Valine polypeptide, 0-form

FIGURE 19.18 Sickle-shaped red blood cells form when only one amino acid (glutamic acid) in a polypeptide chain is replaced by another amino acid (valine). These cells are less able to take up oxygen than normal cells. 

Sickle hemoglobin A defective form of hemoglobin produced as a result of a single substitution of the amino acid valine for glutamic acid at position 6 of the (1-polypeptide chain. 

Of the various isomeric amino-caprok acids only leucine and isoleucine occur in the protein molecule both of them, combined with tyrosine and valine in the form of polypeptides, from which they are easily split off by enzymes, seem to form a very important part of most proteins. 

When the number of amino acids in a polypeptide chain reaches more than fifty, a protein exists. The structure of both polypeptides and proteins dictate how these biomolecules function. There are several levels of structure associated with polypeptides and proteins. The sequence of the amino acids forming the backbone of the protein is referred to as the primary structure. A different order or even a minor change in an amino acid sequence creates an entirely different molecule. Just reversing the order of amino acids in a dipeptide changes how the dipeptide functions. An example of this is sickle-cell anemia. Sickle-cell anemia is a genetic disorder that occurs when the amino acid valine replaces... 

From such a background, some kinds of polypeptide blend samples have been studied by solid state NMR.27,72 74 Especially, detailed information for four kinds of blend samples such as poly(L-alanine) (PLA)/poly(L-valine) (PLV), PLA/poly(L-isoleucine) (PLIL), poly(D-alanine) (PDA)/PLV and polyglycine (PG)/PLV blends, have been reported. Here, let us describe some reasons why PLA/PLV, PDA/PLV, PLA/PLIL and PG/PLV blends are interesting systems. PLA and PDA in the solid-state can take the a-helix and (3-sheet forms due to intra- and intermolecular HBs, respectively. PG in the solid-state can take the 3j-helix (PG-II) and (3-sheet (PG-I) forms due to intra- and intermolecular HBs, respectively. However, PLIL and PLV in the solid state can predominantly take the (3-sheet form as the stable conformation. For this reason, it is interesting to know whether an isolated a-helix or 3i-helix form polypeptide surrounded by a major polypeptide in the (3-sheet form can take the helical conformation, or not, due to the balance between intramolecular and intermolecular hydrogen bonds. In addition, we would like to know whether a polypeptide in the (3-sheet form surrounded by a major polypeptide in the a-helix or 3 -helix form can take the (3-sheet form. 

Pinna nobilis tropomyosin contains no proline (Bailey, 1957), which is consistent with a helical molecule. As might be expected from the study of a 5% copolymer of L-tyrosine with L-glutamic acid (see Section III, F), the presence of small amounts of tyrosine and histidine in this protein has no appreciable rotatory effect. This protein contains relatively large amounts of amino acids that do form standard helical polypeptides—glutamic acid, 21 % lysine, 8% alanine, 12% and leucine, 12.5 %—but it also has 4 % valine, 6 % serine, and 13 % aspartic acid. If these residues behaved as they and their analogues appear to do in synthetic polypeptides, either pre-... 

How does the amino acid sequence of a protein specify its three-dimensional structure How does an unfolded polypeptide chain acquire the form of the native protein These fundamental questions in biochemistry can be approached by first asking a simpler one What determines whether a particular sequence in a protein forms an a helix, a (3 strand, or a turn One source of insight is to examine the frequency of occurrence of particular amino acid residues in these secondary structures (Table 2.3). Residues such as alanine, glutamate, and leucine tend to be present in a helices, whereas valine and isoleucine tend to be present in (3 strands. Glycine, asparagine, and proline have a propensity for being present in turns. 

It has been demonstrated that the isotropic chemical shifts of systems, such as (Gly) (polyglycine(PG)), (Ala) , (Leu) (poly(L-leucine)), (Ileu) -(poly(L-isoleucine)), (Val) (poly(L-valine)), (Phe) (poly(L-phenylalanine)), (Glu(OMe)) (poly(y-methyl L-glutamate)), (Asp(OBzl)) (poly()8-benzyl l-aspartate)) and (Pro) in the peptide backbone of polypeptides in the solid state, exhibit a significant conformation-dependent change [5,6]. Both experimental observation and theoretical calculations confirm this. The Sjso for the a-helix form (97.0-99.2 ppm) appears to low frequencies by about 1.2-... 

Table 22.4 shows the values of Rn for some solid polypeptides, determined by using Equations (22.1a-e) through the observation of the amide carbonyl-carbon chemical shift as listed are solid polyglycine[(Gly) ], poly(L-alanine)[(Ala) ], poly(L-valine)[(Val) ], and poly(L-leucine)[(Leu) ] with several conformations such as righthanded a-helix (aR-helix), j8-sheet, 3i- and (UL-helix. In these homopolypeptides, the Rn o values determined for the /3-sheet form are constant in the range of 3.0-3.1 A, regardless of amino acid residue species. The Rn-.-o value for the 3i-helix in (Gly) is... 

Elastin - Elastin is a highly elastic fiber present in ligaments and arterial blood vessels. The polypeptide is rich in glycine, alanine, and valine. Its secondary structure is the most random of the fibrous proteins described here. Like collagen, elastin contains lysine groups involved in cross-links between the chains. In elastin, however, four lysine chains can be combined to form a desmosine cross-link (see here). Thus, fewer cross-links are needed to provide strength for the chains and a more elastic network is created. 

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