Disulfides terms Links

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

Except for a small amount of the amino acid methionine, the sulfur in wool occurs in the form of the amino acid cystine. Only approximately 82% of clean wool consists of keratinous proteins, which are characterized by a high concentration of cystine. Approximately 17% of the fiber is protein material of relatively low cystine content (<3%) this has been termed nonkeratinous. As a result of the lower cystine content, the nonkeratinous proteins have a lower concentration of disulfide cross-links compared with the keratinous proteins in the fiber. The nonkeratinous material is, therefore, more labile and less resistant to chemical attack than the keratinous proteins. The nonkeratinous proteins are located primarily in the cell membrane complex between the cortical cells and in the endocuticle (Fig. 3). Approximately 13% of the total nonkeratinous material is also located within the cortical cells, where its distribution differs between the cells of the ortho- and paracortex. 

Wool is a protein fiber that is obtained from the fleece of - sheep. Aside from normal domestic sheep, there are other varieties, such as merino. The term w. covers also the hair of lamas (alpaca), camels, rabbits (angora) and goats (mohair, cashmere). Chemically, wool is a - protein, based on keratin, and consists of 24 different - amino acids. Most important is the content of S-containing amino acids (cystine), which are responsible for the disulfide cross-linking. 

The second class of AChEs exists as heteromeric assemblies of catalytic and structural subunits. One form consists of up to 12 catalytic subunits linked by disulfide bonds to filamentous, collagen-containing structural subunits. These forms are often termed asymmetric, since the tail unit imparts substantial dimensional asymmetry to the molecule. The collagenous tail unit links by disulfide bonding at its proline rich N-terminus through a coiled coil arrangement to the C-terminus of two of the catalytic subunits [30]. The tail unit associates with the basal lamina of the synapse rather than the plasma membrane. 

We need a few words about the disulfide bonds that link the two chains of insulin. The side chain of Cys is —CH2—SH. The —SH group is termed the sulfhydryl group. If two of these side chains come together, the sulfhydryl groups can be linked together (oxidized) to form a disulfide bond ... 

Hair is composed of approximately 65 to 95% protein, 1 to 9% lipid, and small quantities of trace elements, polysaccharides, and water. - - The majority of hair protein is often referred to as keratin, which is a general term used to describe aggregates of protein with low or high sulfur content. These proteins are synthesized in the keratogenous zone of the hair follicle as matrix cells move upward from the hair bulb to form layers of the hair shaft. The cuticle, cortex, and medulla are comprised largely of keratin, although it is structurally different in each layer. Keratin in the exocuticle contains a high concentration of cysteine, which forms disulfide bonds which link the A-layer to the exocuticle, and this makes the cuticle... 

MPO is a covalently linked dimer which is ellipsoidal in shape with overall dimensions of 110 x 60 x 50 A. The dimer can be cleaved by reduction of a disulfide bond into two identical halves. Each half of the dimer termed hemi-MPO has the same optical and catalytic properties of the dimer. Hemi-MPO consists of two polypeptides of466 and 108 amino acid residues, and a heme prosthetic group covalently bound to the large polypeptide. Like CcP and LIP, MPO is largely a helical bundle protein with very little -sheet stmcture. The bulk of the large polypeptide folds into five separate domains and one... 

FIGURE 16.10 Hapten determinants formed by penicillins that contain a p-lactam ring linked to various side chains (R). The primary route of haptenation involves acylation of the E-amino group of lysine residues of serum or cell surface proteins to form a penicilloyl or major antigenic determinant. Isomerization of penicillin leads to the generation of compounds that form disulfide bonds wdth the cysteine sulfhydryl groups of proteins. These epitopes are termed minor determinants. 

How do these fragments relate to the three-dimensional structure of whole IgG molecules Immunoglobulin G consists of two kinds of polypeptide chains, a 25-kd light (L) chain and a 50-kd heavy (H) chain (Figure 33.5), The subunit composition is L2H2. Each L chain is linked to an H chain by a disulfide bond, and the H chains are linked to each other by at least one disulfide bond. Examination of the amino acid sequences and three-dimensional structures of IgG molecules reveals that each L chain comprises two homologous domains, termed immunoglobulin domains, to be... 

The crystal structure of canine MPO has recently been determined to 3-A resolution (7, 14). This mammalian enzyme is a covalently linked dimer of molecular weight 140 kDa that can be cleaved into two identical halves by reduction of a single disulfide bond. Each half of the dimer, termed hemi-MPO, has the same optical properties and catalytic activity as the parent. Hemi-MPO consists of two polypeptides of 466 and 108 amino acid residues, and a heme-type prosthetic group is covalently bound to the larger polypeptide in a crevice 15 A below the protein surface. Like CCP and LIP, the secondary structure of MPO is dominated by a-helices with relatively little /3-sheet structure (Fig. 

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