Everything about Protein Disulfide Isomerase totally explained
Protein disulfide isomerase or
PDI is an
enzyme in the
endoplasmic reticulum in eukaryotes or
periplasmic space of prokaryotes 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.
In contrast, reduced (dithiol) form of PDI is able to catalyse a reduction of mispaired thiol residues of a particular substrate, acting as an
isomerase. Therefore, PDI is capable of catalyzing the post-translational
disulfide exchange. Such exchange reactions can occur intramolecularly, leading to the rearrangement of disulfide bonds in a single protein.
Another major function of PDI relates to its activity as a
chaperone, for example, it aids wrongly-folded proteins to reach a correctly-folded state without the aid of enzymatic disulfide shuffling.
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 monoctyes. 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 isn't available on the cell surface, but instead is found in significant amounts in the blood plasma.
Oxidized PDI can catalyze the formation of a disulfide bridge. This reduces PDI and a protein called
Ero1 oxidizes it again.
Other functions
PDI helps load antigenic peptides into
MHC class I molecules. These molecules (MHC I) are related to the peptide presentation by
antigen presenting cells in the immune response.
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 620 nm, which is indirectly used monitor PDI activity. Sensitivity of this assay is in micromolar range.
ScRNase assay: PDI converts scrambled (inactive)
RNase into native (active) RNase that further acts on its substrate. The sensitivty 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.
Further Information
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