Carbohydrates decomposition pathway

Levulinic Acid. Dehydration of glucose or other monomeric and polymeric C6 sugars leads to the direct formation of levulinic acid (LA) as a potential primary building block for the biorefinery, and several reviews have described its potential commercial utility 477,478 The preparation of levulinic acid is not difficult, although the mechanism of its formation from carbohydrates is complex, and offers several alternative decomposition pathways (equation 3).479... 

Conventionally, central and special metabolic pathways are distinguished. Central pathways are common to the decomposition and synthesis of major macromolecules. Actually, they are much alike in all representatives of the living world. Special cycles are characteristic of the synthesis and decomposition of individual monomers, macromolecules, cofactors, etc. Special cycles are extremely diversified, especially in the plant kingdom. For this reason, the plant metabolism is conventionally classified into primary and secondary metabolisms. The primary metabolism includes the classical processes of synthesis and deeradation of major macromolecules (proteins, carbohydrates, lipids, nucleic acids, etc.), while the secondary metabolism ensuing from the primary one includes the conversions of special biomolecules (for example, alkaloids, terpenes, etc.) that perform regulatory or other functions, or simply are metabolic end byproducts. 

Carbohydrates ranging from cellulose to simple sugars are subject to thermal alteration. Factors such as temperature, pH, compound concentration, and other reactants present can alter both the rate and complexity of decomposition reactions. Carbohydrate types are reviewed relative to degradation/carameli-zation pathways and endproducts. Some of the resulting typical food flavors produced are also discussed. 

There are two major pathways responsible for decomposing organics (plant residue, manures, synthetic organics, sewage sludges, etc.) in the soil-water environment. One pathway involves aerobic processes, and a second involves anaerobic processes. During aerobic decomposition, carbohydrates are converted to carbon dioxide and water as follows ... 

The decomposition reaction proceeds in nature by two biosynthetic routes the mevalonate pathway, via mevalonic acid (MVA) (Scheme 102.2), and the recently discovered mevalonate-independent pathway, via deoxyxylulose phosphate (DXP) (Scheme 102.3), the latter taking place in plants. In Scheme 102.2, a schematic representation of the mevalonate pathway is shown in which three molecules of acetyl-coenzyme A derived by carbohydrate, fat, or protein catabolism yield by aldol-type reaction p-hydroxy-(3-methylglutaryl-coenzyme A (HMG-CoA), which is irreversibly reduced to -mevalonic acid (MVA), whose phosphorylation and elimination reaction afford IPP. This molecule is converted by an isomerase into an equilibrium mixture with DMAPP. 

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