Purple acid phosphatases phosphate complex

The effect of phosphate on the purple acid phosphatases is complex and... 

Figure 35. Active site structures of (A) Fe VZH red kidney bean purple acid phosphatase in complex with phosphate (PDB code 4KBP), (B) phosphorylcholine esterase domain of the virulence factor choline-binding protein E from Streptococcus pneumoniae (PDB code IWRA), and (C) rubredoxinioxygen oxidoreductase (ROO) from Desulfovibrio gigas (PDB code 1E5D).

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

The purple acid phosphatases can occur in two diferric forms—one as the tightly bound phosphate complex (characterized for the bovine and porcine enzymes) (45, 171, 203) and the other derived from peroxide or ferricyanide oxidation of the reduced enzyme (thus far accessible for only the porcine enzyme) (206). These oxidized forms are catalytically inactive. They are EPR silent because of antiferromagnetic coupling of the two Fe(IIl) ions and exhibit visible absorption maxima near 550-570 nm associated with the tyrosinate-to-Fe(III) charge-transfer transition. The unchanging value of the molar extinction coefficient between the oxidized and reduced enzymes indicates that the redox-active iron does not contribute to the visible chromophore and that tyrosine is coordinated only to the iron that remains ferric in agreement with the NMR spectrum of Uf, (45). 

Schenk G, Gahan LR, Carrington LE, Mitic N, Valizadeh M, Hanfilton SE, de Jersey J, Guddat LW. 2005. Phosphate forms an unusual liipodal complex with the Fe-Mn center of sweet potato purple acid phosphatase. Proc Natl Acad Sci USA 102 273-278. 

Pinkse MWH, Merkx M, Averill BA. 1999. Fluoride inhibition of bovine spleen purple acid phosphatase characterization of a ternary enzyme-phosphate-fluoride complex as a model for the active enzyme-substrate-hydroxide complex. Biochemistry 38 9926-9936. 

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