Hexose

Hexoses, glycerol, fatty acids, and amino acids render acetyl-CoA, which is oxidized in the citric acrid bicycle.

From: Medical Biochemistry , 2017

Carbohydrates

W. Lee Adair , in xPharm: The Comprehensive Pharmacology Reference, 2007

The Monosaccharides

The monosaccharides, which are highly water soluble and sweet to the sense of taste, are named using the suffix "ose". This group of molecules has ii or more hydroxyl groups and contains either an aldehyde group, which are known as the aldoses, or a ketone group, which are known as the ketoses. Monosaccharides are also classified on their number of carbon atoms. Thus, monosaccharides containing iii, iv, v, and six carbons are referred to as the trioses, tetrooses, pentoses, and hexoses, respectively.

The simplest aldose monosaccharide is the triose glyceraldehyde (Fig. 1). Glyceraldehyde is institute in diverse phosphorylated forms in cells, where it is a primal intermediate in glycolysis and gluconeogenesis. Information technology also serves as a precursor for the glycerol courage of the triglycerides and of membrane phospholipids. Glyceraldehyde, past virtue of its asymmetry at carbon-ii, is potentially optically agile, with d and l enantiomers. Naturally occurring sugars including glyeraldehyde are grouped into D and L classes based on whether their last three carbons are equivalent to d or l glyceraldehyde. Except for L-fructose, sugars plant in mammals are denantiomers.

Fig. 1.. Construction of Glyceraldehyde.

The tetroses are rarely institute equally complimentary sugars. When phosphorylated, the tetrose erythrose is an of import intermediate in sugar metabolism.

The most common pentoses are ribose and deoxyribose, the phosphorylated forms of which comprise the courage of RNA and Deoxyribonucleic acid, respectively (Fig. 2). Ribose-five-phosphate is also an important intermediate in hepatic carbohydrate metabolism.

Ribose

2-Deoxyribose

Fig. ii.. Structures of Common Pentoses, Ribose and ii-Deoxyribose.

Simple sugars containing five or more than carbons are capable of forming cyclic structures in solution. The aldehyde or ketone group of the sugar reacts with an internal hydroxyl grouping to course a five- or 6-member band. For most hexoses, the six-member band is the near common, whereas a five-member ring is characteristic of pentoses, such as ribose and deoxyribose.

The most mutual hexoses are glucose, galactose, mannose, and fructose. The first 3 are aldoses, whereas fructose is a ketose. Glucose, galactose and mannose are epimers in that they differ from one another at a single eye of asymmetry. Thus, glucose is the 4-epimer of galactose because it differs from galactose past the position of the hydroxyl group at carbon-four. Enzymes that interconvert sugars in this mode are therefore called epimerases.

Glucose

Fig. iii.. Structures of Common Hexoses: Glucose, Galactose, Mannose and Fructose.

Galactose

Mannose

Fructose

Of the common hexoses, fructose is the sweetest to the taste. The development of a process to produce fructose from cornstarch has led to an increased use in foodstuffs, especially beverages. For example, nearly all non-diet soft drinks contain "loftier fructose corn syrup" instead of table saccharide (sucrose). Metabolically, fructose is handled much the same every bit glucose and has the same energy yield. Fructose is metabolized past 2 unique enzymes non institute in the glycolytic pathway for glucose.

Galactose is generally plant combined with glucose in the disaccharide lactose (milk sugar). Information technology is also an important component of the oligosaccharide side chains of many glycoproteins. Galactose is converted into common metabolic intermediates. Because mammalian cells tin can convert (epimerize) glucose to galactose, information technology is not an essential dietary component. In-born defects in galactose metabolism tin upshot in galactosemia, a disorder associated with mental retardation and blindness.

The hexose mannose is a pocket-size dietary saccharide in humans. It is institute principally in the glycan portion of near all glycoproteins and is a major component of many fungal cell walls. Its metabolism is similar to glucose.

