We show here that the amino acid residues contributing to the active sites of the vanadate containing haloperoxidases are conserved within three families of acid phosphatases; this suggests that the active sites of these enzymes are very similar. This is confirmed by activity measurements showing that apochloroperoxidase exhibits phosphatase activity. These observations not only reveal interesting evolutionary relationships between these groups of enzymes but may also have important implications for the research on acid phosphatases, especially glucose-6-phosphatase-the enzyme affected in von Gierke disease-of which the predicted membrane topology may have to be reconsidered.Haloperoxidases are enzymes catalyzing the two electron oxidation of a halide (X Ϫ ) to the corresponding hypohalous acid according to Eq. 1.HOX may further react with a broad range of nucleophilic acceptors to form a diversity of halogenated compounds. These haloperoxidases are named after the most electronegative halide they are able to oxidize, and thus a chloroperoxidase (CPO) is able to oxidize chloride, bromide, and iodide. Three classes of haloperoxidases have been identified. One of these consists of enzymes without a prosthetic group and as such have been detected in a number of bacteria (1, 2). The remaining two classes are the so-called heme-containing haloperoxidases-exemplified by the CPO from the fungus Caldariomyces fumago (3) or myeloperoxidase which is present in white blood cells (4)-and the vanadium-containing haloperoxidases that bind a vanadate ion (VO 4 3Ϫ ) as a prosthetic group. Enzymes representing these two classes not only differ in the nature of their prosthetic group but also in at least two other aspects: catalytic mechanism and stability. Hemecontaining peroxidases catalyze the formation of the hypohalous acid by a redox mechanism, whereas in vanadatecontaining peroxidases the transition metal does not change its redox state (5, 6) but may function as a Lewis acid. Vanadatecontaining haloperoxidases not only posses a very high stability (7, 8) but they also resist a high concentration of their substrate (H 2 O 2 ) (9) and their product (HOX) (7) that would readily inactivate the heme-containing peroxidases.Vanadate-containing peroxidases were first discovered in seaweeds about a decade ago (10, 11); at present they are being discovered not only in an increasing number of seaweeds (12, 13) but also in fungi (8) and in a lichen (14).The fungus Curvularia inaequalis has been shown to express a vanadium-containing CPO (V-CPO) (15), and recently the gene encoding this enzyme was cloned and sequenced (16). The V-CPO gene codes for a protein of 609 amino acids with a calculated molecular mass of 67,488 kDa. Database searches using the entire V-CPO sequence showed very little sequence similarity to other known proteins (16). The recent determination of the crystal structure of this enzyme at 2.1-Å resolution (17) revealed that the protein has an overall cylindrical shape and measures approximately 50 ϫ 80 Å. Th...