Transcobalamin

Transcobalamins are carrier proteins which bind cobalamin (vitamin B₁₂).

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. Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum cobalamin.

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.