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Microbial Metabolism

Catrin F Williams , in Reference Module in Biomedical Sciences, 2021

Anabolic pathways: Biosynthesis

Monomers, hexoses such as glucose and its half dozen-carbon sugar derivatives, are the building blocks of the polymers required for cellular growth. The majority of the ATP molecules synthesized during the catabolic processes described higher up are directed towards protein and nucleic acid synthesis. A insufficiently smaller amount of energy is required for lipid and polysaccharide biosynthesis. In the absence of monomers in their surrounding environment, microbes must synthesize these building blocks from simpler constituents through anabolic processes.

Gluconeogenesis is an example of an anabolic process, whereby compounds containing 2–5 carbon atoms are used every bit substrates to synthesize glucose. In full general, the reverse steps of glycolysis are performed from phosphoenolpyruvate, which can be synthesized from oxaloacetate derived from the Krebs bike. For polysaccharide biosynthesis activated forms of glucose, uridine diphosphoglucose (UDPG) or adenosine diphosphoglucose (ADPG), are used at starting material. UDPG is a precursor for N-acetylglucosamine or Northward-acetylmuramic acid, which are present in peptidoglycan or lipopolysaccharide in the Gram-negative cell wall. ADPG is a precursor for glycogen, used in long-term free energy storage. Pentose (5-carbon) sugars, e.g., ribose and deoxyribose, are required for nucleic acid synthesis. For this a carbon atom is removed from a hexose carbohydrate and released equally CO2. The NADPH-dependent ribonucleotide reductase enzyme reduces ribose to deoxyribose, which are precursors of RNA and DNA, respectively.

Synthesis of amino acids, the edifice blocks of proteins and nucleic acids, consist of complex, multi-step processes. Most of the carbon skeleton required to synthesize all amino acids are derived from either glycolysis or the Krebs cycle. The amino group is derived from a source of inorganic nitrogen in the surround. Glutamate dehydrogenase and glutamate synthase, for example, incorporate NH3 into the amino acids glutamate or glutamine, respectively. These amino acids can then shuttle NH3 through transaminase (via oxaloacetate) or aminotransferase (via α-ketoglutarate) reactions into carbon skeletons which undergo further biosynthetic reactions culminating in the synthesis of all 21 amino acids which class the building blocks of proteins in the cell. Purines and pyrimidines, the monomeric precursors of nucleotides, are constructed from several different carbon and nitrogen sources. Inosinic acid and uridylate are the precursors for purine (i.e., adenine and guanine) and pyrimidine (i.e., cytosine, thymine and uracil) nucleotide synthesis, respectively.

Fatty acids, a major component of lipids in bacteria and eukaryotes, are synthesized via an acyl carrier poly peptide (ACP). This holds the fat acid chain in position and binds with a 3-carbon compound, malonate, which donates 2-carbon atoms at a time to build the chain. The remaining carbon atom is released equally CO2. ACP releases the saturated (even carbon number chain length) fatty acrid when the desired chain length has been accomplished (around C12-C20 in bacteria). Unsaturated (odd carbon number chain length) fatty acids are synthesized through the desaturation of a saturated fatty acrid and contain at least one double bond. Different microbial species vary in their fat acid composition and this is temperature-dependent: the higher the temperature, the longer the chain. Lipid assembly in bacteria and eukaryotes involves the addition of glycerol to fatty acids through esterification. Circuitous lipids may likewise have phosphate, ethanolamine or a sugar attached to one of the glycerol carbon atoms. Instead of fatty acids, the lipids of archaea contain phytanyl sidechains. In all cases however, the glycerol caput is hydrophilic and the tail is hydrophobic. These characteristics allow phospholipids, where the glycerol backbone is bound to a phosphate grouping and two fatty acid tails, to play a key structural role in cellular membranes.

