Disulfide bonds, protein folding mechanism

A pathway analogous to the DsbA-DsbB system was recently shown to operate in the endoplasmic reticulum of eukaryotes (Frand and Kaiser, 1998 1999 Pollard et al., 1998). This pathway includes the membrane-bound protein EROl, which keeps protein disulfide isomerase (PDI) in an oxidized state in vivo. Consequently, PDI plays a role somewhat similar to that of DsbAin catalyzing the formation of disulfide bonds during the folding of secreted proteins. The mechanisms driving oxidative protein folding in eukaryotes are described in more detail in another chapter in this volume. 

Cirilli et al.11171 cloned the gene of diaminopimelate epimerase from Haemophilus influenzae, and purified and crystallized the enzyme. The enzyme is monomeric and has a unique protein fold, in which the amino terminal and carboxyl terminal halves of the molecule fold into structurally homologous and superimposable domains (Fig. 17-13). Cys 73 of the amino terminal domain is found in the disulfide linkage, at the domain interface, with Cys 217 of the carboxy terminal domain 117. Thus, it is most conceivable that these two cysteine residues stay in reduced form in the active enzyme and function as the acid and base in the mechanism. Koo and Blanchard 118 explored a number of kinetic and isotope approaches to clarify the mechanism of the enzyme. However, which of the two cysteine residues is responsible for proton abstraction from the two enantiomeric Ca-H bonds is not yet known. 

The numerous specialized bacterial secretion systems that have been identified can be classified into four general types based on their mechanism of operation. Both the type I and the type II secretion systems involve two steps. First, substrate proteins are translocated across the inner membrane into the periplasmic space, where they fold and often acquire disulfide bonds. Second, the folded proteins are translocated from the periplasmic space across the outer... 

By analogy with animal systems, the function of signal sequences in plant storage proteins is to facilitate the translocation of the storage protein into the lumen of the endoplasmic reticulum (ER) as the first step in intracellular transport. Protein folding and disulfide bond formation are considered to occur within the lumen of the endoplasmic reticulum, and may be assisted by molecular chaperones and by the enzyme protein disulfide isomerase respectively [83]. The precise mechanism of intracellular transport of storage proteins from their site of synthesis to their site of deposition are still largely unknown but a two-way hypothesis has been proposed by Shewry [45]. 

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