w8pedia:Nutrition:Glucose

Glucose (Glc), a monosaccharide, is one of the most important carbohydrates. The cell uses it as a source of energy and metabolic intermediate. Glc is one of the main products of photosynthesis and starts cellular respiration. The natural form (D-glucose) is also referred to as dextrose, especially in the food industry. This article deals with the D-form of Glc (see Isomers-section bellow)

Structure

Glc contains six carbon atoms and an aldehyde group and is therefore refered to as an aldohexose. Glc molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of a intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal. In water solution both forms are in equilibrium, and at pH 7 the cyclic one is the predominant. As the ring contains 5 carbon and one oxygen atoms, which resembles the structure of pyran, the cyclic form of Glc is also refered to as glucopyranose. In this ring, each carbon is linked to hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH₂OH group.

Isomers

Glc has 4 optic centers which means that in theory Glc can have 15 optical stereoisomers. In living organisms only 7 of them are found, of which Gal and Man are the most important. These eight isomers (including Glc) are all diastereoisomers in relation to each other and all belong to the D-series.

An additional asymetric center at C-1 (called the anomeric carbon atom) is created when Glc cyclizes and two ring structures, called anomers, can be formed - ?-Glc and ?-Glc. They structurally differ in the orientation of the hydroxyl group linked to C-1 in the ring. When D-Glc is drawn as a Haworth_projection, the designation ? means that the hydroxyl group attached to C-1 is bellow the plane of the ring, ? means - it is above. The ? and ? forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of ?:? 36:64, in a process called mutarotation.

Production

Natural

1. Glucose is one of the products of photosynthesis in plants and some prokaryotes.
2. In animals and fungi, glucose is the result of the breakdown of glycogen, a process known as glycogenolysis. In plants - the breakdown substrate is starch.
3. In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol, by a process known as gluconeogenesis.

Commercial

Glc is produced commercially via the enzymatic hydrolysis of starch. Many crops can be used as the source of starch Maize, rice, wheat, potato, cassava, arrowroot, and sago are all used in various parts of the world. In the United States, cornstarch (from maize) is used almost exclusively.

This enzymatic process has two stages. Over the course of 1-2 hours near 100 °C, these enzymes hydrolyze starch into smaller carbohydrates containing on average 5-10 Glc units each. Some variations on this process briefly heat the starch mixture to 130 °C or hotter one or more times. This heat treatment improves the solubility of starch in water, but deactivates the enzyme, and fresh enzyme must be added to the mixture after each heating.

In the second step, saccharification, the partially hydrolyzed starch is completely hydrolyzed to Glc using the glucoamylase enzyme from the fungus Aspergillus niger. Typical reaction conditions are pH 4.0­4.5, 60 °C, and a carbohydrate concentration of 30­35% by weight. Under these conditions, starch can be converted to Glc at 96% yield after 1­4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and processing a greater volume of water, and is not generally economical. The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator. Solid D-Glc is then produced by repeated crystallizations.

Function

We can speculate on the reasons why Glc, and not another monosaccharide such as Fru, is so widely used. Glc can form from formaldehyde under abiotic conditions, so it may well have been available to primitive biochemical systems. Probably more important to advanced life is the low tendency of Glc, by comparison to other hexose sugars, to nonspecifically react with the amino groups of proteins. This reaction (glycosylation) reduces or destroys the function of many enzymes. The low rate of glycosylation is due to Glc's preference for the less reactive cyclic isomer. Nevertheless, many of the long-term complications of diabetes (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycosylation of proteins.

As an energy source

Glc is a ubiquitous fuel in biology. Carbohydrates are the human body's key source of energy, providing 4 calories (17 kilojoules) of food energy per gram. Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which is Glc. Through glycolysis and later in the reactions of TCAC, Glc is oxidized to eventually form CO₂ and water, yielding energy, mostly in the form of ATP.

As a precursor

Glc is critical in the production of protein and in lipid metabolism.

Glc is used as a precursor for the synthesis of several important substances. Starch, cellulose, and glycogen ("animal starch") are common Glc polymers (polysaccharides). Lactose - the milk sugar, is a Glc-Gal disaccharide. In sucrose, another important disaccharyde, Glc is joined to Fru.

Sources and absorbtion

All major dietary carbohydrates contain Glc, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose. In the lumen of the duodenum and small intestine the oligo- and polysaccharides are broken down to monosaccharides by the pancreatic and intestinal glycosidases. Glc is then transported across the enterocytes and into the bloodstream, first at the apical membrane by Na⁺-dependent transporter protein (GLUT2) and then at the basal membrane by a totally different protein. Some of Glc goes directly to fuel brain cells and erythrocytes, while the rest makes its way to the liver and muscles, where it is stored as glycogen, and to fat cells, where it is stored as fat. Glycogen is the body's auxiliary energy source, tapped and converted back into Glc when there is needs for energy.

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