Disulfide bridge stabilization

At the dimer interface Gly-51 and Gly-114 are invariant residues that form hydrogen bonds with the nonconserved residue at position 151 Cys-57 and Cys-146 form a disulfide bridge stabilizing a region of the protein structure involved in monomer/monomer contact (see Section II). Four invariant residues, two Gly (16 and 147), one Phe (45), and one Leu (106) seem to contribute to maintaining the stable Greek key )8-barrel fold (see Section II). 

Disulfide bridges stabilize proteins, but they may also be imdesirable, so cells contain reducing agents that prevent or reverse this reaction. The most conamon of these redox agents is the tripeptide y-L-glutamyl-L-cysteinylglycine, or glutathione (Fig. 3-57). Its abbreviation is GSH the SH emphasizes the reactive part of the peptide. 

K. P. van den Bergh, P. Proost, J. van Damme, J. Coosemans, E. J. van Damme, and W. J. Peumans, Five disulfide bridges stabilize a hevein-type antimicrobial peptide from the bark of spindle tree (Euonymus europaeus L.), FEES Lett., 530 (2002) 181-185. 

The ability of proline to adopt a cis conformation in its peptide bond, for example, can create hairpin turns along the length of the protein. The thio groups of a cysteine side chain can be oxidized to form a covalent disulfide bridge between two adjacent cysteines. Disulfide bridges stabilize protein structure and can form large loops in proteins. 

The three dimensional shapes of many proteins are governed and stabilized by S—S bonds connecting what would ordinarily be remote segments of the molecule We 11 have more to say about these disulfide bridges m Chapter 27... 

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. 

Lysozyme from bacteriophage T4 is a 164 amino acid polypeptide chain that folds into two domains (Figure 17.3) There are no disulfide bridges the two cysteine residues in the amino acid sequence, Cys 54 and Cys 97, are far apart in the folded structure. The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is inactivated during reversible beat denat-uration. For the wild-type T4 lysozyme the Tm is 41.9 °C. 

The stabilizing activity of polyamine disulfides is specified by the imine group and the disulfide bridge, which participate simultaneously in the breaking of the chain by suppressing the radicals and in the free radical destruction of peroxides and hydroxides. 

Also important for stabilizing a protein s tertiary stmcture are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein. 

Disulfide bridges formation ChEs contain 8-10 cysteines six of these form three internal disulfide bridges. The cysteine that is located four amino acids upstream the carboxyl terminus forms a disulfide bridge with a cysteine of an identical subunit, creating an interchain disulfide bridge, which stabilizes the dimeric structure. 

Covalent bridging of biopolymers is one of the widely occurring prindples in nature for increasing the stability of the tertiary structure, for example the disulfide bridges in keratin and ribonuclease. 

In the Rieske proteins from bci or b f complexes, loops (34-/35 and (36-/37 both contain an additional cysteine residue (Cys 144 and Cys 160 in the ISF and Cys 112 and Cys 127 in RFS) these cysteines form a disulfide bridge connecting the two loops (Fig. 3b). These cysteines are not present in the sequences of Rieske-type proteins, that is, in neither NDO nor Rieske-type ferredoxins. In Rieske proteins, the disulfide bridge appears to be important for the stabilization of the fold around the cluster as the two loops are not shielded by other parts of the protein in NDO, the Rieske cluster is stabilized without a disulfide bridge since it is completely buried by surrounding a and (3 subunits. 

In summary, it appears that the protein has to adopt the correct fold before the Rieske cluster can be inserted. The correct folding will depend on the stability of the protein the Rieske protein from the thermoacidophilic archaebacterium Sulfolobus seems to be more stable than Rieske proteins from other bacteria so that the Rieske cluster can be inserted into the soluble form of the protein during expression with the help of the chaperonins. If the protein cannot adopt the correct fold, the result will be either no cluster or a distorted iron sulfur cluster, perhaps using the two cysteines that form the disulfide bridge in correctly assembled Rieske proteins. 

Nilsson, M., Wang, X., Rodziewicz-Motowidlo, S., Janowski, R., Lindstrom, V., Onnerfjord, P., Westermark, G., Grzonka, Z., Jaskolski, M., and Grubb, A. (2004). Prevention of domain swapping inhibits dimerization and amyloid fibril formation of cystatin C Use of engineered disulfide bridges, antibodies, and carboxymethylpapain to stabilize the monomeric form of cystatin C.J. Biol. Chem. 279, 24236- 24245. 

Disulfide bridges are, of course, true covalent bonds (between the sulfurs of two cysteine side chains) and are thus considered part of the primary structure of a protein by most definitions. Experimentally they also belong there, since they can be determined as part of, or an extension of, an amino acid sequence determination. However, proteins normally can fold up correctly without or before disulfide formation, and those SS links appear to influence the structure more in the manner of secondary-structural elements, by providing local specificity and stabilization. Therefore, it seems appropriate to consider them here along with the other basic elements making up three-dimensional protein structure. 

Despite their lack of stabilizing disulfide bridges Potl inhibitors feature a common, stable fold. The N-terminus is coiled, although in some structures a small /3-strand has been identified. After a turn the structure adopts an a-helical structure, followed by a turn and an other /3-strand. The sequence then features an extended turn or loop motif that contains the reactive site of the inhibitor before it proceeds with a /3-strand running almost parallel to the /3-strand after the a-helix. After another turn and coiled motif a short /3-strand antiparallel to the other /3-strands precedes the coiled C-terminus. Usually the N-terminal residue in the reactive site is an acidic residue followed by an aromatic amino acid, that is, tyrosine or phenylalanine. Figure 11 shows the complex of chymotrypsin inhibitor (Cl) 2 with subtilisin, the hexamer of Cl 2 from H. vulgare and a structural comparison with a trypsin inhibitor from Linum usitatissimum ... 

The global structure of Potll inhibitors is stabilized by disulfide bridges. The N-terminus features a coiled structure followed by two antiparallel /3-strands connected with a turn motif The second /3-strand is linked to the N-terminal region via a disulfide bond. After the second /3-strand the structure adopts an extended coiled loop that is attached to the N-terminal part through two disulfide bonds. This loop also contains the reactive site, which is positioned between the two disulfide bridges. The structure continues with a third /3-strand... 

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