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DIGESTION AND Assimilation

Margaret Due east. Smith PhD DSc , Dion G. Morton MD DSc , in The Digestive Organisation (2d Edition), 2010

Physiological regulation of hexose absorption

The transport of hexoses by the enterocyte can be regulated by diet. Thus a diet high in glucose or fructose results in upregulation of the GLUT2 transporter in the basolateral membrane, and increased ship of hexoses into the blood. Thus claret glucose levels are regulated in role past alterations in the absorptive capacity of the enterocyte.

Glucose send into the blood tin can besides be regulated past claret glucose concentrations. Transport of glucose beyond the castor border membrane, but not the basolateral membrane, is stimulated past low blood sugar (hypoglycaemia). The mechanism may involve an increase in the concentration of circulating glucagon, a hormone which stimulates cAMP germination in the cell. This hormone is released into the claret during starvation (see Ch. 9).

Paradoxically, in diabetes, chronic hyperglycaemia (high blood sugar) also stimulates abdominal glucose transport. This is partly due to an increased surface area for assimilation, resulting from an increase in the number of enterocytes. All the same, glucagon levels are also high in diabetes and this may stimulate glucose ship past the same mechanism as during starvation. In addition there is also an upregulation of the GLUT2 transporter in the basolateral membrane in diabetic hyperglycaemia.

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Cherries (Prunus spp.): The Fruit and Its Importance

Westward. Loescher , in Encyclopedia of Food and Health, 2016

Quality Factors

Soluble solids (primarily hexose sugars and sorbitol) and fruit color (depending on the type) are the best indicators of quality for both sweet and tart cherries, although fruit acid level may be of import in tart cherry. Except for soluble carbohydrates, vitamins A and C, and certain flavonoids that may be of import as antioxidants in some cultivars, cherries are relatively depression in nutrients, but calcium, fe, magnesium, phosphorus, and copper contents are high compared to apple tree, peach, grape, and strawberry. Loftier-quality tart cherry fruit typically has at to the lowest degree 15% soluble solids, while sugariness cherries should accept nearly 20% (or higher). Standards for harvesting and marketing may, however, often exist lower. Optimum conditions also oft vary with use. To facilitate brining (bleaching in sulfur dioxide solutions for maraschino cherries), fruit may be picked prematurely before color and soluble solids are adequate for the fresh market. Stalk fruit removal force is carefully monitored for tart cherries that will exist mechanically harvested, and abscission may be brought on past treatment with ethephon, which releases ethylene, expediting abscission and fruit drop in response to mechanical shaking.

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Pathophysiology of diabetic retinopathy

Chris Steele BSc (Hons) FCOptom DCLP DipOC FBCLA , ... Colin Waine OBE MBBS FRCGP FRCPath , in Diabetes and the Eye, 2008

The polyol pathway

The polyol pathway converts hexose sugars such as glucose into sugar alcohols (polyols). For example glucose can be converted into sorbitol via the action of the enzyme aldose reductase. Aldose reductase is the rate-limiting enzyme for this pathway. Under normal atmospheric condition glucose is metabolized via the hexokinase pathway. In the presence of hyperglycaemia high glucose levels saturate the hexokinase pathway and glucose is then metabolized by the polyol pathway. This and so has a knock-on consequence for other metabolic processes. Increased aldose reductase activity and aggregating of sorbitol have been constitute in diabetic animal models. Every bit sorbitol does not easily deliquesce across cell membranes this increases cellular osmolarity, ultimately leading to jail cell impairment. Increased polyol pathway activity also alters the redox state of the pyridine nucleotides NADP+ and NAD+, thus reducing their concentrations. Since these are important factors in many enzyme-catalysed reactions, many other metabolic pathways may be as well affected. The decreased concentration of these cofactors leads to decreased synthesis of reduced glutathione, nictric oxide, myoinositol and taurine. Myoinositol is especially required for the normal function of fretfulness. Sorbitol may too glycate nitrogens on proteins, such every bit collagen, producing AGE products.

