Basic artichoke (Cynara scolymus L.) peroxidase (AKP-C), when purified from the plant, has an unusually intense and sharp Soret absorption peak. The resonance Raman spectrum [López-Molina, D., et al. (2003) J. Inorg. Biochem. 94, 243-254] suggested a mixture of pentacoordinate high-spin (5cHS) and 6-aquo hexacoordinate high-spin (6cHS) ferric heme species. The rate constant (k(1)) of compound I formation with hydrogen peroxide (H(2)O(2)) was also lower than expected. Further stopped-flow studies have shown this reaction to be biphasic: a nonsaturating fast phase and a slow phase with complex H(2)O(2) concentration dependence. Addition of calcium ions (Ca(2+)) changed the absorption spectrum, suggesting the formation of a fully 5cHS species with a k(1) more than 5 orders of magnitude greater than that in the absence of Ca(2+) using the chelator ethylenediaminetetraacetic acid. Ca(2+) titrations gave a dissociation constant for a single Ca(2+) of approximately 20 microM. The circular dichroism spectrum of AKP-C was not significantly altered by Ca(2+), indicating that any structural changes will be minor, but removal of Ca(2+) did suppress the alkaline transition between pH 10 and 11. A kinetic analysis of the reaction of Ca(2+)-free AKP-C with H(2)O(2) supports an equilibrium between a slow-reacting 6cHS form and a more rapidly reacting 5cHS species, the presence of which was confirmed in nonaqueous solution. AKP-C, as purified, is a mixture of Ca(2+)-bound 5cHS, 6-aquo 6cHS, and Ca(2+)-free 5cHS species. The possibility that Ca(2+) concentration could control peroxidase activity in the plant is discussed.