Protein chains, bond between

Figure 3. Types o/ bonds between protein chains...

The use of oxidizers, such as potassium bromate, potassium iodate, and calcium peroxide as dough modifiers in the baking industry dates back many years. However, their mechanism has not been fully explained. Among authorities, there are at least two major viewpoints. It has been proposed that oxidizers inhibit proteolytic enzymes present in flour. It has also been proposed that the number of —S—S—bonds between protein chains is increased, forming a tenacious network of molecules, This action... 

Figure 11.4 Pleated sheets of fibrous proteins. Parallel pleated sheets are composed of polypeptide chains which all have their N-terminal amino acid at the same end whereas anti-parallel pleated sheets involve polypeptide chains which are alternately reversed in direction. Both forms of sheet show a high degree of hydrogen bonding between the chains.

The nature of the amino acid residues is of prime importance in the development and maintenance of protein structure. Polypeptide chains composed of simple aliphatic amino acids tend to form helices more readily than do those involving many different amino acids. Sections of a polypeptide chain which are mainly non-polar and hydrophobic tend to be buried in the interior of the molecule away from the interface with water, whereas the polar amino acid residues usually lie on the exterior of a globular protein. The folded polypeptide chain is further stabilized by the presence of disulphide bonds, which are produced by the oxidation of two cysteine residues. Such covalent bonds are extremely important in maintaining protein structure, both internally in the globular proteins and externally in the bonding between adjacent chains in the fibrous proteins. 

Disulfide bonds in the amino -acid cystine are important to the properties of many proteins by maintaining covalent intramolecular bonds and crosslinks between protein chains (16). 

The large size of soluble glutenln molecules is due to limited disulfide bonds between polypeptide chains. The insolubility of residue protein is attributable to extensive Intermolecular disulfide crosslinks. 

The collagen superfamily of proteins includes more than twenty colla gen types, as well as additional proteins that have collagen-like domains. The three polypeptide a-chains are held together by hydro gen bonds between the chains. Variations in the amino acid sequence of the a-chains result in structural components that are about the same size (approximately 1000 amino acids long), but with slightly dif ferent properties. These a-chains are combined to form the various types of collagen found in the tissues. For example, the most common collagen, type I, contains two chains called a1 and one chain called... 

The existence of electrostatic interactions between oppositely charged residues and hydrogen bonding between side chains agrees with the observations in protein helices that (1) helix probability correlates with the frequency of occurrence of oppositely charged residues spaced i, i + 4 apart in proteins 88 and (2) there is a strong tendency for nearby, oppositely charged, side chains to point toward each other. 89 In the case of C-peptide, the side-chain interactions were also evident in the crystal structure of RNase A. 

The p pleated sheet structure occurs commonly in insoluble structural proteins and only to a limited extent in soluble proteins. It is characterised by hydrogen-bonding between polypeptide chains lying side by side, as illustrated in Fig. 5.A3b. 

In a similar manner to a helices, extended structures can be held together by hydrogen bonds with the hydrogen bonds running perpendicular to the chain axis. Silk is an example of a protein that is found in the p structure. The amino acid composition of silk is rich in glycine (44.5%), alanine (29.3%), and serine (12.1%) amino acids with small hydrocarbon side chains that form sets of antiparallel hydrogen bonds between molecular chains. Models of polypeptide chains with sequences of poly(gly-ala) and poly(ala-gly-ala-gly-ser-gly) show that the most probable structure contains all the gly residues on one side of the chain and all the ala residues on the other side of the chain, and therefore by packing the chains in... 

The antibodies are similar in structure to other globulin proteins which are present in serum of vertebrates [9], The antibody molecule consists of two light polypeptide chains and two heavy polypeptide chains [10]. The amino acids are present in all the chains but the number of residues and the sequence will vary in different antibody multiforms [9], Light chains contain approximately 220 amino add residues and heavy chains about 450 residues. The complete sequence of the chains of human IgG myeloma protein has been determined by Edelman [11], The chains are held together in the unique conformational structure of the antibody molecule by a few covalent disulfide bonds between the chains and many electrostatic bonds between the amino groups of one chain and the hydroxyl groups of another chain. The covalent bonds are represented in Formula 1, 2 and 3 [peptide, disulfide and... 

Table 19.3. Intermolecular and intramolecular (long-range, local) hydrogen bonds between main-chain and side-chain atoms in the 15 proteins listed in Table 19.1 [596]... 

Water molecules are small probes. They can hydrogen-bond to protein functional groups with few steric limitations, in contrast to the hydrogen bonds between protein main-chain and side-chain atoms which have been the topic of Part III, Chapter 19. The hydrogen bonds involving water are therefore considered to reflect the intrinsic hydrogen-bonding properties of a protein. 

When hydrogen bonding occurs between protein chains rather than within them, a stable structure (the pleated sheet) results. This structure contains many protein chains and is found in natural fibers, such as silk, and in muscles. 

Figure 11.18. Glycosidic Bonds between Proteins and Carbohydrates. A glycosidic bond links a carbohydrate to the side chain of asparagine (TV-linked) or to the side chain of serine or threonine (O-linked). The glycosidic bonds are shown in red.

How have selenium-accumulators been able to absorb so much Se without any damage to themselves These plants are able to separate inorganic S (as sulphate) from inorganic Se (as selenate or selenite), when they enter in the plants, and to channel the Se into the synthesis of nonprotein amino acid analogues, which are not therefore incorporate into protein synthesis. The adapted plants then sequester them in the vacuoles of the leaves, where they are perfectly harmless to the plants but intensely harmful to any unsuspecting grazing animals. In the non-adapted plants selenium toxicity may be attributed to the replacement of cysteine by selenocysteine and the production of disfunctional proteins in which S-S bond between polypeptide chains are replaced by the more labile Se-Se bonds. Fig (14). 

Figure 36.2. Hypothetical flat sheet structure for a protein. Chains fully extended adjacent chains head in opposite directions hydrogen bonding between adjacent chains. Side chains (R) are crowded.

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