Post-transitional modification

Another form of post-transitional modification that may add carbohydrate to a polypeptide is nonenzymatic glycation. This reaction occurs between the reducing ends of sugar molecules and the amino groups of proteins and peptides. See Section 2.1 for further details and the reaction sequence behind this modification. 

A diverse range of factors has been shown to influence, often profoundly, the aggregation of amyloidogenic proteins and peptides. These include amino acid composition and sequence peptide length, concentration, pi and solubility and environmental conditions such as pH, temperature, solvent, metal-binding, macromolecular packing, membrane interactions and post-transitional modifications. Some of these effects are outlined below. 

Moreover, overexpression of the same mutants inhibit DNA replication and block the cells at the Gl/S-phase transition (Kim et al, 2005), emphasizing the potential role of nucleolin mobilization. It is therefore highly probable that two different processes help the formation of RPA-nucleolin complexes after a genotoxic stress a post-transcriptional modification of nucleolin that renders the GAR domain of nucleolin accessible to RPA, and its p53-dependent relocalization to the nucleoplasm where a higher amount of RPA is available. Of importance, nucleolin relocalization is transient and lasts far less than replication inhibition (Daniely and Borowiec, 2000). This means that nucleolin-RPA interaction is only an initial event and that other mechanisms account for prolonged replication inhibition. 

Although the effects of y-H2AX foci are still poorly understood, two theories have been proposed that may not be mutually exclusive [17,18], The first model suggests that the post-translational modification may impart structural transitions to DSB domains, and the second implicates the histone variant as a signaling intermediate in the repair pathway. 

Similarly, zeotype molecular sieves are synthesized by mixing the basic ingredients with the organic template, e.g. aluminophosphates are prepared from alumina and phosphoric acid. Other main group or transition elements can be incorporated into the framework by adding them to the initial sol-gel. Alternatively, different elements can be introduced by post-synthesis modification (see later), e.g. by dealumination followed by insertion of the new elements into the framework position [31]. 

J.P.M. Niederer and W.F. Hoelderich, Isomorphous Substituted Early Transition Metal Containing BEA Vin Post-Synthesis Modification of H-[B]-BEA. Proceedings of the 12th International Zeolite Conference, Part III, ed. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, and... 

The change of composition of mesoporous materials can be done by direct synthesis and post-synthesis modification. Now, the composition of mesoporous materials can be extended to nonsilica oxides, phosphates, sulfides, even metals. The study of nonsilica mesoporous materials started much later than that for silica-based materials. The main reasons include the hydrolysis and condensation reactions of transition metal precursors is difficult to control the inorganic wall easily crystallizes and results in the loss of mesostructures the synthetic procedure is difficult to repeat. 

Some Other Post-Transitional Enzymatic Modifications of Proteins... 

Early in our studies it was expected that the post-translational modification of proline hydroxylation, so important to proper collagen structure and function, would raise the value of the temperature, T, for the onset of the inverse temperature transition for models of elastin. Accordingly, hydroxyproline (Hyp) was incorporated by chemical synthesis into the basic repeating sequence to give the protein-based polymers poly[fvs,i(Val-Pro-Gly-Val-Gly), fHyp( al-Hyp-Gly-Val-Gly)], where f sl -i- fnyp = 1 and values of fnyp were 0, 0.01, and 0.1. The effect of prolyl hydroxylation is shown in Figure 7.49. Replacement of proline by hydroxyproline markedly raises the temperature for hydrophobic association. Prolyl hydroxylation moves the movable cusp of... 

It was emphasized (Kuhn 2004) that, in selecting peptides for use as internal standards, their hydropho-bicity, number of amino acid residues, peptide location within the protein sequence and lack of sites that could be subject to potential post-translational modification should be considered in addition to a high degree of uniqueness in the protein-peptide connectivity (Section 11.6.1). Although multiple MRM transitions were not required (Kuhn 2004) to obtain data of good accuracy and precision for more abundant target proteins, use of relative intensities of more than one (up to five) MRM signals increased the specificity of detection for peptides derived from low abundant proteins in complex proteomics mixtures and thus also increased the confidence of protein identification. 

Transit peptides amino-terminal extensions of precursors to chloroplast proteins which are encoded by nuclear DNA and synthesized in the cytoplasm. The T.p. are removed by Post-translational modification (see) before the protein takes on its mature configuration inside the chloroplast. The overall amino acid sequences of the T.p. are not conserved between species, but the positions of proline and the charged amino acids are highly conserved. The T.p. appear to mediate transport of the chloroplast protein precur-... 

Location of transition metals in the framework and extra framework positions can be deduced from their coordination. Tetrahedrally coordinated species should be detected for framework position, whereas the octahedral coordination can point to the extra framework ones. The UV-Vis spectra of the samples prepared in the one-pot syrrlhesis show the bands characteristic of tetrahedrally coordirrated trarrsition metals, especially for the tantalum containing samples in which the efficiency of metal incorporation was higher (see Table 1). Moreover, the spectra of these samples differ from those recorded for materials prepared via post-synthesis modification. Figure 2 presents the UV-Vis spectra of tantalum oxide, and tantalum incorporated into Y zeolite by synthesis and post-synthesis methods. It is clearly seen, that the latter technique leads to the formation of octahedrally coordinated species, which is likely to be bulk tantalum oxide. However, in the one-pot synthesis method all tantalum is characterized by a band at c.a. 221 nm, typical of tetrahedrally coordinated Ta species. Therefore, one can conclude about the location of Ta in the zeolite skeleton. 

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