Carbohydrates as substrates

This is because (l)most pyruvate comes from aerobic glycolysis, (2)much of the decarbosy-lated a-ketoglutarate originates from carbohydrate sources, and (3) the transketolase re-i action uses carbohydrates as substrates. 

Fig. 12.2 Neutralization of bacterial acids by salivary carbonic anhydrase Saliva contains proteins that form the acquired pellicle to which saccharolytic bacteria attach and grow anaerobically using dietary carbohydrate as substrate. Lactic acid is produced and reacts reversibly with salivary sodium bicarbonate to make sodium lactate and carbonic acid. Traces of salivary carbonic anhydrase in saliva contribute to the acquired pellicle and also remain in solution. The enzyme converts carbonic acid to water and carbon dioxide. This step is irreversible because carbon dioxide is lost to the environment whenever the mouth is opened. Thus, lactic acid is transformed into its salt, sodium lactate. (Original Figure)...

Pederamide (12) is an important intermediate in the projected total synthesis of pederin (13), a component of the vesicant of the beetle Paederus fuscipes. /rart5-2-Butene epoxide has been converted (in 16 steps) into pederamide. One of these involved the conversion of the tetrahydrofuran (14) into the pyran-2-one (15) in 96% yield.Synthetic studies on lasalocid A (16) have continued with the use of carbohydrates as substrates for the construction of both furanoid and pyranoid rings of the antibiotic. ... 

The effect of high pressure on cycloaddition reactions involving carbohydrates as substrates has been extensively investigated by Jurczak and coworkers (6-S). In their studies of Diels-Alder reactions of pyranoid enones, improvements in stereoselectivity were observed when the reactions were carried out at high pressure (6). 

Monascus pigments have been used in Asian countries for centuries as food colorants and spices and in traditional medicine. These pigments are produced by the fungi of Monascus genus cultivated on carbohydrate-rich substrates such as rice, wheat, com, potatoes, and soybeans. Three species of Monascus identified are pilosus. 

HPAEC analyses were carried out to determine the oligomeric products released from various pectic substrates after depolymerization by the PL isoenzymes. Action pattern analyses for the concerted action of PL isoenzymes utilized 68% esterified pectin as substrate. One-ml reaction mixtures in a buffer system as detailed in section 2.2. comprising 0.5% (w/v) substrate and 5 U of enzyme were incubated for 30 s to 18 h, and then thermoinactivated. Samples of 750 pi were applied to a Carbopac PA-1 (Dionex) column before the carbohydrates were eluted over a period of 70 min using a gradient of 0.2 M KOH, 0.05 M K-acetate to 0.2 M KOH, 0.7 M K-acetate. Detection employed a Pulsed Electrochemical Detector (PED, Dionex) in the integrated amperometry mode according to the manufacturer s recommendations. 

In addition to terpenes (as described above), carbohydrates have also been used as substrates in domino metathesis reactions, the aim being to synthesize enan-tiopure polyhydroxylated carbocyclic rings. These structures are components of several biologically active compounds such as aminoglycoside antibiotics [254], inositol phosphates [255], and carbanucleosides [256]. An efficient entry to this skeleton was developed by Madsen s group using a domino RCM/CM of the carbohy-... 

Fluorescent pseudomonads are capable of synthesizing poly(3HAMCL)s from a large number of substrates. Work on the biotechnological production of poly(3HAMCL) has focused mainly on two model systems - Pseudomonas oleo-vorans and P. putida. P. oleovorans is able to use alkanes and alkenes as substrate due to the presence of the OCT-plasmid while P. putida, which does not have this plasmid, cannot. In contrast to P. oleovorans, however, P. putida can use carbohydrates such as glucose and fructose for the production of poly(3HAMCL). 

Valuable information regarding the specificity requirements of enzymes acting on carbohydrates may be expected from a discussion of fructofuranosides as substrates for enzyme action. Not only is the hydrolysis of sucrose by /3-fructofuranosidase (saccharase, invertase) one... 

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

K. Bock and K. Adelhorst, Derivatives of methyl p-lactoside as substrates for and inhibitors of p-D-galactosidase fromil. coli, Carbohydr. Res., 202 (1990) 131-149. 

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7... 

Knowledge of the composition of sugars in solution is fundamental to carbohydrate chemistry. The physical and chemical properties of the sugars in solution depend on the proportions of their various forms and their biological properties may also show such dependence. Enzymes that utilize these sugars as substrates may not be able to use each of the forms. Where only a single form is utilized, the other forms may either be converted into the reactive form or may function as inhibitors. The latter is especially important if the reactive form is present in very low proportion at equilibrium. It is possible that the substrate form is utilized faster than it is generated from the other forms the observed rate of the reaction is then that of the tautomeric interconversion.4... 

The second and third of these steps depend on a supply of appropriate carbohydrate substrates, most favorably sucrose, in the mouth. The latter can become available either directly (sugar ingested in food or drink) or be derived from dietary starch by the action of bacterial or salivary amylases, or both. Of particular relevance in this context is the trapping of carbohydrates as or on food particles remaining in the mouth for considerable periods. 

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