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Transcobalamin

transcobalamin I (vitamin B12 binding protein, R binder family)
Identifiers
SymbolTCN1
NCBI gene6947
HGNC11652
OMIM189905
RefSeqNM_001062
UniProtP20061
Other data
LocusChr. 11 q11-q12
Search for
StructuresSwiss-model
DomainsInterPro
transcobalamin II; macrocytic anemia
Identifiers
SymbolTCN2
NCBI gene6948
HGNC11653
OMIM275350
PDB2BB5
RefSeqNM_000355
UniProtP20062
Other data
LocusChr. 22 q11.2-qter
Search for
StructuresSwiss-model
DomainsInterPro

Transcobalamins are carrier proteins which bind cobalamin (B12).

Types

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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

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  1. ^ 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.
  2. ^ 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.
  3. ^ 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.
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