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Chemical
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Glucose |
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6-(hydroxymethyl)oxane-2,3,4,5-tetrol |
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Synonym
for D-glucose |
dextrose |
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Varieties
of D-glucose |
α-D-glucose; β-D-glucose |
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Abbreviations |
Glc |
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180.16
g mol−1 |
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α-D-glucose: 146°C |
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1.54
g cm−3 |
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50-99-7
(D-glucose) |
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921-60-8
(L-glucose) |
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C(C1C(C(C(C(O1)O)O)O)O)O |
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Glucose (Glc), a monosaccharide
(or simple sugar),
is one of the most important carbohydrates. The cell uses it as a source of energy
and metabolic intermediate. Glucose 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 glucose (The mirror-image of the molecule
is called L-glucose. See isomers
below)
Glucose
contains six carbon
atoms and an aldehyde group
and is therefore referred to as an aldohexose. The glucose molecule can exist in an open-chain
(acyclic) and ring (cyclic) form, the latter being the result of an
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 five carbon atoms and
one oxygen atom, which resembles the structure of pyran, the cyclic
form of glucose is also referred to as glucopyranose. In this ring, each carbon
is linked to an hydroxyl side group with the exception of the fifth atom, which
links to a sixth carbon atom outside the ring, forming a CH2OH
group.
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.
An
additional asymmetric center at C-1 (called the anomeric carbon atom) is
created when glucose cyclizes and two ring structures, called anomers, can be
formed — α-glucose and β-glucose. They differ structurally in the
orientation of the hydroxyl group linked to C-1 in the ring. When D-glucose is drawn as a Haworth projection, the designation α
means that the hydroxyl group attached to C-1 is below the plane of the ring, β
means it is above. The α and β forms interconvert over a
timescale of hours in aqueous solution, to a final stable ratio of α:β
36:64, in a process called mutarotation.
Glucose shifting from Fischer projection to Haworth projection.
|
The Fischer projection of the chain form of D-glucose |
The chain form of D-glucose |
α-D-glucopyranose |
β-D-glucopyranose |
|
Chain form:
ball-and-stick model |
Chain form:
space-filling model |
α-D-glucopyranose |
β-D-glucopyranose |
Glucose
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 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, and
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 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-glucose is then produced by repeated crystallizations.
We can
speculate on the reasons why glucose, and not another monosaccharide such as fructose (Fru)
, is so widely used in evolution/the ecosystem/metabolism. Glucose 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
glucose, by comparison to other hexose sugars, to non-specifically react with
the amino groups
of proteins.
This reaction (glycation) reduces or destroys the function of many enzymes. The low
rate of glycation is due to glucose'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 glycation of proteins or lipids. Glycosylation
is another important type of reaction undergone by glucose.
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.
Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which is
glucose. Through glycolysis and later in the reactions of the Citric
acid cycle (TCAC), glucose is oxidized to eventually form CO2
and water,
yielding energy, mostly in the form of ATP. The insulin reaction, and other
mechanisms, regulate the concentration of glucose in the blood. A high fasting
blood sugar level is an indication of prediabetic and diabetic conditions.
Use of
glucose as an energy source in cells is via the glycolysis metabolic
pathway. The first step of this is the phosphorylation
of glucose by hexokinase to prepare it for later breakdown to provide
energy.
Compound C00031 at KEGG
Pathway Database [1]. Enzyme 2.7.1.1 at KEGG Pathway Database [2]. Compound C00668 at KEGG Pathway Database [3]. Reaction R01786 at KEGG Pathway Database [4].
The
major reason for the immediate phosphorylation of glucose by a hexokinase
is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate
group so the glucose 6-phosphate cannot easily cross the cell
membrane.
Glucose
is critical in the production of proteins and in lipid metabolism. Also, in plants and most animals, it is a
precursor for vitamin
C (ascorbic acid) production.
Glucose
is used as a precursor for the synthesis of several important substances. Starch, cellulose,
and glycogen
("animal starch") are common glucose polymers (polysaccharides).
Lactose, the
predominant sugar in milk, is a glucose-galactose
disaccharide. In sucrose,
another important disaccharide, glucose is joined to fructose.
All
major dietary carbohydrates contain glucose, 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. Glucose is then transported across the apical
membrane of the enterocytes by SLC5A1 and later across
their basal membrane by SLC2A2 (ref). Some of
glucose 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 glucose when there is need for energy.