| Summary: | Over the last two decades several crystal structures of Antarctic fish hemoglobins (AFH) have been determined. Here we review the most relevant results. The hemichrome formation has been proved as a property common to every AFH, and the current results are here presented in contrast to the mammalian hemoglobins (Hbs). Furthermore, many AFH exhibit the Root effect, a drastic reduction in oxygen-carrying capacity and cooperativity occurring at acidic pH, a property common to many fish Hbs. Despite the longstanding problem, a general structural explanation of the Root effect is still pending. Recent advances have re-opened the question and new perspectives leading us toward a solution are presented.
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