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Lifestyle Approaches

May Loo MD , in Integrative Medicine for Children, 2009

Vitamin C

Vitamin C is a hexose derivative synthesized by plants from glucose and galactose and serves as an antioxidant; as a coenzyme in the synthesis of collagen, carnitine, norepinephrine, and serotonin; and as a coenzyme to reduce atomic number 26 and copper, facilitating in their assimilation. Vitamin C also has an indirect role as a reducing amanuensis in producing many hormones and polypeptides. Vitamin C prevents oxidation of LDH-C, lowers total cholesterol, raises HDL-C, and inhibits platelet aggregation; therefore it is helpful in preventing atherosclerosis. Vitamin C likewise promotes resistance to infection through immunologic activity within leukocytes, the production of interferon, its role in the inflammatory procedure, and promotion of mucous membrane integrity. 28

The best sources of vitamin C are citrus fruits, melons, strawberries, tomatoes, peppers, and potatoes. Refrigeration and quick freezing helps retain the vitamin content in these foods, but prolonged exposure to air and the cooking process destroys vitamin C. Information technology has been estimated that as much as 45% of the vitamin is lost from prepared vegetables refrigerated for 24 hours, and as much equally 52% is lost in frozen products. 45

The RDA is 30 to 90 mg/day for children and l to 90 mg/day for adults. These levels may not provide adequate reserves for this vitamin, specially for smokers and those who are under stress or with increased metabolic demands (burns), taking oral contraceptives, or being treated for sure affliction processes. For these patients, doses as high as 2000 mg/day may be recommended. 37

Vitamin C deficiency results in scurvy, the symptoms of which are swollen, bleeding gums; languor; fatigue; peel lesions; and psychological manifestations (depression, hysteria, hypochondria). Infants can develop this effectually vi months of age if they are breastfed without maternal supplementation or do not receive any other sources of vitamin C. Moeller-Barlow disease occurs when maternal stores of vitamin C are depleted and is characterized past poor wound healing, edema, weakness of bones and connective tissue, or hemorrhages. 28

Doses greater than 2000 mg may cause flatulence, diarrhea, and urinary frequency or urgency. Patients with a renal rock predisposition need to be cautious in taking vitamin C, as it may increase the risk of oxalate stone formation in the kidneys.

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Carbohydrates

Antonio Blanco , Gustavo Blanco , in Medical Biochemistry, 2017

Amino Sugars

When one hydroxyl group of a monosaccharide is replaced by an amine grouping, an amino-sugar is formed. Glucosamine and galactosamine, in which the amine group is attached to carbon 2, are the well-nigh common ones in nature. They found glycolipids and circuitous polysaccharides and are frequently acetylated on their amine group. An acetylated derivative of glucosamine is the basic component of chitin, a polysaccharide arable in the exoskeleton of arthropods and insects, besides as in the cell wall of fungi.

Other nitrogen containing compounds related to hexoses are neuraminic and muramic acids. Neuraminic acrid is an important component of polysaccharide chains in glycoproteins and glycolipids of cell membranes. This ix-carbon compound is formed past the amino-sugar mannosamine and pyruvic acid; generally the Northward is acylated, forming sialic acids. The most common sialic acid is North-acetyl-neuraminic, one of the strongest organic acids in living organisms (pG a = 2.6).

*N indicates that the acetyl moiety is attached to the nitrogen.

Muramic acid is formed past d-glucosamine with its C3 bonded to C2 of lactic acid (ether bond). An acetyl derivative of muramic acrid, N-acetyl-muramic, is a component of the polysaccharide bacterial jail cell wall.

*Due north indicates that the acetyl moiety is fastened to the nitrogen.

