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Since 1933, carbonic anhydrase research has focused on enzymes from mammals (α class) and plants (β class); however, two additional classes (γ and δ) were discovered recently. Cam, from the procaryote Methanosarcina thermophila, is the prototype of the γ class and the first carbonic anhydrase to be characterized from either an anaerobic organism or the Archaea domain. All enzymes characterized from the four classes have been purified aerobically and are reported to contain a catalytic zinc. Herein, we report the anaerobic reconstitution of apo-Cam with Fe²⁺ which yielded Cam with an effective k_cat which exceeded that for the Zn²⁺-reconstituted enzyme. Mössbauer spectroscopy showed that the Fe²⁺-reconstituted enzyme contained high-spin Fe²⁺ that when oxidized to Fe⁺³ inactivated the enzyme. Reconstitution with Fe³ was unsuccessful. Reconstitution with Cu², Mn²⁺, Ni²⁺, or Cd²⁺ yielded enzymes with effective k_cat values that were 10% or less than the value for the Zn²⁺-reconstituted Cam. Cam produced in Escherichia coli and purified anaerobically contained iron with effective k_cat and k_cat/Km values exceeding the values for Zn²⁺-reconstituted Cam. The results identify a previously unrecognized biological function for iron.