Acid phosphatase dimer

Corrano and co-workers have characterized phosphate ester species with a mixed donor heteroscorpionate tripod zinc species 452 Krebs and co-workers synthesized purple acid phosphatase mimics including a structurally characterized FemZnn phosphate bridged dimeric species.453... 

Acid phosphatase (acid phosphomonoesterase, EC 3.1.3.2) also catalyzes the hydrolysis of phosphoric acid monoesters but with an acidic pH optimum. It has broad specificity and catalyzes transphosphorylations. Acid phosphatases are a quite heterogeneous group with monomeric, dimeric, larger glycoprotein, and membrane-bound forms. Acid phosphatase activity is present in the heart, liver, bone, prostate, and seminal fluid. Prostate carcinomas produce large quantities of acid phosphatase, and the enzyme is, therefore, used as a biomarker [141]. 

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. 

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. 

The purple acid phosphatases (PAPs) are a class of phosphoprotein phosphatases which possess a p-oxo(hydroxo)-bridged dinuclear iron centre. An enzyme has been isolated from beef spleen which is purple in colour, while a violet phosphatase has been characterised from red kidney beans (KBPase). This latter enzyme consists of two subunits with M = 58200 and contains two equivalents of Zn(II) and Fe(III) per dimer which are essential for catalytic activity. KBPase hydrolyses nucleosidetriphos-phates as well as activated phosphomonoesters such as 4-nitrophenylphosphate or a-naphthyl phosphate (Beck et al., 1986). As with the beef spleen enzyme, KBPase is inhibited by tetrahedral oxoanions such PO and AsO . 

In general there are three phosphatase families alkaline, acid, and protein phosphatases. Alkaline phosphatases are typically dimers that contain three metal ions per subunit and have a pH optimum pH above 8. Acid phosphatases exhibit an optimum pH<7 and are usually divided into three classes low molecular weight acid phosphatases (<20 kDa), high molecular weight acid phosphatases (50-60 kDa), and purple acid phosphatases (which contain an Fe-Fe or Fe-Zn center at the active site). Phosphatases specific for I-l-P appear to be most similar (in kinetic characteristics but not in mechanism) to the alkaline phosphatases, but their structures define a superfamily that also includes inositol polyphosphate 1-phosphatase, fructose 1, 6-bisphosphatase, and Hal2. The members of this superfamily share a common structural core of 5 a-helices and 11 (3-strands. Many are Li+-sensitive (York et al., 1995), and more recent structures of archaeal IMPase proteins suggest the Li+ -sensitivity is related to the disposition of a flexible loop near the active site (Stieglitz et al., 2002). 

Purple acid phosphatases (PAPs) catalyze the hydrolysis of phosphate monoesters with mildly acidic pH optima (5-7) utilizing a binuclear metal center containing a ferric ion and a divalent metal ion. PAPs are also characterized by their purple color, the result of a tyrosine (Tyr) to Fe3+ charge transfer transition at about 560nm.113 All known mammalian PAPs are monomeric and have a binuclear Fe3+-Fe2+ center, whereas the kidney bean and soybean enzymes are dimeric and have an Fe3 + -Zn2+ center in each subunit. The X-ray structures for kidney bean PAP114 and the PAP115 from rat bone reveal that despite a sequence similarity of only 18%, they share very similar catalytic sites. The structure of the kidney bean PAP shows the two metal ions at a distance of 3.1 A, with a monodentate bridging Asp-164. These and other residues involved in metal coordination can be seen in Fig. 21. 

The next step in coenzyme M formation is the dephosphorylation of phosphosulfolactate by a Mg(II)-dependent acid phosphatase, ComB. The third enzyme in the pathway, sulfolactate dehydrogenase (ComC), has also been structurally characterized with the bound reaction product NADH. ComC is present in solution as a dimer, and in the crystal the asymmetric unit contains a tetramer of tight dimers. The dimer is the enzymatically active unit and a portion of each monomer binds NADH at the active site. As a result of this interaction, ComC does not contain the classic Rossmann-Fold topology for NADH binding but instead defines a novel fold for NADH binding. 

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). 

Although reduction and activation are synonymous for the vast majority of the purple acid phosphatases, several exceptions exist. The Fe-Zn forms of uteroferrin and bovine spleen phosphatase do not require prior reduction to exhibit enzymatic activity . The Fe-Cu and Fe-Hg derivatives of uteroferrin also do not require activation and in fact, the Fe-Cu preparation is inactivated by reducing agents The recently described high molecular weight pink form of uteroferrin has enzymic properties identical to those of purple uteroferrin treated with 2-mercaptoethanoP. Finally, the sweet potato acid phosphatase, which may exist as an 02 dimer with separate mononuclear iron centers, does not require the addition of reductant to promote its enzymatic activity. ... 

Acid phosphatase is found in bee venoms as a dimer of a protein chain of about 49000 molecular weight (61, 98, 99, 100, 101, 102). Acid phosphatase activity cannot be consistently demonstrated in venoms from vespid wasps. The N-terminal sequence and the sequences of a number of proteolytic peptides from honeybee venom acid phosphatase have recently been reported (103). The venom acid phosphatase is more closely related to mammalian prostatic type acid phosphatases (104, 105, 106, 107, 108) than to lysosomal type acid phosphatases (109, 110), as is illustrated in Fig. 5. 

Alkaline phosphatase can be reversibly denatured by thiol reduction in the presence of urea (88), a treatment which dissociates the dimer. Proteins purified from alkaline phosphatase-negative mutants that are antigenically related to alkaline phosphatase are readily and reversibly dissociated by acid (65). Normal alkaline phosphatase is more stable but at a lower pH, less than 3.0, it too forms monomers with release of zinc ions. However, chelating agents that remove zinc do not cause... 

Figure 18.5 summarizes some of the salient control features that affect glycogen phosphorylase activity. The enzyme is a dimer that exists in two forms, the inactive T (taut) form and the active R (relaxed) form. In the T form (and only in the T form), it can be modified by phosphorylation of a specific serine residue on each of the two subunits. The esterification of the serines to phosphoric acid is catalyzed by the enzyme phosphorylase kinase the dephosphorylation is catalyzed by phosphoprotein phosphatase. The phosphorylated form of glycogen phosphorylase is called phosphorylase a, and the dephosphorylated form is called phosphorylase h. The switch from phosphorylase b to phosphorylase a is the major form of control over the actiHty of phosphorylase The response time of the changes is on the order of seconds to nunutes. Phosphoiylase is... 

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