Cystine structure

Also the trityl group was applied for the masking of SH functions. The sulfides were prepared by the reaction of the thiol with trityl chloride (75% yield) or from trityl alcohol and the thiol in the presence of anhydrous TFA (85-90% yield). The cleavage of this group can be carried out under several conditions (Scheme 58). It is sensitive to acids (e.g. trifluoroacetic acid/ethanethiol 1 1) and to heavy metals. Thiocyanogen (SCN)2 oxidizes 5-trityl ethers to the disulfides and iodine converts 5-tritylcy -teine derivatives to cystine structures. 

Tertiary structure also refers to the overall shape of a molecule, especially to structures stabilized by disulfide bridges (cystine) formed by the oxidation of cysteine mercapto groups. 

Properties and Structure. a -Acid glycoprotein (a -AGP) has a molecular mass of about 41,000 and consists of a peptide chain having 181 amino acid residues and five carbohydrate units (14,15). Two cystine disulfide cross-linkages connect residues 5 and 147 and residues 72 and 164. The carbohydrate units comprise 45% of the molecule and contain siaUc acid, hexosamine, and neutral hexoses. In phosphate buffer the isoelectric point of the... 

Describe the stereochemical aspects of the structure of cystine, the structure that is a disulfide-linked pair of cysteines. 

One of the amino adds commonly found in protein hydrolysates is called cystine it has the following structure ... 

The Rp toxins also have an additional residue at the N-terminus which extends the sheet one residue in that direction. This may be an indication that the structural difference arises from the substitution of F17 for L17 in RpII, because there is also leucine at position 17 in the A. xanthogrammica toxins. The structure of the core sheet is remarkably similar among the different toxins, considering that there is essentially no conservation of the residues involved in forming it, aside from the cystines. 

Figure 33 The cyclo(Adm-Cyst)3 adopts a figure-eight-like helical structure. The chiral amino acid, cystine, configuration determines the helix disposition (rigjit-handed or left-handed helix). Adamantane plays an important role as a ring size controlling agent. Taken from Ref. [163] with permission.

One of the most convenient ways of generating sulfhydryl groups is by reduction of indigenous disulfides. Many proteins contain cystine disulfides that are not critical to structure or activity. 

Fig. 45. The structure of L-cystine dihydrobromide, seen down the crystallographic 2-fold axis. The disulfide is in a left-handed spiral conformation.

Now let us examine the distribution and position of disulfides in proteins. The simplest consideration is distribution in the sequence (see Fig. 51), which is apparently quite random, except that there must be at least two residues in between connected half-cystines. Even rather conspicuous patterns such as two consecutive halfcystines in separate disulfides turn out, when the distribution is plotted for the solved structures (Fig. 51), to occur at only about the random expected frequency. The sequence distribution of halfcystines is influenced by the statistics of close contacts in the three-dimensional structures, but apparently there are no strong preferences of the cystines that could influence the three-dimensional structure. 

There is a correlation between the backbone conformations which commonly flank disulfides and the frequency with which disulfides occur in the different types of overall protein structure (see Section III,A for explanation of structure types), although it is unclear which preference is the cause and which the effect. There are very few disulfides in the antiparallel helical bundle proteins and none in proteins based on pure parallel /3 sheet (except for active-site disulfides such as in glutathione reductase). Antiparallel /3 sheet, mixed /8 sheet, and the miscellaneous a proteins have a half-cystine content of 0-5%. Small proteins with low secondary-structure content often have up to 15-20% half-cystine. Figure 52 shows the structure of insulin, one of the small proteins in which disulfides appear to play a major role in the organization and stability of the overall structure. 

Thionins are cystine-rich, cationic small peptides ( 5 kDa) found in monocots and eudicots. They are divided into the families of o //3-thionins and 7-thionins. As is now generally accepted practice, we will refer to 7-thionins as plant defensins, as they are structurally more closely related to mammalian and insect defensins... 

Plant defensins are cystine-rich, cationic peptides ranging in size from 45 to 54 amino acids, of which eight are cysteine. They were first discovered in wheat and barley ° and were proposed to form a novel subclass of thionins, the 7-thionins. As it became clear that they closely resemble mammalian and insect defensins in primary and secondary structure, the term plant defensins was introduced to describe these peptides. It is generally assumed that all plants express plant defensins " and that they are expressed in a wide range of plant tissue, that is, leaves, floral tissue,tubers,bark, root, pods, and seeds,with seeds in particular being from where most plant defensins have been isolated. ... 

The backbone cyclization and cystine knot motif together render cyclotides as a class of structurally well-defined peptides that show exceptionally high stability against chemical, thermal, or enzymatic degrada-tion. °° ° This stability has meant that they have been proposed to have applications as protein engineering templates. 

The arsenal of plant defense peptides contains members capable of binding carbohydrate residues, namely /31-4 linked A -acetyl glucosamine residues that form the biopolymer chitin. The actual mode of action remains unclear. Antifungal and antimicrobial activity has been shown in vitro. For example Ac-AMP2 is a small disulfide-rich chitin-binding peptide isolated from the seeds of Amaranthus caudatus with antimicrobial activity. It differs from Ac-AMP 1 by one additional arginine residue at the C-terminus. The structure was determined by NMR and contains a cystine knot motif. Ac-AMP2 displays a so-called hevein domain partly... 

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