Disulphide links

An alternative possibility is a vertical stacking structure where the p-strands are arranged on top of each other, again satisfying electrostatic and hydrophobic packing considerations (Fig. 20). Whichever is more likely, this is still far from a complete picture because the exact arrangement of the two disulphide-linked peptide chains is not yet known. 

Intrastrand bonding via disulphide links cause the molecule to fold into globular domains and it is these that direct the biological activity of the molecule. 

Additional BHT-derived 5-LO inhibitors bear heteroatom-linked 4-substituents. Searle s SC-45662 (50) was selective (25 1) for 5-LO over CO in cRBL (3.7 /iM) and in A23187-stimulated RBL-1 cells (7.1 yuM) [146]. Besides NSAID-like activity in RAA (down to 10 mg/kg p.o.), SC-45662 also inhibited GPB (ED30 16.7 mg/kg p.o.), and LTB4 release from ulcerative colitis rectal mucosal biopsy samples was decreased [147]. Several patents have described similar compounds where the alkyl substituent on sulphur is varied quite widely [148 151]. Oxidation of the distal sulphur was consistent with activity, while replacement of this sulphur with oxygen gave reduced potency. Simple alkyl groups, alkylene-linked esters and amides, and disulphide-linked alkanoic esters were also active in cRBL with similar potency free carboxylic acids were somewhat less potent. Oxidation of the sulphur attached to the phenolic ring destroyed the activity. 

Appendix 4A. We have retained the term cystine to indicate two disulphide-linked cysteines. 

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. 

All the major caseins associate with themselves and with each other. In unreduced form, K-casein is present largely as disulphide-linked polymers. K-Casein also forms hydrogen and hydrophobic bonds with itself and other caseins but these secondary associations have not been studied in detail. 

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. 

Plasmin is a serine proteinase (inhibited by diisopropylfluorophosphate, phenylmethyl sulphonyl fluoride and trypsin inhibitor) with a high specificity for peptide bonds to which lysine or arginine supplies the carboxyl group. Its molecular weight is about 81 Da and its structure contains five intramolecular disulphide-linked loops (kringles) which are essential for its activity. 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of / -lg with K-casein, and probably as2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

Reduction of disulfide bonds followed by alkylation of the free cysteines to prevent re-oxidation, while not essential for the digestion of most proteins, generally gives better results. This is due to increased susceptibility of the reduced/ alkylated protein to tryptic digestion and the absence of any disulphide-linked peptides (which are not matched in the database search) from the PMF data (6). 

Several plant proteins have been isolated that inhibit the metalloprotease carboxypeptidase A [205-217] (Table 7), notably potato carboxypeptidase inhibitor PCI [207-217] (Table 7). PCI is a small, cysteine-rich protein with a compact knotted structure determined by 3 disulphide links. The C-terminal region inserts into the active site of the carboxypeptidase. The C-terminal glycine is cleaved and remains trapped in the active site, this representing an example of suicide inactivation [207-216]. 

A variety of cereal (Poaceae) BBPIPs have been isolated and characterized [224, 271-284] (Table 9). Whereas the legume (Fabaceae) BBPIPs are typically double-headed, 7-9 kDa proteins with 7 disulphide links (Table 8), many cereal BBPIPs are 7-9 kDa and single-headed with 4-5 disulphides such as the Coix, Hordeum Bsil, Setaria, type II Triticum and Zea BBPIPs (Table 9). However some cereal BBPIPs are of the double-headed kind including particular Hordeum, Oryza and Triticum BBPIPs (Table 9). 

Various non-legume, non-Poaceae Bowman-Birk protease inhibitors have been isolated that have sequence homology to the other BBPIPs [137-140, 285-288] (Table 10). The Ananas BBPIPs are disulphide-linked heterodimers [137-140]. The Helianthus BBPIP SFTI-1 is a small cyclic peptide (cyclotide) [285, 286] whereas the Solanum tuberosum BBPIP is similar to the single-headed Poaceae BBPIPs [287, 288]. 

The mustard (Brassicaceae) 7 kDa trypsin inhibitors have molecular masses of about 7 kDa and have 8 cysteines (i.e. 4 disulphide links). For other details see the legend to Table 4. 

Rg. 1.3 Parts of the disulphide-linked A and B chains of bovine insulin. (Reconstructed from data deposited in the Brookhaven protein databank, with permission of Professor D. L D. Caspar and the Biophysical Journal.)... 

Platelet-derived growth factors (PDGFs) exist in three different dimeric compositions, containing disulphide-linked A-, or B-, or A- and B-chains (AA, BB, or heterodimeric... 

Its use with bradykinin and desmopressin (Zubarev et al., 1994), illustrated as a case study below, shows the benefits of MS sequence analysis. The latter peptide, desmopressin ( Minirin , a neurohypophysial hormone analogue), involves a disulphide link and a C-terminal amide, as well as a non-coded amino-acid residue (Mpa = mercaptopropionic acid) and these features can be identified by chemical sequencing methods (Chapter 5) only with considerable effort. 

Disulphide links (J = o HC - CH - S - S -1 NH 1 0 = ( CH,-( H Hl 1 Thioglycolic acid, reducing agents, alkalies, steam at high pressure or steam at atmospheric pressure for long periods of time. 

Characterization of the aggregates in heated milk revealed that the serum aggregates are mainly disulphide-linked complexes of whey protein and K-casein (Jean et al., 2006). Increasing the pH of milk from 6.5 to... 

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