Transcobalamin
{{Short description|Group of carrier proteins which bind with Vitamin B12 in the blood}}
{{protein
|Name=transcobalamin I (vitamin B12 binding protein, R binder family)
|caption=
|image=
|width=
|HGNCid=11652
|Symbol=TCN1
|AltSymbols=
|EntrezGene=6947
|OMIM=189905
|RefSeq=NM_001062
|UniProt=P20061
|PDB=
|ECnumber=
|Chromosome=11
|Arm=q
|Band=11
|LocusSupplementaryData=-q12
}}
{{protein
|Name=transcobalamin II; macrocytic anemia
|caption=
|image=
|width=
|HGNCid=11653
|Symbol=TCN2
|AltSymbols=
|EntrezGene=6948
|OMIM=275350
|RefSeq=NM_000355
|UniProt=P20062
|PDB=2BB5
|ECnumber=
|Chromosome=22
|Arm=q
|Band=11.2
|LocusSupplementaryData=-qter
}}
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.{{cite journal |vauthors=McCorvie TJ, Ferreira D, Yue WW, Froese DS |title=The complex machinery of human cobalamin metabolism |journal=J Inherit Metab Dis |volume=46 |issue=3 |pages=406–20 |date=May 2023 |pmid=36680553 |doi=10.1002/jimd.12593 |url=}} Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B12.{{cite journal |vauthors=Ermens AA, Vlasveld LT, Lindemans J |title=Significance of elevated cobalamin (vitamin B12) levels in blood |journal=Clin Biochem |volume=36 |issue=8 |pages=585–90 |date=November 2003 |pmid=14636871 |doi=10.1016/j.clinbiochem.2003.08.004 |url=}}
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.{{cite journal |vauthors=Seetharam B, Li N |title=Transcobalamin II and its cell surface receptor |journal=Vitam Horm |series=Vitamins & Hormones |volume=59 |issue= |pages=337–66 |date=2000 |pmid=10714245 |doi=10.1016/s0083-6729(00)59012-8 |isbn=978-0-12-709859-3 |url=}}
References
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