Purple manganese acid phosphatase

Purple, iron-containing acid phosphatases have been purified from animal sources and from some plant sources.350 However, the purple acid phosphatase from the sweet potato contains manganese, the purple colour arising from an intense absorption band at about 515 nm. There is some doubt over the stoichiometry, in that the dimeric enzyme may contain one351 or two352 Mn2+, apparently depending on the variety of sweet potato. The iron acid phosphatases contain two Fe atoms. 

A number of purple acid phosphatases821 have been isolated from animal sources, including bovine spleen, rat bone and the enamel organ of rat molars. Other phosphatases may belong to this class but the identification is not yet certain. Purple acid phosphatase from the sweet potato, as noted in Section 62.1.3.6.1, contains manganese. 

The purple acid phosphatases (PAP) catalyze the hydrolysis of phosphate esters under acidic pH conditions (pH optimum 5) (9, 10). They differ from other acid phosphatases in having a distinct purple color due to the presence of iron or manganese and in being uninhibited by tartrate. Diiron units have been found in the active sites of the enzymes from mammalian spleen (171-173) and uterus (173, 174), while a heterodinu-clear FeZn unit has been characterized for the enzyme from red kidney bean (175). Either the Fe2 or the FeZn unit is catalytically competent in these enzymes, since the enzymes from porcine uterus and bovine spleen can be converted into active FeZn forms and the kidney bean enzyme can be transformed into an active Fe2 form (176). There are also enzymes from other plant sources (particularly sweet potato) that have been reported to have either a mononuclear Mn(III) or Fe(III) active site (177), but these are beyond the scope of the review. This section will focus on the enzymes from porcine uterus (also called uteroferrin), bovine spleen, and red kidney bean. 

Purple Acid Phosphatases. Purple acid phosphatases (PAPs) utilize a dinuclear metal center to catalyze the hydrolysis of phosphate monoesters. The characteristic purple color of these enzymes arises from a charge transfer absorption at about 560 nm, between a tyrosinate ligand and the conserved Fe + found in all PAPs. The second metal ion varies with the source of the enzyme and is always divalent. Mammalian PAPs are monomeric and have Fe -Fe " centers, whereas most plant PAPs are dimeric with Fe " -Zn + centers. A PAP isolated from sweet potato contains an Fe +-Mn + center, the first of its kind in any enzyme (26,27). This novel PAP also differs from others by its greater catalytic efficiency toward both activated and unactivated substrates (27), as well as in its strict requirement for manganese in the divalent site (26). 

In order to generate more structurally relevant biomimetics for dinuclear metallohydrolases much effort has been devoted to the synthesis of asymmetric ligands. These ligands are considered to be more suitable models for the asymmetric coordination environment found in enzymatic systems. Nordlander et al. proposed that asymmetric complexes are not only more appropriate functional models for the active site of phosphoesterase enzymes, but also that they exhibit enhanced catalytic rates compared with their symmetric counterparts [1-3]. A selection of ligands used to generate purple acid phosphatase [1, 4, 5, 6-10], phosphoesterase [11], urease [12, 13], catechol oxidase [14] and manganese catalase biomimetics [15, 16] is displayed in Fig. 7.1. 

Tyrosine-bound manganese purple manganese acid phosphatase and ribonucleotide reductase 587... 

The isolation of the first manganese-containing acid phosphatase was reported in 1971 from the juice of the sweet potato (Kokei No. 14) (67). The enzyme was unique in that it was distinctly purple, the color resulting from a broad absorption band with a maximum at 555 nm. The enzyme was determined to be 110 kDa, composed of two 55-kDa subunits. The purple enzyme was capable of hydrolyzing a variety of biologically relevant phosphates as well as inorganic pyrophosphate [Eq. (2)]. Emission spectroscopy revealed the presence of Mn (68). 

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