A disulphide

AVP and OT are cyclic nonapeptides with a disulphide bridge between the cysteine residues 1 and 6, resulting in a six-amino acid ring and a COOH-terminal a-amidated three-residue tail. OT differs only in two amino acids from AVP lie in position 3, which is essential for OT recqrtor (OTR) stimulation and Leu in position 8. AVP has a Phe in position 3 and an Arg in position 8. Arg 8 is essential for acting upon vasopressin receptors (Fig. 1). Lysipressin, found in pigs and some marsupials, has a Lys in position 8 [1]. 

Unsymmetrical thiosulphinates and thiosulphonates are both oxidized by potassium superoxide in pyridine in the presence of 18-crown-6 ether to produce sulphinic and sulphonic acids and a disulphide, under mild conditions (equation 84)200,201. Sulphinic and sulphonic acids were produced from both the R and R substituents whilst the disulphide was derived only from the sulphenyl side of the reactant. Thus, the reaction mixture contained five products, making the reaction not synthetically useful. Pyrolysis of thiosulphinates also produces mixtures of products, one being the thiosulphonate again this is not a synthetically useful reaction202. 

Vasopressin is closely related to oxytocin and both peptides are cyclic in that they contain a disulphide bridge. Although much is known about the peripheral actions of the peptides the extent of our current knowledge of their possible CNS function is that vasopressin appears to act as a cognitive enhancer and has positive effects on learning processes in animals. Vasopressin acts on three receptors, Via and b and a V2 receptor. 

Fig. 1.4 The increase in thymidylate synthase inhibition activity obtained from an initial hit identified as a disulphide-bound enzyme adduct and optimised to a reversible potent 330nM inhibitor using crystal structure-guided design.

TNF- a was first purified from conditioned medium from HL-60 cells. It has a relative molecular mass of 17 kDa when analysed by SDS-PAGE, but 45 kDa when analysed by gel filtration. Thus, the molecule exists as a non-glycosylated trimer with a pi of 5.3. Each monomer comprises 157 amino acids and contains two cysteine residues that form a disulphide bridge. Trimer formation appears to be due to noncovalent interactions between the monomers. Human TNF-a is synthesised as a 233-amino-acid protein that is proteolytically cleaved during processing. Whilst the 17-kDa form is readily secreted (and hence can function as an extracellular mediator), a 26-kDa transmembrane form has also been identified. This form of TNF-a may thus function in cytotoxicity resulting from cell-cell contact. 

Step 3 D-Penicillamine thus obtained is oxidised quantitatively by iodine to give rise to a disulphide, as expressed in the following equation whereas, the excess iodine is back titrated with 0.02 N sodium thiosulphate solution ... 

A disulphide bond is a covalent bond between the sulphur atoms of two cysteine residues, each In a different part of a polypeptide chain. 

With a disulphide compound as acceptor 1.14.14 With reduced flavin or flavoprotein as one... 

Fluorinated base, -dG residue with a disulphide linker at the exocyclic amino group, RT-room temperature. 

Baumann next found that cystine, on reduction with zinc and hydrochloric acid, was converted into a new base, which he called cysteine this gave the same products on decomposition as cystine, into which it was easily reconverted by oxidation. He therefore recognised that these compounds were related to each other, as a mercaptan is to a disulphide consequently the formula... 

N.A. Disulphides, resin, gum, sesquiterpenoid coumarins, foetidin." An expectorant, for digestive problems, bronchitis, bronchial asthma, whooping cough, lower blood pressure and thin the blood. 

Disulphide bonding. The two principal caseins, asl and / , contain no cysteine or cystine but the two minor caseins, as2 and k, each contains two cysteines per mole which normally exist as intermolecular disulphide bonds. Under non-reducing conditions, ocs2-casein exists as a disulphide-linked dimer (previously known as as5 casein) while K-casein exists as a series of disulphide-linked molecules ranging from dimers to decamers. 

When heated in the presence of whey proteins, as in normal milk, K-casein and /Mactoglobulin interact to form a disulphide-linked complex which modifies many properties of the micelles, including rennet coagulability and heat stability. 

