On the other hand, 4-fluorination could suppress oxidative metabolism or modified ring conformation of ligands.The labeled analogues may be used as probes in PET (positron emission tomography) for localization of tumors... 

In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

Gl (MW >5000). Most of the pyrolysis fragments found in this fraction are derived from the three general classes just mentioned. Pyrrole and methylpyrrole originate from proteinaceous material such as polypeptides as well as from single amino acids such as proline and hydroxyproline. A quantitative relationship between amino acid hydrolyzable content and pyrrole abundance was established by Bracewell (20) for some Scottish brown forest soils, and such a correlation probably could be established for water. 

Hydroxyproline and hydroxylysine Collagen contains hydroxy proline (hyp) and hydroxylysine (hyl), which are not present in most other proteins. These residues result from the hydroxylation of some of the proline and lysine residues after their incorporation into polypeptide chains (Figure 4.6). The hydroxylation is, thus, an example of posttranslational modification (see p. 440). Hydroxy proline is important in stabilizing the triple-helical structure of colla gen because it maximizes interchain hydrogen bond formation. 

Collagen-like triple helices also occur within other proteins. One of these is protein Clq, a component of the complement system of blood (Chapter 31). This protein interacts with antibodies to trigger a major aspect of the immune response. Clq has six subunits, each made up of three different polypeptide chains of about 200 residues apiece. Beginning a few residues from the N termini, there are over 80 residues in each chain with collagen-like sequences. The three chains apparently form a triple helix within each subunit. However, the C-terminal portions are globular in nature.200 Collagen-like tails also are present on some forms of the enzyme acetylcholinesterase (see Chapter 12C,10). Tire extensins of plant cell walls contain 4-hydroxyproline and evidently have a structure... 

The principal protein of skin and connective tissue is called collagen and is primarily constituted of glycine, proline, and hydroxyproline. Collagen is made up of tropocollagen, a substance with very long and thin molecules (14 X 2900 A, MW about 300,000). Each tropocollagen molecule consists of three twisted polypeptide strands. When collagen is boiled with water, the... 

Collagen has a most unusual amino acid composition in which glycine, proline, and hydroxyproline are the dominant amino acids. Further characterization of the polypeptide chains shows that these amino acids are arranged in a repetitious tripeptide sequence, Gly-X-Y, in which X is frequently a proline and Y is frequently a hydroxyproline. This unusual amino acid sequence and the unique diffraction pat-... 

In dyeing the hair and the suede portion a number of factors must be allowed for. The keratin of the hair contains basically the same amino acids as the collagen of the skin but in a different ratio. The keratin of the hair includes cysteine, which cross-links the polypeptide chain and imparts stability. The collagen of the skin does not have these substances, and the cross-links are made by the tanning agent. On the other hand, only L-hydroxyproline can be found in the collagen. As a result the thermal stability is different, and in addition the isoelectric points of the two polypeptides diverge. 

Polypeptides, however, are composed of amino acids with side chains that are longer and therefore the area of allowed conformations is reduced when an alanine (Figure 2.12), aspartic acid (Figure 2.13), or a proline (Figure 2.14) is added to the second peptide unit. Finally, the conformational map for a dipeptide of proline-hydroxyproline is dramatically reduced. Rings in the backbone of any polymer reduce the ability of the polymer backbone to adopt numerous conformations and thereby stiffen the structure. 

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