Introduction

Disulfid bridges(or bonds) are among the factors that determine stability and therefore proteins structure. unlike other factors that determine tertiary structure of protein, this factor has covalent nature thus their breakage and rejoining creates greater demand for cells. these particular bonds form under oxidative condition that is existed in extracellular spaces and cell organels. this demand for formation of correct disulfid bonds need to be addressed in cell organels.
The Enzyme
Protein Disulfid Isomerase (PDI) ,E.C. 5.3.4.1 is an ER enzyme in eukaryotes and periplasmic enzyme in prokaryotes. it catalysis disulfide bond formation and switch in proteins in folding process until they reach their native conformation.
Mechanism
Only oxidized form of enzyme can catalyze these function. the enzyme act as a reductant on their target protein while their disulfide bond oxidizes. another factor called Ero1 act on this enzyme to return it back to its active state(Oxidized form).
Assay to measure activity
Insulin Turbidity Assay: PDI brakes disulfide bond between A and B chain therefore the chains dissociate from one another. this may lead to precipitation of B chain that can be measured on 620nm.