Glucose

Glucose
Chemical name	6-(hydroxymethyl)oxane-2,3,4,5-tetrol
Synonym for D-glucose	dextrose
Varieties of D-glucose	α-D-glucose; β-D-glucose
Abbreviations	Glc
Chemical formula	C6H12O6
Molecular mass	180.16 g mol−1
Melting point	α-D-glucose: 146°C β-D-glucose: 150°C
Density	1.54 g cm-3
CAS number	50-99-7 (D-glucose)
CAS number	921-60-8 (L-glucose)
SMILES	C(C1C(C(C(C(O1)O)O)O)O)O

Glucose (Glc) is a monosaccharide (or simple sugar) with the chemical formula C₆H₁₂O₆. It is the major free sugar circulating in the blood of higher animals, and the preferred fuel of the brain and nervous system, as well as red blood cells.

Glucose is of central importance in the metabolism of all life forms, as a universal substrate for the production of cellular energy in the form of ATP. It is both one of the main products of photosynthesis, the process by which plants, algae, some bacteria, and some protists convert energy from sunlight into chemical energy to be used by the cell, and a major starting point for cellular respiration, the process by which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes.

All major dietary carbohydrates contain glucose, either as their only building block, as in starch and glycogen, or in combination with another monosaccharide, as in sucrose (“table sugar”) and lactose, the primary sugar found in milk.

The natural form of glucose (D-glucose) is also referred to as dextrose, especially in the food industry.

The structure of glucose

A space-filling model of glucose.

The open-chain form of D-glucose.

Sugars are classified according to two properties: (1) the number of carbon atoms and (2) the type of functional group (either an aldehyde or ketone group) they contain. Glucose, which has six carbon atoms (i.e., it is a hexose sugar) and contains an aldehyde group (-CHO), is thus referred to as an aldohexose.

The glucose molecule can exist in an open-chain (acyclic) form and a ring (cyclic) form. In solution and at neutral pH, the cyclic form is predominant at equilibrium. When glucose exists in cyclic form, the functional group is not free, making the molecule less reactive. This preference for the less reactive ring form offers a possible explanation for the crucial and widespread use of glucose in metabolism, as opposed to another monosaccharide such as fructose (Fru). The low tendency of glucose, in comparison to other hexose sugars, to non-specifically react with the amino groups of proteins might explain its importance to advanced life.

Isomers

Glucose has 4 optic centers which means that in theory glucose can have (4²-1) = 15 optical stereoisomers. Only 7 of these are found in living organisms, and of these galactose (Gal) and mannose (Man) are the most important. These eight isomers (including glucose itself) are all diastereoisomers in relation to each other, and all belong to the D-series.

Natural sources of glucose

• Glucose is one of the products of photosynthesis in plants and some prokaryotes.
• Glucose can be obtained through the breakdown of glycogen, the storage form of glucose in animals and fungi, through a process known as glycogenolysis. In plants, glucose is stored as starch.
• When glucose is not supplied in the diet and glycogen stores have been depleted, animals may also synthesize glucose in the liver and (to a lesser extent) in the kidneys from non-carbohydrate intermediates. Lactate from active skeletal muscle, amino acids from protein in the diet or protein in muscle, or glycerol, derived from the hydrolysis of fats, may contribute to the synthesis of glucose, which is referred to as gluconeogenesis).

The function of glucose

As an energy source

Glucose is a ubiquitous fuel in biology. Carbohydrates are the human body's key source of energy, providing 4 kilocalories (17 kilojoules) of food energy per gram. The breakdown of carbohydrates, whether stored as glycogen or taken in through the diet, yields mono- and disaccharides, many of which are glucose molecules. In the anaerobic process of glycolysis and later in the reactions of the Citric acid cycle (TCAC), glucose is oxidized to form CO₂ and water, yielding energy, mostly in the form of ATP.

As a precursor to other biological molecules

In plants and most animals, glucose is a precursor for the production of vitamin C (ascorbic acid). Glucose is also critical in the production of proteins and in lipid metabolism. Glycation and glycosylation are two important types of reactions undergone by glucose.

Glucose is used as a precursor for the synthesis of several important carbohydrates. Starch, cellulose, and glycogen are common glucose polymers (polysaccharides). Glycogen is the body's auxiliary energy source, tapped and converted back into glucose when there is need for energy. Lactose, the predominant sugar in milk, is a glucose-galactose disaccharide. In sucrose, another important disaccharide, glucose is joined to fructose.

The body's regulation of glucose

The body regulates the concentration of glucose in the blood through the action of the hormones insulin, which directs the flow of glucose into cells, and [[glucagon[[ and epinephrine, which retrieve glucose from its storage form as glycogen in liver and muscle tissue. Low blood sugar (hypoglycemia). Elevated blood sugar (hyperglycemia) in diabetes.

Commercial production

Glucose is produced commercially through the breakdown of starch in an enzyme-catalyzed process called hydrolysis (a chemical reaction in which a molecule is split into two parts through the addition of water). The enzymatic process has two stages:

• Over the course of 1-2 hours near 100 °C, these enzymes break the starch into smaller carbohydrates containing on average 5-10 glucose 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, so that fresh enzyme must be added to the mixture after each heating.
• In the second step, known as saccharification, the partially hydrolyzed starch is completely hydrolyzed to glucose using the glucoamylase enzyme from the fungus Aspergillus niger. Typical reaction conditions are pH 4.0–4.5, 60 °C, and a carbohydrate concentration of 30–35% by weight. Under these conditions, starch can be converted to glucose at 96% yield after 1–4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and the processing of a greater volume of water, which is not generally economical.

The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator. Solid D-glucose is then produced by repeated crystallizations.

Many crops can be used as sources of starch in the commerical production of glucose, including maize, rice, wheat, potato, cassava, arrowroot, and sago. In the United States, cornstarch (from maize) is used almost exclusively.

External links

• EINECS number 200-075-1 (D-glucose)
• EINECS number 213-068-3 (L-glucose)
• CID 5793 from PubChem (D-glucose)
• CID 206 from PubChem (L-glucose)
• More on the chemistry and function of glucose in biology at EvoWiki
• Computational Chemistry Wiki

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