Phosphate binding mechanism

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

The catalytic center of the protein tyrosine phosphatases includes ca. 230 amino acids and contains the conserved sequence motif HA -C-(X)5-R-S/T-G/A/P (X is any amino acid) which is involved in phosphate binding and in catalysis and is part of a loop known as the P loop. The available structural data on the catalytic domains of protein tyrosine phosphatases indicate that the mechanism shown schematically in Fig. 8.17 is likely (see Tainer and Russell, 1994). The invariant Cys and Arg residues of the P loop have a central function in binding and cleavage of the phosphate residue. 

The iron(II)-iron(III) form of purple acid phosphatase (from porcine uteri) was kinetically studied by Aquino et al. (28). From the hydrolysis of a-naphthyl phosphate (with the maximum rate at pH 4.9) and phosphate binding studies, a mechanism was proposed as shown in Scheme 6. At lower pH (ca. 3), iron(III)-bound water is displaced for bridging phosphate dianion, but little or no hydrolysis occurs. At higher pH, the iron(III)-bound OH substitutes into the phosphorus coordination sphere with displacement of naphthoxide anion (i.e., phosphate hydrolysis). The competing affinity of a phosphomonoester anion and hydroxide to iron(III) in purple acid phosphatase reminds us of a similar competing anion affinity to zinc(II) ion in carbonic anhydrase (12a, 12b). 

Brune, M. et al. 1998. Mechanism of inorganic phosphate interaction with phosphate binding protein from Escherichia coli. Biochemistry 37, 10370-10380. 

This structural model for the mechanism of MIPS is not consistent with the reported structure of the yMIPS/2-deoxyglucitol-6-phosphate complex structure (Stein and Geiger, 2002). In this structure, the inhibitor 2-deoxyglucitol-6-phosphate s phosphate group does not occupy the phosphate-binding site, but instead is modeled into a site on the opposite side of the active site from... 

Since concerted bond breaking and leaving group protonation was found to be considerably favored over a stepwise mechanism in the first part of the reaction, the analogous concerted pathway was also modeled here. Simulation of the first step showed that the protein environment cannot stabilize a negative ligand in the active site outside the phosphate binding loop, which would also be the case for a stepwise proton transfer to Asp 129 and a subsequent in-line attack of a hydroxide ion. 

Figure 17-8 Two-step mechanism of phosphate binding to purple add phosphatase as proposed by Aquino et al. [72].

Besides the role of 1,25-(OH)2D3 in the transfer of calcium across the intestinal membrane it is known that this substance also activates the transfer of inorganic phosphate across intestinal illeum and jejunum 307-3io) process has been shown to be independent of calcium transport and thus represents an entirely different function of the vitamin. Evidence has been presented that 1,25-(OH)2D3 rather than 25-OH-D3 functions in this process Little is known concerning the mechanism of phosphate transfer except that it is a sodium dependent and active process ° So far a specific phosphate binding protein has not yet been found. 

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