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.
[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 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.