Holotranscobalamins in B12 and non B12 requiring prokaryotes and eukaryotes

Transcobalamins, vitamin B12 binding proteins, deliver B12 to cell surface receptors which then permit B12 to cross cell membranes for metabolic use. There is little documentation concerning B12 binding proteins in bacteria and protists. We found that prokaryotes and eukaryotes requiring B12, as well as those protists synthesizing B12, also produce several transcobalamins for functionally transporting B12 similar to humans.