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. Highest expression levels are detected in the eye and brain, especially in the retinal epithelium cells and cortical neurons. Also expressed in skeletal muscle, lung, heart, adrenal gland, kidney, prostate, thyroid and testis. Glycosylated. Mutant mice exhibit inflammation and fibrosis of renal tubuli, glomerular sclerosis, enlarged Bowman capsules, and develop late-onset proteinuria. Vision is compromised, primarily due to impairment of cone function. Contaminating sequence. Sequence of unknown origin in the N-terminal part.