|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
|protein disulfide isomerase family A, member 2|
|Locus||Chr. 16 p13.3|
|protein disulfide isomerase family A, member 3|
|Locus||Chr. 15 q15|
|protein disulfide isomerase family A, member 4|
|Locus||Chr. 7 q35|
|protein disulfide isomerase family A, member 5|
|Locus||Chr. 3 q21.1|
|protein disulfide isomerase family A, member 6|
|Locus||Chr. 2 p25.1|
Protein disulfide isomerase or PDI is an enzyme in the endoplasmic reticulum in eukaryotes and the periplasm of bacteria that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold. This allows proteins to quickly find the correct arrangement of disulfide bonds in their fully folded state, and therefore the enzyme acts to catalyze protein folding.
PDI contains two catalytic thioredoxin-like domains (a, a') containing the canonical CGHC motif and two non catalytic domains (b, b'); it displays oxidoreductase and isomerase activity, both of which depend on the type of substrate that binds to PDI and changes in PDI's redox state. For basic oxidative folding, the first substrate cysteine residue binds to the active site--either the a domain or the a' domain--of PDI to form an enzyme substate complex; a second substrate cysteine residue binds to the complex, whereby after subsequent catalysis, a stable disulfide bridge on the substrate is formed, leaving PDI's cysteine residues reduced. Afterwards, PDI is regenerated into its oxidized form by the ER's environment. The reduced (dithiol) form of PDI is able to catalyze a reduction of misformed disulfide bridge of a particular substrate through either reductase activity or isomerase activity. For the reductase method, misfolded substrate disulfide bonds are converted to reduced cysteine residues by the transfer of electrons from glutathione and NADPH. Afterwards, normal oxidative folding occurs to the correct pairs of substrate cysteine residues, leading to properly folded proteins. For the isomerase method, intramolecular rearrangement of substrate functional groups is catalyzed near the N terminus of the a/a' domain. Therefore, PDI is capable of catalyzing the post-translational modification disulfide exchange.
In the chloroplasts of the unicellular algae Chlamydomonas reinhardtii the PDI RB60 serves as a redox sensor component of an mRNA-binding protein complex implicated in the photo-regulation of the translation of psbA, the RNA encoding for the photosystem II core protein D1. PDI has also been suggested to play a role in the formation of regulatory disulfide bonds in chloroplasts.
PDI has been found to be involved in the breaking of bonds on the HIV gp120 protein during HIV infection of CD4 positive cells, and is required for HIV infection of lymphocytes and monocytes. Some studies have shown it to be available for HIV infection on the surface of the cell clustered around the CD4 protein. Yet conflicting studies have shown that it is not available on the cell surface, but instead is found in significant amounts in the blood plasma.
PDI is critical for thrombus formation.
Assays used for PDI activity
Insulin Turbidity Assay: PDI breaks the two disulfide bonds between two insulin (a and b) chains that results in precipitation of b chain. This precipitation can be monitored at 650 nm, which is indirectly used to monitor PDI activity. Sensitivity of this assay is in micromolar range.
Di-E-GSSG assay: This is the fluorometric assay that can detect picomolar quantities of PDI and therefore is the most sensitive assay to date for detecting PDI activity. Di-E-GSSG has two eosin molecules attached to oxidized glutathione (GSSG). The proximity of eosin molecules leads to the quenching of its fluorescence. However, upon breakage of disulfide bond by PDI, fluorescence increases 70-fold.
Inhibition and Effects of Stress
Redox dysregulation leads to increases in nitrosative stress in the ER. Such adverse changes in the normal cellular environment of susceptible cells, such as neurons, leads to nonfunctioning PDIs. More specifically, PDI can no longer fix misfolded proteins once its thiol group in its active site has a nitric monoxide group attached to it; as a result, accumulation of misfolded proteins occurs in neurons, which leads to neurodegenerative diseases such as Alzheimer's Disease and Parkinson's Disease.
Due to the role of PDI in a number of disease states, small molecule inhibitors of PDI have been developed. These molecules can either target the active site of PDI irreversibly  or reversibly.
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