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Group of carrier proteins which bind with Vitamin B12 in the blood
transcobalamin I (vitamin B12 binding protein, R binder family)
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. Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B12.
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.
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. PMID36680553.