Transcobalamins arecarrier proteins which bindcobalamin (B₁₂).

Transcobalamin I (TC-1), also known ashaptocorrin (HC),R-factor, andR-protein is encoded in the human by theTCN1 gene. TC-1 is a glycoprotein produced by thesalivary glands of the mouth. It primarily serves to protectcobalamin (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 tointrinsic factor for absorption byileal 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 theTCN2 gene. TC-2 binds cobalamin once it has been taken up byenterocytes 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]

• Transcobalamins at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

• Genes on human chromosome 11
• Genes on human chromosome 22
• Vitamin B12

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