Transcobalamins are

carrier proteins A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane A biological membrane, biomembrane or cell membrane is a selectivel ...

which bind

cobalamin Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. One of eight B vitamins, it serves as a vital cofactor (biochemistry), cofactor in DNA synthesis and both fatty acid metabolism, fatty acid and amino a ...

(B₁₂).

Types

Transcobalamin I (TC-1), also known as

haptocorrin Haptocorrin (HC) also known as transcobalamin-1 (TC-1) or cobalophilin is a transcobalamin protein that in humans is encoded by the gene. One essential function of haptocorrin is protection of the acid-sensitive vitamin B12 while it moves thr ...

(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 The salivary glands in many vertebrates including mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibula ...

of the mouth. It primarily serves to protect

(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 Intrinsic factor (IF), also known as cobalamin binding intrinsic factor, or gastric intrinsic factor (GIF), is a glycoprotein produced by the parietal cells (in humans) or chief cells (in rodents) of the stomach. It is necessary for the absorp ...

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

enterocyte Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase ...

s 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

External links

* {{Globulins Vitamin B12