Sucrose Synthesis
by D. DeWitt, PhD
v1.3 10/15/07
A. Introduction
Although it might seem straight forward, the synthesis of sucrose, either
as a simple condensation reaction (a.k.a. dehydration synthesis), or
what actually happens in plants is complicated.
Before we explore sucrose's creation, let's take a look at its
structure. In Figure 1, the space-filling model is pretty but
rather useless at this point in our journey. We need to see the
atoms!
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Figure 1: Space-filling
Model of Sucrose
(Click on image to visit originating
website.)
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In Figure 2, you will find the structural
formula for sucrose. It is a disaccharide made from
the simple sugars glucose (on the left) and fructose (on the right).
I will caution you right now that there are many different view of
sucrose on the internet, so if this one does not agree with your
impression, there are reasons: 1) errors (e.g., Fructose
has 7 oxygens) and 2) synthesis from different forms (isomers) of
glucose and fructose. |
Figure 2: Structural Model of Sucrose
Note: Carbon atoms are assumed at all angles of each geometric shape
(hexagons and pentagons) unless an atom is shown (e.g. oxygen)
(Click on image to visit
originating website.)
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The first issue is the
process by which two sugars can be chemically joined.
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The answer is found in the fact
that these sugars are literally covered
with hydroxyl (-OH) groups. As shown in Figure 3 on the right, a
hydroxyl
group on one sugar (-OH) can be connected to another
hydroxyl group (HO) on a second sugar by removing
an -OH from the first sugar and a H- from the second leaving a -O-
bond connecting the two sugars in a disaccharide.
In addition, the released -OH and H- join to form H2O (HOH). The name of the -O- bond
is called a glycosidic bond. The bond gets its
name from the fact that the disaccharide is also called a glycoside.
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sugar1-OH
+ HO-sugar2
sugar1- + -O-sugar2 + -OH + H-
sugar1-O-sugar2 + HOH
Figure 3: Glycosidic Bond
Formation
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The overall reaction is called
either a a) condensation reaction because water appears, or b) dehydration
synthesis because
water is removed (dehydration). Therefore the formation of the
disaccharide sucrose occurs via a condensation reaction. Because
it occurs within cells, the reaction occurs only with the aide
of an enzyme.
ACTUALLY... in plant cells, it occurs
differently, but I will address the real world later in this discussion.
Let's take a closer look at the specifics (and therefore, the sources
of confusion) about condensation reactions.
Monosaccharide ISOMERS
Two molecules are isomers if they have the same molecular formula, but
different structural formulas. Therefore glucose and fructose are
structural isomers because they both have the same
molecular formula of C6H12O6
but they are arranged differently. (E.g., Glucose is an aldose
and fructose is a ketose.) In Figure 4 below, you can see how
glucose and fructose carbons are numbered. Reference to carbons
1, 2 and 4 will be forthcoming. |
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Figure 4: Glucose (left)
and Fructose (right)
(Click on image to visit
originating website.)
Note: This website's fructose has 7 oxygens! Look at Carbon 5.)
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With
each named monosaccharide, alternative forms exist as well. These
are also isomers, based on more subtle differences. In other
words, each aldose or ketose has different forms possible which are
also isomers. Examples are D and L isomers and more
useful to this discussion, alpha and beta isomers (a.k.a., anomers).
THE major source of confusion and the source of
the abundance of alternate structures
for sucrose, is the lack of attention to alternate forms (a.k.a.
isomers) of the
building block monosaccharides glucose and fructose.
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Alpha and Beta Isomers
(a.k.a., alpha and beta anomers)
If you examine Figure 4 above, and find the Oxygen atom in each ring,
you will find a carbon to the right in the ring, denoted as carbon 1 in
glucose and carbon 2 in fructose. This carbon is called the anomeric
carbon.
So normally, a glycosidic bond is made between the -OH of an anomeric
carbon of one monosaccharide and an -OH connected to any carbon in the
second monosaccharide. The glycosidic bonds of interest in my
courses, are 1-->4, 1-->6, 1-->2.
Anomeric forms of glucose
In Figure 5 below, you can see that the anomeric forms of glucose
differ ONLY at the anomeric carbon #1. When the hydroxyl group is
found below carbon 1, it is called the alpha anomer.
When the hydroxyl group is above carbon 1, it is called the beta
anomer.
Note: Sometimes they are just called alpha and beta isomers of
glucose. In some of my course discussions (A&P), I also refer
to this as "alpha and beta-ness"... to avoid annoying my students with
ANOTHER confusing term. In Mol. Bio. and Bio. Chem., I DO refer
to anomeric isomers.
Figure 5: Alpha-glucose
(left) and Geta-glucose (right)
(Click on image to visit
originating website.)
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Let's make disaccharides using
just alpha and beta glucoses.
An classic example of how important the correct
use of alpha and beta forms of glucose is, is the difference in
disaccharides (and polysaccharides) produced using only alpha-glucose
or only beta-glucose.
