Catalyzes the post-translational formation of 4-hydroxyproline in hypoxia-inducible factor (HIF) alpha proteins. Hydroxylates HIF1A at 'Pro-402' and 'Pro-564'. May function as a cellular oxygen sensor and, under normoxic conditions, may target HIF through the hydroxylation for proteasomal degradation via the von Hippel-Lindau ubiquitination complex. 2-oxoglutarate + L-prolyl-[hypoxia-inducible factor alpha subunit] + O2 = CO2 + succinate + trans-4-hydroxy-L-prolyl-[hypoxia-inducible factor alpha subunit] Binds 1 Fe(2+) ion per subunit. Homodimer. Widely expressed with highest levels in adult pancreas, heart, skeletal muscle, brain, placenta, kidney and adrenal gland. Expressed at lower levels in epiphyseal cartilage and in fibroblasts. By hypoxia in many cultured cell lines. Glycosylated. The disease is caused by variants affecting the gene represented in this entry. Intron retention. Intron retention.