Cobalamin transport protein

Cobalamin transport proteins are a group of specialized carrier proteins responsible for the binding, protection, plasma transport, intestinal absorption, lysosomal export, and cellular delivery of cobalamin (vitamin B12) in mammals. The three main extracellular human cobalamin-binding proteins are haptocorrin, intrinsic factor, and transcobalamin. Each has a distinct role: • **Haptocorrin** binds dietary cobalamins in saliva/stomach. • **Intrinsic factor** binds dietary cobalamins after gastric passage; this complex is absorbed by ileal enterocytes via the cubilin receptor. • **Transcobalamin** binds newly absorbed or circulating free cobalamins for delivery to tissues through interaction with the transcobalamin receptor on cell surfaces. Additionally, intracellular trafficking involves lysosomal exporters such as ABCD4 and its escort LMBD1 that move internalized vitamin B12 from lysosomes into the cytosol for metabolic use. These proteins share a two-domain architecture that creates a high-affinity binding site at their domain interface. Dysfunction—whether genetic or acquired—in any step involving these carriers or their associated receptors leads to clinical syndromes characterized by impaired red blood cell formation and neurological deficits due to defective tissue supply of active vitamin B12. Note on correctness: "Cobalamin transport proteins" is not itself a single canonical target but rather refers collectively to several distinct but related carrier molecules—each with its own gene/protein name. For structured data purposes it is preferable to specify individual members such as "Transcoblamin," "Intrinsic factor," etc., rather than using this plural/generic form.

Other names

HaptocorrinIntrinsic factorTranscobalamin (sometimes specified as Transcobalamin II)Lysosomal cobalamin transporter (ABCD4)Lysosomal cobalamin transport escort protein (LMBD1)

Facilitate the binding, transport, and receptor-mediated endocytosis of vitamin B12 from dietary sources into circulation and then into target cells via specific carrier proteins and their receptors