Cysteine modifying agents

Several laboratories have replaced consecutive amino acids with cysteine residues as part of a technique called the substituted cysteine accessibility method (SCAM) (Akabas et al., 1992). In this technique, which is described elsewhere in this volume, the accessibihty of the inserted cysteine residues to an aqueous environment is measured by their accessibihty to covalent chemical modifying agents. The effects of the addition of these bulky agents can then be used to explore protein functional domains such as those that line ion channels or ligand binding sites. 

Figure 24. Exterior morphologies of extrusion texturized reducing or oxidizing agent-modified soy flours. Note that both reducing agents (Na>SO, and cysteine) -modified flours produced more puffed products, whereas the oidant (KIO,) modified flour showed opposite effect. (Mag. 1.5.)...

C) In the presence of a nucleophilic exomolecular agent such as H20, cysteine, amino acids, or amino groups, the nucleophilic agent is added to the o-quinone structure that evolves to a modified diphenol in position 5, which produces several polymeric compounds. 

This covalent bonding arises as a result of biochemical oxidation of the thiol groups in two cysteine residues, and it may also be achieved chemically with the use of mild oxidizing agents. This modification of thiol groups may thus loop a polypeptide chain or cross-link two separate chains. It also significantly modifies the properties of a... 

Strain K4 was tested in the absence of oxygen at various concentrations of glycerol (10,20,30,40 g L ).Two serum bottles were used with a modified Hungate technique one ofthe bottles contained resazurin and a reducing agent, namely cysteine-H Cl, and the other did not contain resazurin. The gas was analyzed by GC with TCD. 

Asparagine is unique as an acceptor amino acid for ADP-ribosylation by C3-like transferases (Sekine et ai, 1989). In contrast, pertussis toxin modifies cysteine residues, and cholera toxin and C. botulinum C2 toxin specifically ADP-ribosylate arginine residues (for details see the relevant chapters). The ADP-ribose-asparagine bond formed by C3-like transferases is stable towards neutral hydroxyl-amine [0.5 M, 2h) and mercury ions (2m/Vl, 1 h), whereas cysteine and arginine-specific ADP-ribosylation, respectively, are sensitive towards these agents (Aktories ef a/., 1988a). 

Acetylation can also affect Lys5, Lys8 and Lys12. The e-amino group of Lys20 is also modified, but by mono- or di-methylation. The biochemical methylating agent is S-adenosylmethionine (Section 8.5). The biosyntheses of coenzyme A and. S -adcno-sylmethionine provide additional examples of the biochemical utilisation of cysteine and methionine, respectively. 

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