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Sugar Metabolism in the Central Nervous System

I. Simpson , Southward.J. Vannucci , in Encyclopedia of Biological Chemistry (Second Edition), 2013

Glucose Transporters

The transport of glucose and other hexoses into most mammalian cells is mediated by the SLC2 family of 14 ship proteins, GLUT1 to GLUT14, which includes HMIT, the proton-driven myoinositol transporter. With the exception of GLUT1, which is ubiquitously expressed, the other family members have adequately distinct tissue- and cell-specific patterns of expression. As illustrated in Figure 3 , the brain with its complex cellular heterogeneity expresses about, if not all, SLC2 family members. Much of the data supporting the indicated cellular distribution have been reviewed previously and will not be further discussed here. However, it is important to note that many of the transporters indicated in Figure three either are unable to ship glucose (GLUT5, -6, and -11, and HMIT have very low analogousness for glucose) or have very express localization and concentrations (GLUT2 and -4), whereas for others, their location, power, or capacity to send glucose take even so to be fully assessed (GLUT8, -9, and -ten). Thus, with all of the above exclusions, the predominant transporters in mammalian brain involved in cerebral glucose utilization are GLUT1 and -3, which are expressed throughout the brain and whose localization and kinetic characteristics are well established. Two molecular forms of GLUT1 have been identified: a 55-kDa grade in the endothelial cells and a 45-kDa form in the various glia (astrocytes, oligodendrocytes, and microglia) and choroid plexus. The difference in their relative molecular weight is deemed for by a differing extent of glycosylation, but this additional glycosylation does not appear to modify their protein structure or kinetic characteristics. GLUT3 is expressed predominantly in neurons and is singled-out from GLUT1 in that its affinity for glucose is higher (M m, 2–3 vs. 5–8   mM) and the ability to transport glucose is faster (K cat, 6512   vs. 1166   s−1 @ 37   °C). An interesting feature of neuronal GLUT3 is that both messenger RNA (mRNA) and protein levels in vivo announced to exist regulated in direct relation to changes in neuronal activity and rates of cerebral glucose utilization in several conditions underscoring the importance of glucose to neuronal energy demand.

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Genetics and Biosynthesis of Lipopolysaccharide

Miguel A. Valvano , in Molecular Medical Microbiology (2d Edition), 2015

Assembly of the Outer Core

A big multifariousness of glycosyltransferases that transfer hexose units from their nucleotide-activated precursors to glycosylated lipid A acceptor molecules are fundamental to the biosynthesis of many cadre oligosaccharide structures [26]. The functional consignment of these enzymes is typically based on structural analyses of the lipid A-core of defined mutants combined with structural studies on the truncated core derivatives [11,26,109].

In E. coli and Salmonella, glucose is the first saccharide in the outer core [x], which is incorporated to the last heptose of the inner core by WaaG [10]. WaaG is a UDP-glucose:(heptosyl) LPS α-ane,3-glucosyltransferase that remains well conserved in E. coli and Salmonella strains expressing each of the core types. Boosted glycosyltransferases transfer glucose or galactose and in general, the enzymes catalysing conserved linkages in E. coli and Salmonella cores are closely related, and their corresponding genes occupy like positions in the loci, with a few exceptions [10]. The genetics and biosynthesis of outer-core structures in many other leaner have revealed increased complexity and variability in the outer-cadre associates (recently reviewed in [26]).

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URONIC ACIDS

F.M. Huffman , in Encyclopedia of Nutrient Sciences and Diet (Second Edition), 2003

Hemicellulose

Hemicellulose is a branched polymer of pentose and hexose sugars, found in the constitute cell wall. The uronic acrid limerick is mainly d-glucuronic acrid and four-O-methyl-d-glucuronic acid. There are two singled-out hemicelluloses in plants: the acidic and the neutral. Acidic hemicelluloses comprise a larger number of uronic acids than neutral hemicelluloses. Hemicelluloses are partially fermented by the microorganisms of the colon, producing some volatile fat acids. Hemicelluloses are insoluble in water but soluble in alkaline solutions. They, forth with other insoluble dietary fibers, decrease the intestinal transit time; hemicelluloses besides increase fecal weight and slow down starch hydrolysis. Acidic hemicelluloses may bind to cations. These characteristics of hemicellulose may exist responsible for its physiological effects.

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