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  Domain Name: PDI_b'_family
Protein Disulfide Isomerase (PDIb') family, redox inactive TRX-like domain b'; composed of eukaryotic proteins involved in oxidative protein folding in the endoplasmic reticulum (ER) by acting as catalysts and folding assistants. Members of this family include PDI, calsequestrin and other PDI-related proteins like ERp72, ERp57 (or ERp60), ERp44, P5 and PDIR. PDI, ERp57, ERp72, P5 and PDIR are all oxidases, catalyzing the formation of disulfide bonds of newly synthesized polypeptides in the ER. They also exhibit reductase activity in acting as isomerases to correct any non-native disulfide bonds, as well as chaperone activity to prevent protein aggregation and facilitate the folding of newly synthesized proteins. These proteins contain multiple copies of a redox active TRX (a) domain containing a CXXC motif, and one or more redox inactive TRX-like (b) domains. The molecular structure of PDI is abb'a'. Also included in this family is the PDI-related protein ERp27, which contains only redox-inactive TRX-like (b and b') domains. The redox inactive domains are implicated in substrate recognition with the b' domain serving as the primary substrate binding site. Only the b' domain is necessary for the binding of small peptide substrates. In addition to the b' domain, other domains are required for the binding of larger polypeptide substrates. The b' domain is also implicated in chaperone activity.