This gene encodes the beta subunit of prolyl 4-hydroxylase， a highly abundant multifunctional enzyme that belongs to the protein disulfide isomerase family. When present as a tetramer consisting of two alpha and two beta subunits， this enzyme is involved in hydroxylation of prolyl residues in preprocollagen. This enzyme is also a disulfide isomerase containing two thioredoxin domains that catalyze the formation， breakage and rearrangement of disulfide bonds. Other known functions include its ability to act as a chaperone that inhibits aggregation of misfolded proteins in a concentration-dependent manner， its ability to bind thyroid hormone， its role in both the influx and efflux of S-nitrosothiol-bound nitric oxide， and its function as a subunit of the microsomal triglyceride transfer protein complex.