Alpha-glucose oligomers and polymers
As shown below in Figure 6, if you join two alpha-glucoses together you
form the disaccharide alpha-maltose, and it is linked via an alpha-1,4-glycosidic
bond. It is called alpha-maltose because the "free anomeric
carbon" (i.e., the carbon 1 on the right side of maltose), is in the
alpha form.
Figure 6: Alpha-Maltose
Synthesis from Two Alpha-glucoses
(Click on image to visit
originating website.)
If you
used one
alpha-glucose on the left, and one beta-glucose on the right, you would
end up with a maltose, called beta-maltose, because the "free anomeric
carbon" (i.e., the carbon 1 on the right side of maltose), is in the
beta form.
If you
continue this process to make a polysaccharide, you end up with the
starch amylose. Note
also that you can eat and digest maltose or amylose and gain valuable
energy by absorbing the resultant alpha-glucoses into your blood.
Beta-glucose oligomers and polymers
As
shown below in Figure 7, if you join two beta-glucoses
together you form the disaccharide cellobiose, and it is
linked via a beta-1,4-glycosidic bond. If you continue
this
process to make a
polysaccharide, you
end up with the cellulose.
Figure 7: Cellbiose
Synthesis from Two Beta-glucoses
(Click on image to visit
originating website.)
Note
also that you CAN eat, but you CAN NOT digest cellobiose or
cellulose.
Therefore, you will NOT gain valuable
energy from this meal. Why not? Because your digestive
system does
not make the necessary enzyme cellobiase or cellulase that will break
beta-1,4-glycosidic bonds.
And so... such as small difference between alpha and beta isomers of
glucose, changes life on earth! What would life be like if all
animals could digest beta bonds?
One more thing about beta bonds...
As you can see in Figure 7, if you were to synthesize a long polymer of
beta-glucoses, the string would move upward at an angle.. which takes
up a lot of space on paper..... so there are other ways to show the
molecule that keep it horizontal.
In Figure 8, the "zig-zag" method shows a section of cellulose with ...
used to show that the molecule extends with many more glucoses.
Figure 8: Cellulose
"zig-zag" Representation of the Glycosidic Bonds
(Click on image to visit
originating website.)
In
Figure 9, another method shows alternating upside down glucoses.
In addition, this version shows the ends, and indicates MANY monomers
by bracketing the repeating cellobiose dimer.
Figure 9: Cellulose
"alternating UPSIDE DOWN" Representation of the Glycosidic Bonds
(Click on image to visit
originating website.)
And... now you are educated to the level that will allow for an
easy discussion of sucrose synthesis.
Anomeric forms of fructose
In Figure 10 below, you can see that the anomeric forms of fructose
differ ONLY
at the anomeric carbon #2. When the hydroxyl group is found below
carbon 1, it is called the alpha anomer. When the hydroxyl group
is
above carbon 1, it is called the beta anomer.
Figure 10:
Figure 5: alpha-Fructose
(left) and beta-Fructose(right)
(Click on image to visit
originating website.)
Note:
This website's fructose has 7 oxygens! Look at Carbon 5.)
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It is now time to
synthesize sucrose!
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B.
Sucrose Synthesis by Condensation Reaction (a.k.a., Dehydration
Synthesis)
Now
that you understand that glucose and fructose are available in anomeric
forms, the obvious
question is: Is sucrose made from alpha or beta
glucose and alpha or beta fructose?
If
we examine sucrose, maybe we can figure it out.
Figure
11: Structural Model of Sucrose
(Click on image to visit
originating website.)
In Figure 11, the glucose on the
left appears to be an alpha-D-glucose because a -H (at *1) is above carbon 1... which means
it had a -OH below before the condensation reaction ran.
And now for the confusing part....
On the far right in Figure 11, there is no -OH...at *2 so how does one decide if the
fructose is alpha or beta?
Let's bring fructose back and put it next to sucrose...
Figure
12: Structural Model of Sucrose (left) and beta-Fructose (right)
(Click on image to visit
originating website.)
So where did the -OH go on carbon 2 of fructose in sucrose?
Answer is: The carbon on the far right of sucrose is NOT
carbon 2. It is carbon 5!
The proof is to look at carbons 3 and 4 in fructose. Then compare
their up/down positions to sucrose. You will see that they are
opposite in sucrose in comparison to fructose!
CONFUSION!
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In Figure 13, you will see
another way of looking at sucrose which may help solve this problem.
In this view, the fructose appears in the same orientation that it does
alone. (Figure 14 below)
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Figure 14: Structural
Model of Beta-Fructose
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but... how
does Figure 13's sucrose relate to Figure 12's sucrose?
Actually.. they represent the same molecule.
Figure 15 shows how beta-D-fructose, is first flipped back to front,
followed by a rotation about the ring oxygen, and then condensed with
alpha-D-glucose.
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Figure 13: Structural
Model of Sucrose
(Upright Version)
(Click on image to visit
originating website.)
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Figure
15: Synthesis of Sucrose via a Condensation Reaction
(Source: D. DeWitt)
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