Mammalian hypoxia-inducible factors. HIF-1α, HIF-2α, HIF-3α, and HIF-1β are each encoded by a distinct gene, with alternative mRNA splicing generating multiple isoforms (not shown) of HIF-1α, HIF-3α, and HIF-1β. HIF-1α, HIF-2α, and HIF-3α dimerize with HIF-1β to form the functional proteins HIF-1, HIF-2, and HIF-3, respectively. The basic-helix-loop-helix (bHLH)–PAS domains mediate dimerization and DNA binding. All known HIF-1 binding sites contain the core DNA sequence 5′-RCGTG-3′ (R, A, or G). Hydroxylation of prolyl residues (402 and 564 in human HIF-1α) is required for the binding of the von Hippel-Lindau protein (VHL), which targets the proteins for ubiquitination and proteasomal degradation. Hydroxylation of an asparagine residue (803 in human HIF-1α) blocks the binding of coactivators p300 and CBP. The HIF prolyl and asparaginyl hydroxylases use O₂ and α-ketoglutarate and generate CO₂ and succinate as by-products.