Oxidation of amino acid residues

Low-density lipoprotein complexes (LDLs), which are the primary means of transporting cholesterol in the blood, are readily oxidized. These oxidations include peroxidation of unsaturated fatty acids, hydroxylation of cholesterol, and oxidation of amino acid residues in the apoprotein. A class of white blood cells recognizes the oxidation and absorbs the LDL through its scavenger receptor. After a white blood cell has absorbed numerous LDLs containing cholesterol, it becomes engorged and is referred to as a foam cell. Foam cells attract other white blood cells, which leads to accumulation of more cholesterol. Ultimately, this accumulation of cholesterol becomes one of the chief chemical constituents of the atherosclerotic plaque that forms at the site. 

Oxidation of amino acid residues - Conditions that generate oxygen radicals cause many proteins to undergo mixed-function oxidation of particular residues. Conditions require Fe + and hydroxyl radical, and the amino acids most susceptible to oxidation are lysine, arginine, and proline. E. coli and rat liver each contain a protease that cleaves oxidized glutamine synthetase in vitro, but does not attack the native enzyme. Presumably, other oxidized proteins are also targets for this enzyme. 

The oxidative mechanism of RNR R2 differs from that of MMOH in requiring an additional electron, since Tyrl22 provides only a single electron. This electron is needed to convert P to X. It has been shown that external reductants such as excess Fe(II) or ascorbate can provide this electron in in vitro reconstitution reactions [87,97], Since the diiron site is buried 10 A below the protein surface, a long-range electron transfer pathway is required to deliver this extra electron to the diiron center. Such a pathway involving a number of amino acid residues has been proposed from examining the crystal structure of R2 [98],... 

Alteration of amino acid residues can be obtained by heating at acid or alkaline pH. Main classes of reactions used to chemically modify the side-chain of amino acids are acylation, alkylation, oxidation and reduction (Figure 1). Some of them are described in this chapter. 

Another consequence of DNA oxidation is the formation of DNA protein crosslinks between carbon radicals and the carbon chains of amino acid residues. This is problematic for two reasons First, it leads to errors in DNA replication, and second, it disrupts the structure and function of the protein. 

Fig. 1. a-Oxidation of amino acids. Hydroxyl radical (or other reactive radical) abstracts hydrogen atom from the a-carbon. The C-centered free radical formed may react with other amino acid residues or dimerize in the absence of oxygen, which leads to protein aggregation. In die presence of oxygen the carbon-centered radical forms peroxyl radical. Reduction of peroxyl radical leads to protein hydroperoxide. Decomposition of hydroperoxide leads to formation of carbonyl compounds via either oxidative deamination or oxidative decarboxylation. Oxidation of the new carbonyl group forms a carboxyl group. 

A9. Amici, A., Levine, R. L., Tsai, L., and Stadtman, E. R., Conversion of amino acid residues in proteins and amino acid homopolymers to carbonyl derivatives by metal-catalyzed oxidation reactions. J. Biol. Chem. 264, 3341-3346 (1989). 

The subphase contained 10 mM Tris-HCl buffer (pH 7.4). CaCl2 (2 mM) or PLL (0.5 mM, concentration of amino acid residues) was added to the subphase. The temperature of the subphase was adjusted to 18°C. Hydrogenase solution of 20 pi (1.1 mg/ml) was spread on the air/water interface of the subphase (surface area of 248 cm2) using the glass-rod method (Hirata et al., 1992). After 10 min, the monolayer was compressed at a rate of 7.5 cm2/min. The layer of hydrogenase was transferred to hydrophilic quartz plates and indium tin oxide (ITO) electrodes (40 x 5 x 1 mm). The vertical deposition onto the substrate was performed a surface pressure of 30 mN/m. The dip-ping and withdrawal rates were 50 and 5 mm/min, respectively. The density of hydrogenase molecules in an LB film was calculated based on the number of molecules spread on the subphase and the transfer ratio (TR). 

Studies on the structure of apoferritin are advancing rapidly to the stage where the primary structure of the horse spleen molecule should soon be established. The quaternary structure of apoferritin from a variety of organisms seem to be similar the protein is in all cases made up of 24 identical polypeptide chains of molecular weight 18,500 daltons. Studies on the subunit-subunit interactions are progressing, and a number of amino acid residues at the subunit interface have been identified. The process of oligomer formation from subunits is amenable to study at low pH. Apoferritin catalyses the oxidation of Fe2+ in the presence of... 

Many enzymes are active in hydrolyses, and operate by acid-base catalysis. This is because the polypeptide chain can provide high concentrations of protons from readily available amino and carboxyl functional groups. However, many of the reactions which occur in metabolism involve chemical processes other than acid-base catalysis, which cannot be mediated by the side chains of amino acids. These include oxidation-reduction, group transfer, isomerization, and bond-breaking and bond-making reactions. Proteins consisting only of amino acid residues cannot catalyze these processes. 

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