transcobalamin
Transcobalamin refers to a family of vitamin B12–binding proteins that mediate the transport of cobalamin (vitamin B12) in the bloodstream and to tissues. In humans, the main members are transcobalamin I (TCN1, also known as haptocorrin), transcobalamin II (TCN2, transcobalamin), and transcobalamin III (TCN3). TCN1 is produced in various tissues and secreted into plasma and other fluids; it binds corrinoids including vitamin B12 but does not efficiently transfer them to cells, instead helping to protect cobalamin in the circulation and in secretions. TCN2 is the principal transport protein for cobalamin in blood and forms holo-transcobalamin (holo-TC) when bound to a molecule of cobalamin. The holo-TC–cobalamin complex is recognized by the cellular receptor CD320 (transcobalamin receptor), allowing uptake of cobalamin into cells where it is converted to the active cofactors methylcobalamin and adenosylcobalamin. The holo-transcobalamin fraction represents the bioavailable portion of vitamin B12 in the circulation; total serum B12 includes holo-TC as well as B12 bound to TCN1 and other proteins.
TCN3’s role is less clearly defined; some studies suggest tissue-associated or regulatory functions with limited involvement