Enzymes disulfide bonds

In enterokinase, which is a membrane-bound enzyme, disulfide bonds holding the light and heavy chains together are not essential for the activity (Savithi and Light, 1980). 

The shape of a large protein is influenced by many factors including of course Its primary and secondary structure The disulfide bond shown m Figure 27 18 links Cys 138 of carboxypeptidase A to Cys 161 and contributes to the tertiary structure Car boxypeptidase A contains a Zn " ion which is essential to the catalytic activity of the enzyme and its presence influences the tertiary structure The Zn ion lies near the cen ter of the enzyme where it is coordinated to the imidazole nitrogens of two histidine residues (His 69 His 196) and to the carboxylate side chain of Glu 72... 

Figure 6.6 Schematic diagram of the structure of the enzyme lysozyme which folds into two domains. One domain is essentially a-helical whereas the second domain comprises a three stranded antiparallel p sheet and two a helices. There are three disulfide bonds (green), two in the a-helical domain and one in the second domain.

Enzymes assist formation of proper disulfide bonds during folding... 

Figure 6.8 Schematic diagram of the enzyme DsbA which catalyzes disulfide bond formation and rearrangement. The enzyme is folded into two domains, one domain comprising five a helices (green) and a second domain which has a structure similar to the disulfide-containing redox protein thioredoxin (violet). The N-terminal extension (blue) is not present in thioredoxin. (Adapted from J.L. Martin et al.. Nature 365 464-468, 1993.)...

Enzymes assist formation of proper disulfide bonds during folding Isomerization of proline residues can be a rate-limiting step in protein folding Proteins can fold or unfold inside chaperonins GroEL is a cylindrical structure with a... 

Disulfide bonds are hardly formed in cytosol of unmodified E. coli strains. Such enzymes can be expressed in E. coli Origami strain, in cells coexpressing helper proteins, such as PDI or DsbC, or they can be directed to the more oxidative periplasm. 

Most poly(HA) depolymerases are inhibited by reducing agents, e.g., dithio-erythritol (DTT), which indicates the presence of essential disulfide bonds, and by serine hydrolase inhibitors such as diisopropyl-fluoryl phosphate (DFP) or acylsulfonyl derivates. The latter compounds covalently bind to the active site serine of serine hydrolases and irreversibly inhibit enzyme activity [48]. 

The poly(3HO) depolymerase differs from poly(HASCL) depolymerases in several of its biochemical properties it is specific for poly(HAMCL) and for artificial esters such as p-nitrophenylacyl esters with six or more carbon atoms in the fatty acid moiety. Poly(3HB) and other poly(HASCL) are not hydrolyzed. The enzyme is not inhibited by dithioerythritol or by EDTA and therefore apparently does not contain essential disulfide bonds. It is also not dependent on Ca2+ or other divalent cations. 

Figure 20.4 Reduction of the disulfide bonds within the hinge region of an IgG molecule produces half-anti-body molecules containing thiol groups. Reaction of these reduced antibodies with a maleimide-activated enzyme creates a conjugate through thioether bond formation.

The inactivation of enzymes containing the zinc-thiolate moieties by peroxynitrite may initiate an important pathophysiological process. In 1995, Crow et al. [129] showed that peroxynitrite disrupts the zinc-thiolate center of yeast alcohol dehydrogenase with the rate constant of 3.9 + 1.3 x 1051 mol-1 s-1, yielding the zinc release and enzyme inactivation. Later on, it has been shown [130] that only one zinc atom from the two present in the alcohol dehydrogenase monomer is released in the reaction with peroxynitrite. Recently, Zou et al. [131] reported the same reaction of peroxynitrite with endothelial NO synthase, which is accompanied by the zinc release from the zinc-thiolate cluster and probably the formation of disulfide bonds between enzyme monomers. The destruction of zinc-thiolate cluster resulted in a decrease in NO synthesis and an increase in superoxide production. It has been proposed that such a process might be the mechanism of vascular disease development, which is enhanced by diabetes mellitus. 

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