Transcobalamin I (TCN1)

There are 3 types of human vitamin B12-binding proteins: gastric intrinsic factor (GIF), also known as transcobalamin III; plasma transcobalamin II (TCN2); and transcobalamin I (TCN1), an R binder protein. R binder proteins are a class of immunologically identical but electrophoretically distinct glycoproteins found in many secretions, such as saliva, gastric juice, tears, milk and leukocytes, and differ only in their carbohydrate content; """R""" denotes """rapid,""" referring to electrophoretic mobility. Transcobalamin I is also found in neutrophils, where it is restricted to the secondary granules.The deduced 433-residue protein showed extensive areas of homology with rat gastric intrinsic factor, which implicated regions potentially important for vitamin B12 binding.Comparison of the genomic structures of TCN1 and GIF suggested that the 2 genes arose by a gene duplication.