|MadSci Network: Biochemistry|
I am not an expert on transferrin, but I hope I can answer your question to your satisfaction. Ferric iron (Fe3+) is found under conditions of neutral or alkaline pH, while ferrous iron (Fe2+) is found under acidic pH. This in itself would suggest that transferrin would mostly only come into contact with ferric iron in the body. Anyway, transferrin binds to a number of different metals, but its highest affinity is for ferric iron. It does not bind at all to ferrous iron. This would explain the colour change / absorbance when mixed with Fe3+, but no change when transferrin is mixed with Fe2+.
In terms of the bicarbonate versus carbonate question, I can see the problem, because a lot of people are lazy and do not distinguish between the two. I looked up a research article that definitively said that transferrin binds to bicarbonate (aka hydrogen carbonate) but not carbonate.
Hope this helps!
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