Processing pathways

The reactions of primary amines and maleic anhydride yield amic acids that can be dehydrated to imides, polyimides (qv), or isoimides depending on the reaction conditions (35—37). However, these products require multistep processes. Pathways with favorable economics are difficult to achieve. Amines and pyridines decompose maleic anhydride, often ia a violent reaction. Carbon dioxide [124-38-9] is a typical end product for this exothermic reaction (38). 

Lohmann V, Koch JO, Bartenschlager R (1996) Processing pathways of the hepatitis C vims proteins. J Hepatol 24(Suppl 2) 11-19... 

Finding new cherrtical process pathways that can make large advances in the production of liqtrid fuels from solid and gaseous resources. 

Immunoelectron microscopic and biochemical fractionation data on specific processing enzymes have permitted the localization of specific processing steps in the processing pathway for proteins in the Golgi complex. The distribution of many remaining enzymes and steps has not yet been determined. The distribution for each step indicated in this table reflects a significant enrichment rather than an exclusive localization. 

Similarly, five closely related melanocortin receptors that respond to various peptides derived from the POMC precursor have been identified (Fig. 18-7) [24]. As expected, the receptor on adrenal cortical cells responds best to ACTH, which normally stimulates adrenal steroidogenesis, and the receptor on melano cytes responds best to aMSH, which causes skin darkening. However, the pattern of melanocortin receptor expression in the brain is not simply explained by the known patterns of peptide expression in the brain or by the known effects of POMC-derived peptides when applied to various brain regions. With this number of peptide receptors, it is obvious that production of final peptide products must be precisely controlled and that different biosynthetic processing pathways can dramatically affect the biological activity observed (Figs 18-5,18-7). 

Rate equation (1) indicates that ku should be inversely proportional to the activity of water for solvolysis by the AAil mechanism and independent of it if the bimo-lecular processes (pathways (i) and (ii)) pertain. Fig. 12 illustrates that acid independent rate constants at different volume fractions of D20 in CD3CN, /cH, were linearly dependent upon the inverse of ud2o in CD3CN as determined from the corresponding activities of H20 in CH3CN.142 This is in accord with the AA]1 mechanism (pathway (iii), Scheme 6). 

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

All of the intermediates in this processing pathway are collectively known as hnRNA. 

Seliger, B., Ritz, U., Abele, R., Bock, M., Tampe, R., Sutter, G., Drexler, I., Huber, C., and Ferrone, S., 2001, Immune escape of melanoma First evidence of structural alterations in two distinct components of the MHC class 1 antigen processing pathway. Cancer Res. 61 8647-8650. 

Rehfeld JF Cholecystokinin expression in the central nervous system. Eur Neuropsychopharmacol 2(3) 189-191, 1992b Rehfeld JF The molecular nature of cholecystokinin in plasma—an in vivo immunosorption study in rabbits. Scand J Gastroenterol 29 110-121, 1994 Rehfeld JF, Hansen HF Characterization of preprocholecystokinin products in the porcine cerebral cortex evidence of different processing pathways. J Biol Chem 261 5832-5840, 1986... 

Portion of the Chart of the Nuclides showing s-process and r-process pathways. The s-process pathway, shown by the dark line in the center of the valley of p-stability, shows how a nuclide that successively captures individual neutrons would evolve. Each added neutron moves the nuclide to the right on the diagram, until it reaches an unstable nuclide, in which case it will p-decay to the stable nuclide with a higher Z. In contrast, in situations where nuclides capture neutrons very rapidly ( -process), they will be driven far to the right of the valley of p-stability until the timescale for neutron capture matches that for p-decay. They will then move to higher Z and capture more neutrons until they either reach a size that causes them to fission (break) into smaller nuclei (which can then capture more neutrons) or until the neutrons disappear, in which case they will p-decay back to the first stable isotope along paths of constant A (arrows). 

Most eukaryotic mRNA transcripts produce only one mature mRNA and one corresponding polypeptide, but some can be processed in more than one way to produce different mRNAs and thus different polypeptides. The primary transcript contains molecular signals for all the alternative processing pathways, and the pathway favored in a given cell is determined by processing factors, RNA-binding proteins that promote one particular path. 

Based on these data, two pathways for the degradation of geraniol (20) were proposed by Madyastha [32], Fig. (7). Pathway A involves an oxidative attack on the 2,3-double bond resulting in the formation of an epoxide. Opening of the epoxide yields the triol (31) which upon oxidation forms a ketodiol (32). The ketodiol (32) is then converted to 6-methyl-5-hepten-2-one (33) by an oxidative process. Pathway B is... 

Processing pathway of preproopiomelanocortin. This precursor polypeptide is cleaved into a variety of active peptides. With the exception of the signal peptidase cleavage site, the cleavage sites are generally pairs of basic amino acids, although one site contains four. Which active peptides are produced depends on the processing pathway, which varies in different cell types. Thus, in the anterior... 

The processing pathways that lead to the two forms of mRNA are illustrated in Figure 8. To produce mRNA encoding ps, a primary transcript is cleaved and polyadenylated at a proximal poly(A) site located 3 to the exon encoding C 4 and the terminal segment of the secreted chain. To produce mRNA encoding pm, the transcript is cleaved and polyadenylated at a distal poly(A) site 3 to the membrane exons. The primary transcript for pm is spliced from the border between... 

Early, P., Rogers, J., Davis, M., Calame, K., Bond, M., Wall, R., Hood, L. (1980b). T vo mRNAs can be produced from a single immunoglobulin p gene by alternative RNA processing pathways. Cell 20,313-319. 

The proposed mechanism of oxidative fluorination of unsaturated compounds by halogen fluorides [84-86] and VF5 [33] includes electron transfer from substrate to halogen fluoride or VF5 as a first step, followed by addition of F" to a radical-cation, leading to formation of a radical and its further oxidation to carbocation (see Eq. 12, pathways A,B). It should be pointed out that this is not the only direction, and the actual mechanism may depend strongly on the substrates and reaction conditions. Other mechanisms, such as a radical process (pathway C), cannot be ruled out. 

Lipids, unlike many excipients, whether present in food or as discreet pharmaceutical additives, are processed both chemically and physically within the GIT before absorption and transport into the portal blood (or mesenteric lymph). Indeed, most of the effects mediated by formulation-based lipids or the lipid content of food are mediated by means of the products of lipid digestion—molecules that may exhibit very different physicochemical and physiological properties when compared with the initial excipient or food constituent. Therefore, although administered lipids have formulation properties in their own right, many of their effects are mediated by species that are produced after transformation or activation in the GIT. An understanding of the luminal and/or enterocyte-based processing pathways of lipids and lipid systems is therefore critical to the effective design of lipid-based delivery systems. 

Moremen, K.W., Trimble, R. B., and Herscovics, A. (1994). Glycosidases of the asparagines-linked oligosaccharide processing pathway. Glycobiology, 4, 113-125. 

Decarboxylation of an amino acid is an important reaction, catalyzed by a pyridoxal-dependent decarboxylase, that affords an amine as product (Scheme 2.6). It is very attractive to learn how to mimic this process to generate various amines from a-amino acids. Unfortunately, our previous studies established that treatment of a-alkyl amino acids with pyridoxal afforded only ketone and pyridoxamine as products, by a transamination-dependent oxidative decarboxylation process (pathway b in Scheme 2.5) [41]. Consequently, non-oxidative decarboxylation, using pyridoxals to generate amines, remains elusive. 

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