PHD3 (Egl nine homolog 3; Hypoxia-inducible factor prolyl hydroxylase 3; HIF-prolyl hydroxylase 3; HIF-PH3; HPH-1; Egln3; Prolyl hydroxylase domain-containing protein 3;)Catalyzes the post-translational formation of 4-hydroxyproline in hypoxia-inducible factor (HIF) alpha proteins. Hydroxylates HIF-1 alpha at 'Pro-564', and HIF-2 alpha. Functions as a cellular oxygen sensor and, under normoxic conditions, targets HIF through the hydroxylation for proteasomal degradation via the von Hippel-Lindau ubiquitination complex. May play a role in cell growth regulation in muscle cells and in apoptosis in neuronal tissue. Promotes cell death through a caspase-dependent mechanism. [Catalytic Activity] An HIF alpha chain L-proline + 2-oxoglutarate+ O(2) = An HIF alpha chain trans-4-hydroxy-L-proline + succinate + CO(2). [Subcellular Location] Cytoplasm. Nucleus. Widely expressed at low levels. Expressed athigher levels in heart (cardiac myocytes, aortic endothelial cells and coronary artery smooth muscle) and placenta.