Disulfides helix

Mihara and co-workers (120, 123) have shown in a self-assembly system composed of a helix-disulfide-helix and an iron(III) mesoporphyrin... 

The interaction of dioxygen has been observed in several systems, mostly due to autooxidation of ferrous hemes with dioxygen, but only characterized in a few instances. Sakamoto et al. (119) have illustrated peroxidase-type activity using a helix-disulfide-helix system that binds a single heme as shown in Fig. 13. The initial communication illustrated that the addition of an organic cosolvent, trifiuoroethanol, increases the helical content of the peptide, the affinity for heme (1.7 DM IQ at maximal affinity, 15% TFE), and the peroxidase activity (conversion of... 

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

One long a helix connects the two domains (purple). Thermostable mutants of this protein were constructed by introducing disulfide bridges at three different places (yellow). 

In addition to classification based on layer structure, proteins can be grouped according to the type and arrangement of secondary structure. There are four such broad groups antiparallel a-helix, parallel or mixed /3-sheet, antiparallel /3-sheet, and the small metal- and disulfide-rich proteins. 

Fig. 2.17 A / -peptide (92) two-helix bundle [1 79]. The parallel bundle was designed by dimerizing a 3,4-helical peptide via a disulfide bond. The interaction interface of the bundle consist of four hydrophobic residues ((S)-amino valeric acid, / -HLeu and...

Polyacrylamide gel electrophoresis results suggest that p-LG undergoes a greater conformational loss as a fimction of extrusion temperature than a-LA, presumably due to intermolecular disulfide bond formation. Atomic force microscopy indicates that texturization results in a loss of secondary structure of aroimd 15%, total loss of globular structure at 78 °C, and conversion to a random coil at 100 °C (Qi and Onwulata, 2011). Moisture has a small effect on whey protein texturization, whereas temperature has the largest effect. Extrusion at or above 75 °C leads to a uniform densely packed polymeric product with no secondary structural elements (mostly a-helix) remaining (Qi and Onwulata, 2011). 

The essential distinction between the approaches used to formulate and evaluate proteins, compared with conventional low molecular weight drugs, lies in the need to maintain several levels of protein structure and the unique chemical and physical properties that these higher-order structures convey. Proteins are condensation polymers of amino acids, joined by peptide bonds. The levels of protein architecture are typically described in terms of the four orders of structure [23,24] depicted in Fig. 2. The primary structure refers to the sequence of amino acids and the location of any disulfide bonds. Secondary structure is derived from the steric relations of amino acid residues that are close to one another. The alpha-helix and beta-pleated sheet are examples of periodic secondary structure. Tertiary... 

Bachinger, H.P., Bruckner, P., Timpl, R., Prockop, D.J. and Engel, J. (1980) Folding mechanism of the triple helix in type-III collagen and type-III pN-collagen role of disulfide bridges and peptide bond isomerisation. European Journal of... 

Figure 5.1 Schematic diagram of the lactoferrin molecule. The positions of carbohydrate attachment are marked with a star. O, ovotransferrin T, human serotransferrin L, human lactoferrin R, rabbit serotransferrin M, melanotransferrin A, the connecting helix B, the C-terminal helix. The disulfide bridges are indicated by heavy bars, and the iron and carbonate binding sites by filled or open circles, respectively. Reprinted from Baker et al., 1987. Copyright (1987), with permission from Elsevier Science.

Govaerts et al. (2004) proposed a parallel /1-helix model for prion rods that is consistent in overall dimensions with their low-resolution EM studies of two-dimensional PrP 27-30 crystals (Wille et al, 2002). In this model, residues 89-174 form 4 coils, or complete helical turns (Jenkins and Pickersgill, 2001), of a left-handed, parallel /Hielix (Fig. 5B). The coils of one monomer are proposed to stack on the coils of another to form an extended triangular -structure. Three of these triangular units pack together to form the fibril (Fig. 5G and D). The G-terminal a-helices (a2 and a3) of monomeric PrP are proposed to retain their native structure in the fibril and pack around the outside of the trimer (Fig. 5G and D). The presence of these helices in the prion rods is consistent with antibody binding studies (Peretz et al, 1997), the presence of a disulfide bond (Turk et al, 1988), and FTIR measurements (Wille et al, 1996). 

The differences in reactivity between the three Asn residues has been explained by their molecular environment [134], AsnA18 appears protected from deamidation by being flanked at the C-terminal side with a bulky Tyr, and by being positioned in an a-helix and close to a disulfide bridge. In contrast, AsnA21 is at the C-terminus of chain A and appears readily accessible for acid catalysis. As for AsnB3, it is located in a flexible part of the peptide sequence and can, thus, react at neutral pH to form the intermediate succinimide (Fig. 6.29, Pathway e). 

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