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transcobalamin I (vitamin B₁₂ binding protein, R binder family)

Identifiers

Symbol

TCN1

Other data

Chr. 11 q11-q12

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Structures	Swiss-model
Domains	InterPro

transcobalamin II; macrocytic anemia

Identifiers

Symbol

TCN2

Other data

Chr. 22 q11.2-qter

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Structures	Swiss-model
Domains	InterPro

Transcobalamins are carrier proteins which bind cobalamin (B₁₂).

Types

Transcobalamin I (TC-1), also known as haptocorrin (HC), R-factor, and R-protein is encoded in the human by the TCN1 gene. TC-1 is a glycoprotein produced by the salivary glands of the mouth. It primarily serves to protect cobalamin (Vitamin B12) from acid degradation in the stomach by producing a HC-Vitamin B₁₂ complex. Once the complex has traveled to the more pH-neutral duodenum, pancreatic proteases degrade haptocorrin, releasing free cobalamin, which now binds to intrinsic factor for absorption by ileal enterocytes.

Separate from the digestive absorption function, serum TC-1 binds 80-90% of circulating B₁₂, rendering it unavailable for cellular delivery by TC-2.^[1] Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B₁₂.^[2]

Transcobalamin II (TC-2), a nonglycoprotein secretory protein of molecular mass 43 kDa, is encoded in the human by the TCN2 gene. TC-2 binds cobalamin once it has been taken up by enterocytes of the terminal ileum and the "Intrinsic Factor-Vitamin B12" complex has been degraded. TC-2 is then involved with the transport of Vitamin B12 to the tissues, where it binds to its plasma membrane receptor (TC-2R), a heavily glycosylated protein with a monomeric molecular mass of 62 kDa, and releases cobalamin to the cells.^[3]

References

^ McCorvie TJ, Ferreira D, Yue WW, Froese DS (May 2023). "The complex machinery of human cobalamin metabolism". J Inherit Metab Dis. 46 (3): 406–20. doi:10.1002/jimd.12593. PMID 36680553.
^ Ermens AA, Vlasveld LT, Lindemans J (November 2003). "Significance of elevated cobalamin (vitamin B12) levels in blood". Clin Biochem. 36 (8): 585–90. doi:10.1016/j.clinbiochem.2003.08.004. PMID 14636871.
^ Seetharam B, Li N (2000). "Transcobalamin II and its cell surface receptor". Vitam Horm. Vitamins & Hormones. 59: 337–66. doi:10.1016/s0083-6729(00)59012-8. ISBN 978-0-12-709859-3. PMID 10714245.

External links

Transcobalamins at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

v t e Proteins: carrier proteins
Fatty acid	FABP1 FABP2 FABP3 FABP4 FABP5 FABP6 FABP7
Hormone	peptide hormone: Follistatin Growth hormone binding protein Insulin-like growth factor binding protein IGFBP1 IGFBP2 IGFBP3 IGFBP4 IGFBP5 IGFBP6 IGFBP7 Neurophysins Neurophysin I II steroid hormone: Sex hormone binding globulin/Androgen binding protein Transcortin Thyroxine-binding globulin Transthyretin
Metal/element	calcium Calcium-binding protein Calmodulin-binding proteins copper Ceruloplasmin iron Iron-binding proteins Transferrin receptor
Vitamin	Retinol binding protein 1 2 3 4 Transcobalamin
Pigment	Plant Light-Harvesting Complex Orange Carotenoid Protein Phycobiliprotein Photosynthetic Reaction Centers Phytochrome Rhodopsin
Other	Acyl carrier protein Adaptor protein Cholesterylester transfer protein F-box protein GTP-binding protein Latent TGF-beta binding protein Major urinary proteins Membrane transport protein Odorant binding protein

Globular proteins

Serum globulins

Alpha globulins

serpins:	alpha-1 (Alpha 1-antichymotrypsin, Alpha 1-antitrypsin) alpha-2 (Alpha 2-antiplasmin) Antithrombin (Heparin cofactor II)
carrier proteins:	alpha-1 (Transcortin) alpha-2 (Ceruloplasmin) Retinol binding protein
other:	alpha-1 (Orosomucoid) alpha-2 (alpha-2-Macroglobulin, Haptoglobin)

Beta globulins

carrier proteins:	Sex hormone-binding globulin Transferrin
other:	Angiostatin Hemopexin Beta-2 microglobulin Factor H Plasminogen Properdin

Gamma globulin

Immunoglobulins

Other

Other globulins

Albumins

Egg white	Conalbumin Ovalbumin Avidin
Serum albumin	Human serum albumin Bovine serum albumin Prealbumin
Other	C-reactive protein Lactalbumin (Alpha-lactalbumin) Parvalbumin Ricin Vitamin D-binding protein Extracellular matrix protein 1

see also disorders of globin and globulin proteins

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