Broken chain pathway

The means by which the cone conformer (u,u,u,u) of a calix[4]arene is transformed to its inverted cone conformer (d,d,d,d) remains somewhat uncertain, although it clearly involves a lower rim through the annulus pathway (see Section 4.5.1), One possibility is that the process is concerted (a continuous chain pathway ) without intermediates and only one transition state. Another possibility is that the process is stepwise (a broken chain pathway ) with intermediates and two or more transition states, e.g. cone partial cone - 1,2- or... 

The strategy pursued to elucidate the metabolism of AG was to postulate a mechanism based on the known degradation pathway of the structurally related surfactant LAS. This exclusively proceeds via co-oxidation of the alkyl chain, resulting in the formation of carboxylic acids, which are then further broken down through (3-oxidations as described previously. The corresponding pathway relevant to AG is shown in Fig. 5.1.17. From the putative carboxylic acids that are theoretically possible, only the C4-glucamide acid could be detected (Fig. 5.1.18) [38] 3... 

An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process B), in which a covalent bond along the polymer main chain is broken and a stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for example, homolytic carbon-carbon cleavage, have obviously high activation energies. The stress-free rates of these reactions are therefore typically extremely low. 

Scheme 5. Various Glycosyltransfer Reactions Related to the Biosynthesis of Glycosphingolipids having Globo-series Sugar Chains. (Broken arrows indicate the hypothetical, biosynthetic pathway.)...

Immediate consequences of a point mutation in a biosynthetic pathway. When the mutation leads to an inactive enzyme, the chain of reactions leading to the end product in the pathway is broken, and frequently large amounts of intermediate are produced, accumulate in the cell, and may leak to the environment. 

In kinetic diagrams, the kinetic irreversibility is usually indicated with a single arrow ( ), while the potential kinetic reversibility is shown by a double arrow (t ). In any complex pathway with the known drops of chemical potentials at individual stages, the transformation chain can be broken down into kineticaUy reversible and kineticaUy irreversible steps (Figure 1.6). A priori consideration of some elementary steps of a stepwise reaction as kineticaUy irreversible may cause some serious mistakes in making conclusions via classical kinetic analysis of the scheme of chemical transformations. 

Threonine can be broken down by two separate pathways. Serine dehydratase catalyzes the conversion of threonine to 2-ketobut ate plus an ammonium ion 2-ketobutyrate is then converted by branched-chain keto add (BCKA) dehydrogenase to propionyl-CoA plus carbon dioxide. Propionyl-CoA catabolism is described later in this chapter. Threonine can also be broken down by a complex that has been suggested to be composed of threonine dehydrogenase and acetoacetone synthase (Tressel et ah, 1986). Here, threonine catabolism results in the production of acetyl-CoA plus glycine. 

Chain reactions are the major pathway by which hydrocarbon polymers as used in packaging are broken down, with the radicals for initiation arising from photoinduced... 

Reactions can be accelerated by the formation of intermediates, provided that such intermediates are rapidly formed and broken down. This then provides a new reaction pathway of lower activation energy. Many enzyme-catalyzed reactions proceed via covalent catalysis, involving side chains such as Cys, Ser, Eys. or His acting as nucleophiles in the formation of intermediates such as acyl-enzymes or Schiff bases. Changes in environment... 

Glycolysis The initial pathway in the catabolism of carbohydrates, by which a molecule of glucose is broken down to two molecules of pyruvate, with a net production of ATP molecules and the reduction of two NAD molecules to NADH. Under aerobic conditions, these NADH molecules are reoxidized by the electron transport chain under anaerobic conditions, a different electron acceptor is used. An anaerobic metabolic pathway used to break down glucose into pyruvic acid while producing some ATP. 

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