Chemistry:Transcobalamin
Transcobalamins are carrier proteins which bind cobalamin (vitamin 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-cobalamin 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 (IF) for absorption by ileal enterocytes.
Separate from the digestive absorption function, serum TC-1 binds 80–90% of circulating cobalamin, 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 cobalamin.[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 IF-cobalamin complex has been degraded. TC-2 is then involved with the transport of cobalamin 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
- ↑ "The complex machinery of human cobalamin metabolism". J Inherit Metab Dis 46 (3): 406–20. May 2023. doi:10.1002/jimd.12593. PMID 36680553.
- ↑ "Significance of elevated cobalamin (vitamin B12) levels in blood". Clin Biochem 36 (8): 585–90. November 2003. doi:10.1016/j.clinbiochem.2003.08.004. PMID 14636871.
- ↑ "Transcobalamin II and its cell surface receptor". Vitam Horm. Vitamins & Hormones 59: 337–66. 2000. doi:10.1016/s0083-6729(00)59012-8. ISBN 978-0-12-709859-3. PMID 10714245.
External links
- Transcobalamins at the US National Library of Medicine Medical Subject Headings (MeSH)
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