Amino-acid residues methionine

Of the following amino acid residues—methionine, histidine, arginine, phenylalanine, valine, glutamine, glutamic acid—which would you expect to find on the (a) surface of a protein and which would you expect to find (b) in the interior ... 

Of all amino acid residues, methionine in the extramembrane state appears most often at the beginning of the sequence because the codon for this amino acid is at the same time the start codon. However, methionine, similarly to other amino acid residues, also appears inside of the sequence. As the first amino acid residue in the sequence, methionine does not possess left-hand neighbors. They are mimicked by the NH2-terminal space with a constant low propensity value. The lowest propensity value for the local environment X is that for methionine in the extramembrane state p = 0.0771). For this reason, the frequency distributions for methionine in the extramembrane state deviate most from the regular frequency distributions. 

Among the genetically encoded amino acid residues, methionine (Met) and cysteine (Cys) are special because they each contain an atom of sulphur. The present chapter describes how diese residues are incorporated into peptides in the context of an Fmoc/tBu solid-phase synthesis strategy, as well as further considerations once the synthetic peptide is released from the support Of added interest, some manipulations of Cys are advantageously performed at the level of the assembled peptide-resin, prior to cleavage. Many of the aspects discussed here also carry over to the preparation of peptides using a Boc/Bzl strategy. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

HOCl-mediated protein oxidation accelerates under pathophysiological conditions. Thus, proteins from extracellular matrix obtained from advanced human atherosclerotic lesions contained the enhanced levels of oxidized amino acids (DOPA and dityrosine) compared to healthy arterial tissue [44], It was also found that superoxide enhanced the prooxidant effect of hypochlorite in protein oxidation supposedly by the decomposition of chloramines and chlor-amides forming nitrogen-centered free radicals and increasing protein fragmentation [45], In addition to chlorination, hypochlorite is able to oxidize proteins. The most readily oxidized amino acid residue of protein is methionine. Methionine is reversibly oxidized by many oxidants including hypochlorite to methionine sulfide and irreversibly to methionine sulfone [46] ... 

In normal cells, the GDP/GTP-binding proteins, after protein synthesis, move to the cell membrane to which they become hooked by a hydrophobic farnesyl group. The y-subunit is anchored in the membrane by a post-translational modification of the C-terminal CAAX sequence (C - cystein, AA - aliphatic amino acids, X - methionine). This protein is first enzymatically farnesylated by a specific farnesyltransferase, then the AAX part is cleaved by specific proteases and finally the cystein residue is converted to a methyl ester. 

Non-corrin cobalt has a number of interesting applications in the chemical industry, for example in the hydroformylation (OXO) reaction between CO, H2 and olefins. A number of non-corrin Co-containing enzymes have been described, including methionine aminopep-tidase, prolidase, nitrile hydratase and glucose isomerase. We describe the best characterized of these, namely the E. coli methionine aminopeptidase, a ubiquitous enzyme, which cleaves N-terminal methionine from newly translated polypeptide chains. The active site of the enzyme (Figure 15.13) contains two Co(II) ions that are coordinated by the side-chain atoms of five amino acid residues. The distance between the two Co2+ is similar to that between the two Zn2+ atoms in leucine aminopeptidase, and indeed the catalytic mechanism of methionine aminopeptidase shares many features with other metalloproteases, in particular leucine aminopeptidases. 

Distinct changes in several properties of lysozyme occur after reaction with ozone. The lytic activity of the ozonized lysozyme shows the same trend at various pHs as the native enz3mie (Fig. 2) this may suggest that the pK values of the ionizable groups involved in catalysis have not been altered by ozonplysis. The amino acid composition of ozonized lysozyme differs from that of the native enz3mie in three residues — methionine, tryptophan and t3H osine. None of the other amino acids is affected by ozone. The extensive loss of enz5miic activity must be ascribed to the oxidative modification of these three amino acid residues in the lysozyme. 

Myoglobin is a protein of molecular weight of about 17,000 with the protein chain containing 153 amino acid residues folded about the single heme group. This restricts access to the iron atom (by a second heme) and reduces the likelihood of formation of a hematin-like Fe(III) dimer. The micro environment is similar to that in Cytochrome c, but there is no sixth ligand (methionine) to complete the coordination sphere of the iron atom. Thus there is a site to which a dioxygen molecule may reversibly bind. 

Excessive heat can cause destruction of amino acid residues. The amino acid most susceptible to direct heat destruction is cystine. Although not an essential amino acid, cystine does have a sparing effect on the dietary requirement for methionine. As a result, cystine destruction can be nutritionally important. In addition, many vegetable proteins are limiting in the sulfur amino acids. Cystine destruction would be particularly harmful for these proteins. 

Cytochrome c552 from Euglena gracilis (also known as cytochrome / or c6) contains 87 amino acid residues, two hemes and one flavin per molecule.693 NMR studies706 indicate that the chirality of the axial methionine is similar to that of cytochrome c but different from cytochrome c5Sl. Rapid intramolecular transport has been demonstrated by the use of pulsed laser excitation, and the measurement of reduction kinetics. Both flavin and heme groups are reduced simultaneously on a multisecond time scale, with the transient formation of a protein-bound flavin anion radical.707... 

Cytochromes c from different species contain the same heme group but may differ in the nature of the amino acid residues at various positions of the polypeptide chain. The functional importance of those segments of the protein which are invariant among different species has long been recognized (Margoliash and Schejter (71)). The NMR spectra in the reduced and oxidized forms indicate that methionine is an axial ligand in all the mammalian-type cytochromes c studied so far (McDonald et al. (79) Wuthrich (111)). 

All amino-acid residues of proteins are potential targets for attack by reactive oxygen species (ROS) produced in the radiolysis of water however, in only a few cases have the oxidation products been fully characterized. Moreover, under most physiological conditions, cysteine, methionine, arginine, lysine, proline, histidine, and the aromatic amino acids are primary targets for ROS-mediated oxidation. 

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