Bismuth, Interaction with Transferrin
Transferrin is an abundant serum glycoprotein of molecular mass 80 kDa (kilodaltons) (Wally and Buchanan 2007). Sequence similarity between the N- and C-terminal lobes of the molecule indicates that transferrin has evolved by duplication of an ancestral gene. Each lobe binds a single ferric ion with high affinity, via the side chains of two tyrosines, a histidine, and an aspartic acid. The remaining two ligands in the distorted octahedral binding site are provided by the synergistic anion bicarbonate. Transferrin is responsible for the transport of iron around the body, and delivers iron to target cells via interaction with the transferrin receptor TfR1. A related protein, lactoferrin, is found in milk and other secreted fluids, where its ability to bind iron tightly limits bacterial growth.