Enzymes with an heterodinuclear iron–manganese active site: Curiosity or necessity?

This review analyzes the currently available data on true and purported FeMn enzymes with a particular emphasis on their specific physical properties. The characterization of the purple acid phosphatase from sweet potato and the current view of the hydrolysis mechanism are presented. The controversy associated with the discovery of the class Ic ribonucleotide reductase from Chlamydia trachomatis is discussed in the light of its extensive reactivity and physical studies. The amine oxygenase AurF is presented also albeit it is not exactly an FeMn enzyme but its case is particularly enlightening of the difficulties in assessing which is the right metal of an enzyme. Then, the very recent emergence of a new class of FeMn oxidases is highlighted. Lastly, examination of potential model compounds reveals the paucity of reported examples and therefore the need to develop this area. General considerations on biologically active metals and their substitution in hydrolases and redox active proteins are provided and possible reasons for the choice of the peculiar FeMn active site over the more classical diiron center are considered.