Protein anionic groups

When ionic or free urinary calcium was evaluated, high protein meals resulted in equal or slightly depressed calciuric response. The levels of ionic calcium in the urine, as determined by a calcium ion-selective electrode, suggest that a considerable amount of urinary calcium was complexed to anions or compounds with anionic groups. 

Among protein aromatic groups, histidyl residues are the most metal reactive, followed by tryptophan, tyrosine, and phenylalanine.1 Copper is the most reactive metal, followed in order by nickel, cobalt, and zinc. These interactions are typically strongest in the pH range of 7.5 to 8.5, coincident with the titration of histidine. Because histidine is essentially uncharged at alkaline pH, complex-ation makes affected proteins more electropositive. Because of the alkaline optima for these interactions, their effects are most often observed on anion exchangers, where complexed forms tend to be retained more weakly than native protein. The effect may be substantial or it may be small, but even small differences may erode resolution enough to limit the usefulness of an assay. 

Where this factor plays a role, the hydrophobic interaction between the hydrocarbon chains of the surfactant and the non-polar parts of protein functional groups are predominant. An example of this effect is the marked endothermic character of the interactions between the anionic CITREM and sodium caseinate at pH = 7.2 (Semenova et al., 2006), and also between sodium dodecyl sulfate (SDS) and soy protein at pH values of 7.0 and 8.2 (Nakai et al., 1980). It is important here to note that, when the character of the protein-surfactant interactions is endothermic (/.< ., involving a positive contribution from the enthalpy to the change in the overall free energy of the system), the main thermodynamic driving force is considered to be an increase in the entropy of the system due to release into bulk solution of a great number of water molecules. This entropy... 

Carboxyl groups of these side chains are dissociated at neutral pH (piCa values are 4.3-4.7) and provide anionic (-) groups on the surfaces of proteins. 

It has been suggested that the modification of the protein s isoelectric point could result in an alteration of its pharmacokinetic profile. Avidin acylation was performed by lysine amino group derivatization with succinyl anhydride or other anhydrides, which allowed the isoelectric point to be shifted to more acidic values, depending on the level of modification. Indeed, the protein anionization induced a reduction of accumulation in the liver, but resulted only in a limited prolongation of residence time in the circulation [30, 31]. 

The anionic groups observed in basement membrane with cationic stains were identified as heparan sulfate chains on the basis of their sensitivities to heparitinase and to nitrous acid (Kanwar and Farquhar, 1979b) and by chemical analysis (Kanwar and Farquhar, 1979c Parthasarathy and Spiro, 1982). Current concepts suggest that the heparan sulfate chains are linked through a common oligosaccharide terminating in xylose linked to serine residues on the protein core and that certain xylosides can act as initiators for the synthesis of heparan sulfate. In the absence of added xyloside, the synthesis of heparan sulfate is entirely dependent on the synthesis of a core protein. 

Figure 1. Postulated mechanism of racemization and lysinoalanine formation via a common carbanion intermediate. Note that two B-elimination pathways are possible (a) a concerted, one-step process (A) forming the dehydroprotein directly and (b) a two-step process (B) via a carbanion intermediate. The carbanion, which has lost the original asymmetry, can recombine with a proton to regenerate the original amino acid residue which is now racemic. Proton transfer may take place from the environment of the carbanion or from adjacent NH groups, as illustrated. Protein anions and carbanions can also participate in nucleophilic addition and displacement reactions (24, 82, 83).

Krull, H., and Friecbnan, M. (1967). Anionic polymerization of methyl acrylate to protein functional groups. J. Polym. Sci. A-l, 5, 2535-2546. 

Probably the most frequently used spectrophotometric method to detect thiol groups, both for non-protein and protein sulphydryl groups involves the use of Ellman s reagent (Scheme 7.5). 5,5 -Dithiobis(2-nitrobenzoic acid), (DTNB) (Ellman 1959) undergoes disulphide exchange with thiol groups and the formation of 5-thio-2-nitrobenzoate anion (TNB) (Scheme 7.6). 

Carboxyl-containing polysaccharides behave as polyanions at mild acidic and neutral pH values, which are typical of most foods. Electrostatic complexing between proteins and anionic polysaccharides generally occurs in the pH range between the pK value of the anionic groups (carboxyl groups) on the polysaccharide and the protein s lEP. Sulphated polysaccharides are capable of forming soluble complexes at pH values above the protein s lEP. 

Wool and silk contain charged functional groups, such as NH3 and COO . Because of this, they bind to ionic dyes by electrostatic interactions. For example, positively charged NH3 groups bonded to the protein backbone are electrostatically attracted to anionic groups in a dye like methyl orange. 

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