In the direct synthesis of thiophosphonium salts, the oxidizing agent can be a disulphide using either constant-current electrolysis (c.c.e) conditions or by simple addition with benzoic acid433 (reaction 122). 

Sulphur compounds, (a) Disulphides can be purified by extracting acidic and basic impurities with aqueous base or acid, respectively. However, they are somewhat sensitive to strong alkali which slowly cleaves the disulphide bond. The lower-melting members can be fractionally distilled under vacuum. The high members can be recrystallised from alcohol, toluene or glacial acetic acid. 

A disulphide, [(HCS)2S is formed as a yellowish-red precipitate when potassium dithioformate in alcohol solution is cautiously oxidised by the addition of iodine. It decomposes similarly to the acid above 200° C. Both sulphides are insoluble in the common solvents. Dithioformates of silver, lead, zinc and cobalt have also been described.2... 

Pantothenate salts are more stable and less hygroscopic than the free acid (185). In solution, pantothenate salts are most stable at pH 6-7. The pantothenate salts are very soluble in water. Coenzyme A is water soluble and stable at pH 2-6 it can be oxidized to coenzyme A disulphide. 

From biochemical studies after in vitro isolation, NADH NR appears to be a soluble homodimeric enzyme (Pan Nason, 1978 Solomonson Barber, 1990), aggregating into tetramers at high concentration, as has been described in Chlorella and occasionally observed in higher plants. Both subunits (molecular mass 100-110 kDa) have recently been shown to be linked by a disulphide bond which can be reduced without loss of activity (Hyde et al., 1989). There is no direct evidence that a monomer subunit alone is active. Nevertheless, radiation inactivation analysis (Solomonson et al., 1987) provided a target size for Chlorella and spinach NADHrNR of 100 kDa, suggesting that indeed each subunit would behave independently as a functional NADH NR unit. 

Polarography has been successfully applied to the investigation of structural problems involving sulphur compounds. The presence of a disulphide bond has been established by means of the polarographic reduction waves of cytochrome C (156) and lipoic acid (757), and in cyclic disulphides of the oxytocine and vasopressine type (158). The elucidation of the process responsible for the reduction wave of lipoic acid was carried out by comparison with reduction waves of cyclic disulphides, where the disulphide bond was incorporated into rings of various size. The similarity indicated that in lipoic acid an S—S bond which is a part of a larger cyclic system is reduced. 

Another example of a quantal repeat—but with considerable variation in sequence—is seen in the keratin-associated proteins (KAPs). In sheep, these display pentapeptide and decapeptide consensus repeats of the form G—G—Q—P—S/T and C-C-Q/R—P—S/T—C/S/T—C—Q—P/T—S, respectively (Parry et al., 1979). Some of the positions, as indicated by the presence of a consensus sequence, contain residues that occur much more frequently than others, but the absolute conservation of a residue in any position is not observed. The decapeptide consists of a pair of five-residue repeats closely related, but different to that displayed by the pentapeptide. Although the repeats have an undetermined structure, the similarity of the repeat to a sequence in snake neurotoxin suggests that the pentapeptides will adopt a closed loop conformation stabilized by a disulphide bond between cysteine residues four apart (Fig. 5 Fraser et al., 1988 Parry et al, 1979). Relative freedom of rotation about the single bond connecting disulphide-bonded knots would give rise to the concept of a linear array of knots that can fold up to form a variety of tertiary structures. The KAPS display imperfect disulphide stabilization of knots and have interacting... 

Fig. 5. Predicted conformation of the pentapeptide repeat C-X-Y-Z-C in trichocyte keratin-associated proteins. Glutamine and arginine residues are found commonly in the X position, prolines in the Yposition, and serines and threonines in the Z position. The structure is based on the known conformation of a similar repeat in snake neurotoxin. The model shows a disulphide bond-stabilized /5-bend with a potential hydrogen bond (dotted). A string of these /5-bends, linked by bonds about which there is relatively free rotation, has been proposed as a model for this important family of matrix proteins in trichocyte keratin (Fraser et al, 1988). Figure from Fraser et al (1988) with permission from Elsevier.

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