Essential monosaccharide

Figure 25.9 Structures of the eight monosaccharides essential to humans.

All the essential monosaccharides arise from glucose, by the conversions summarized in Figure 25.10. We ll not look specifically at these conversions, but might note that end-of-chapter Problems 25.55 through 25.57 lead you through several of the biosynthetic pathways. 

Figure 25.10 An overview of biosynthetic pathways for the eight essential monosaccharides.

Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-glucose by galactose 4-epimerase, where UDP = uridylyl diphosphate (a ribonucleotide diphosphate Section 28.1). The enzyme requires NAD+ for activity (Section 17.7), but it is not a stoichiometric reactant. and NADH is not a final reaction product. Propose a mechanism. 

Mannose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its 6-phosphate derivative from fructose 6-phosphate. No enzyme cofactor is required. Propose a mechanism. 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

Stevens and coworkers used their c.d. data on the various D-glucans to assign, tentatively, the bands to specific chromophores. They found that derivatives of these polysaccharides that have all of their hydroxyl groups acetylated still exhibit the 177-nm band. They assigned this band (which occurs at somewhat shorter wavelengths for the helical polymers) to the ether of the acetal chromophore. This assignment is essentially consistent with the results obtained by Johnson and coworkers on unsubstituted monosaccharides. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

Proteins, the main constituents of the animals body, are polypeptides, biopolymers consisting of many amino acid molecules (the monomers) combined together (see Chapter 11) collagen, for example, the main component of animal skin, is a complex protein consisting of many molecules of amino acids combined together into polypeptide chains (see Fig. 71). Polysaccharides, the essential constituents of plants, also consist of many monosaccharide molecules combined together. Cellulose, the most abundant biological material on earth, which makes up most of the structural... 

Upon treatment with n-BruiNF, product 97 is converted to 98, which is a key intermediate for the synthesis of the monosaccharide 3-deoxy-D-manno-2-octulosonic acid (KDO)—an essential constituent of the outer membrane lip-op oly s accharide (LPS) of gram-negative bacteria.40... 

Biologically, carbohydrates are most important as energy sources. This is especially true for monosaccharides, disaccharides, and starches. Starches also function as energy storage molecules in plants. Cellulose is an essential component of plant cell walls. 

In addition to the common pathways, glycolysis and the TCA cycle, the liver is involved with the pentose phosphate pathway regulation of blood glucose concentration via glycogen turnover and gluconeogenesis interconversion of monosaccharides lipid syntheses lipoprotein formation ketogenesis bile acid and bile salt formation phase I and phase II reactions for detoxification of waste compounds haem synthesis and degradation synthesis of non-essential amino acids and urea synthesis. 

The structural chemistry of carbohydrates, including the monosaccharides, is complex. We are going to ignore most of the complexity. Let s get started with the essentials by looking at a model of glucose ... 

Carbohydrates serve as a general and easily available energy source. In the diet, they are present as monosaccharides in honey and fruit, or as disaccharides in milk and in all foods sweetened with sugar (sucrose). Meta-bolically usable polysaccharides are found in vegetable products (starch) and animal products (glycogen). Carbohydrates represent a substantial proportion of the body s energy supply, but they are not essential. 

The separation of disaccharides and higher oligomers is not essentially different from the separation of monosaccharides, except that the volatility rapidly decreases with increasing molecular weight. Oligosaccharides may be transformed into volatile derivatives, commonly the trimethylsilyl ethers, either directly or after reduction. Other derivatives, such as trifluoroacetates, have been used, but the acetates have low volatility. Oligosaccharides are not usually converted into their methyl glycosides prior to trimethylsilylation. Detailed examples are listed in Table VII (see p. 130). 

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