Protein disulfide isomerase
Encyclopedia
Protein disulfide isomerase or PDI is an enzyme
in the endoplasmic reticulum
in eukaryotes that catalyzes the formation and breakage of disulfide bond
s between cysteine
residues within protein
s 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
.
. Therefore, PDI is capable of catalyzing the posttranslational modification
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, i.e., it aids wrongly folded proteins to reach a correctly folded state without the aid of enzymatic disulfide shuffling.
Oxidized PDI can catalyze the formation of a disulfide bridge. This reduces PDI and a protein called Ero1
oxidizes it again.
molecules. These molecules (MHC I) are related to the peptide presentation by antigen presenting cells in the immune response.
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 lymphocyte
s 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.
(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 sensitivity 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.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
in the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
in eukaryotes that catalyzes the formation and breakage of disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
s between cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
residues within protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s 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
Protein folding
Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....
.
Protein folding
The reduced (dithiol) form of PDI is able to catalyse a reduction of mispaired thiol residues of a particular substrate, acting as an isomeraseIsomerase
In biochemistry, an isomerase is an enzyme that catalyzes the structural rearrangement of isomers. Isomerases thus catalyze reactions of the formwhere B is an isomer of A.-Nomenclature:...
. Therefore, PDI is capable of catalyzing the posttranslational modification
Posttranslational modification
Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....
disulfide exchange
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
. 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, i.e., it aids wrongly folded proteins to reach a correctly folded state without the aid of enzymatic disulfide shuffling.
Oxidized PDI can catalyze the formation of a disulfide bridge. This reduces PDI and a protein called Ero1
ER Oxidoreductin
ER oxidoreductin 1 is an oxidoreductase enzyme that catalyses the formation and isomerization of protein disulfide bonds in the endoplasmic reticulum of eukaryotes. ER Oxidoreductin 1 is a conserved, luminal, glycoprotein that is tightly associated with the ER membrane, and is essential for the...
oxidizes it again.
Redox signaling
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 photoisystem II core protein D1. PDI has also been suggested to play a role in the formation of regulatory disulfide bonds in chloroplasts.Other functions
PDI helps load antigenic peptides into MHC class IMHC class I
MHC class I molecules are one of two primary classes of major histocompatibility complex molecules and are found on every nucleated cell of the body...
molecules. These molecules (MHC I) are related to the peptide presentation by antigen presenting cells in the immune response.
PDI has been found to be involved in the breaking of bonds on the HIV gp120
Gp120
Envelope glycoprotein GP120 is a glycoprotein exposed on the surface of the HIV envelope. The 120 in its name comes from its molecular weight of 120 kilodaltons...
protein during HIV infection of CD4
CD4
CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...
positive cells, and is required for HIV infection of lymphocyte
Lymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
s 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.
Assays used for PDI activity
Insulin Turbidity Assay: PDI breaks the two disulfide bonds between two insulinInsulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
(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 sensitivity 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
Eosin
Eosin is a fluorescent red dye resulting from the action of bromine on fluorescein. It can be used to stain cytoplasm, collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic....
molecules attached to oxidized glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...
(GSSG). The proximity of eosin molecules leads to the quenching
Quenching (fluorescence)
Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on...
of its fluorescence. However, upon breakage of disulfide bond by PDI, fluorescence increases 70-fold.