Peptides metal-catalyzed

Including EDTA in the final preparation inhibits metal-catalyzed oxidation of the sulfhydryl groups to disulfides. The modified peptide or protein should be used immediately to assure full sulfhydryl reactivity. 

Figure 19.19 shows a plot of the results of such an assay done to determine the maleimide content of activated BSA. This particular assay used 2-mercaptoethanol which is relatively unaffected by metal-catalyzed oxidation. For the use of cysteine or cysteine-containing peptides in the assay, however, the addition of EDTA is required to prevent disulfide formation. Without the presence of EDTA at 0.1 M, the metal contamination of some proteins (especially serum proteins such as BSA) is so great that disulfide formation proceeds preferential to maleimide coupling. Figure 19.20 shows a similar assay for maleimide-activated BSA using the more innocuous cysteine as the sulfhydryl-containing compound. 

Transition metals such as iron can catalyze oxidation reactions in aqueous solution, which are known to cause modification of amino acid side chains and damage to polypeptide backbones (see Chapter 1, Section 1.1 Halliwell and Gutteridge, 1984 Kim et al., 1985 Tabor and Richardson, 1987). These reactions can oxidize thiols, create aldehydes and other carbonyls on certain amino acids, and even cleave peptide bonds. The purposeful use of metal-catalyzed oxidation in the study of protein interactions has been done to map interaction surfaces or identify which regions of biomolecules are in contact during specific affinity binding events. 

Several peroxidative reactions initiated by OH radical that may be generated by either y- and X-radiolysis of aqueous solutions or by transition metal-catalyzed reduction of H2O2 have been identified in free amino acids and short peptides. In this respect we may distinguish oxidizing reactions that involve the polypeptide backbone on the one hand... 

Hydrolysis of peptides,84 amides,85 phosphate esters,86 sulfonate esters87 and acetals88 can also be metal catalyzed. The hydrolysis of a phosphate ester coordinated to cobalt(III) also occurs at an increased rate (Scheme 19).89 A rather similar reaction occurs in the amine exchange of coordinated dithiocarbamates (equation 21).90 The conversion of imidates to amidines has been mentioned previously and is a similar type of reaction (see Section 7.4.2.2.1). 

Enzymatic hydrogenations generate optically pure isomers attempts to initiate such processes are made on metal-catalyzed hydrogenations. Asymmetric hydrogenation can fill the need for asymmetric compounds of which only one enantiomorph is active, e.g., amino acids such as L-lysine, 1 (indispensable in animal feeds), L-phenylalanine, 2 (a sweet peptide component), L-dopa 3 (a drug for Parkinsonism), are required in the L-form for human or animal consumption. Consequently, most of the examples investigated are related to the asymmetric hydrogenation of acrylic acid or cinnamic acid derivatives. 

The reactions of the Cu(II), Fe(III), and Al(III) chelates of Schiff bases formed by the condensation of pyridoxal with amino acids and peptides were found by Snell to have catalytic properties similar to those of the pyridoxal phosphate enzymes. A typical metal-catalyzed reaction of this type would be the transamination of pyridoxal and alanine according to... 

Schoneich C (2000). Mechanisms of metal-catalyzed oxidation of histidine to 2-oxo-histidine in peptides and proteins. J. Pharm. Biomed. Anal. 21 1093-1097. 

Khossravi M, Borchardt R T (2000). Chemical pathways of peptide degradation. X Effect of metal catalyzed oxidation on the solntion strnctnre of a histidine-containing peptide fragment of hnman relaxin. Pharm. Res. 17 851-858. 

An alternative approach to metal-catalyzed peptide synthesis was proposed by Beck and co-workers. Their method uses half-sandwich complexes of Rh, Ir, or In this reaction, TV,TV -coordinated peptides are elongated at the... 

Metal-catalyzed oxidation has been suggested to occur on histidines 13 and 14 of amyloid P peptides. Inoue et al. (2006) used LC—MS/MS to characterize these sites of oxidation on tryptic amyloid p (AP) peptides. These peptides are... 

It should be noted that protein carbonylations occur via many different pathways. The hydrazide-coated glass beads are capable of enriching peptides harboring carbonyl modihcations other than those by HNE (e.g., 4-hydroxy-2-hexenal, 4-oxo-2-nonenal, metal-catalyzed oxidation, and others). These modifications can be included in the database search algorithms for in-depth mining of posttranslational protein carbonylations. 

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