A&Psitemap
Sucrose Synthesis
by D. DeWitt, PhD
v1.5   11/10/12

Introduction
Condensation
Reaction

Plant Synthesis


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!

sucrose
Figure 1: Space-filling Model of Sucrose
(Click on image to visit originating website.)
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
.
sucrose structure

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.)

The first issue is the process by which two sugars can be chemically joined.

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.


sugar1-
OH     +    HO-sugar2

sugar1-   +   -
O-sugar2  +  -OH  +   H-

sugar1
-O-sugar2  +  HOH

Figure 3: Glycosidic Bond Formation

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.
glucose and fructose
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.)

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.

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.
fig5 glucose anomers
Figure 5: Alpha-glucose (left) and Geta-glucose (right)
(Click on image to visit originating website.)

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.
fig   6
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 amyloseNote 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
fig 7
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.

fig8
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.

fig9 
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. 
fig10
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.)

It is now time to synthesize sucrose!
Introduction
Condensation
Reaction
Plant Synthesis

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.
fig11
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...
fig12
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!

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)

fig14
Figure 14: Structural Model of Beta-Fructose
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.


fig13
Figure 13: Structural Model of Sucrose
(Upright Version)
(Click on image to visit originating website.)
fig15

Figure 15: Synthesis of Sucrose via a Condensation Reaction
(Source: D. DeWitt)

Therefore sucrose is made from alpha-glucose and beta-fructose connected via a 1-2 glycosidic bond.

Because both anomeric carbons are connected, a trisaccharide can not be made from sucrose by adding any other monosaccharide.

And now for the real way sucrose is made in plant cells! Return to Top


C.  Sucrose Synthesis in Plants

Sucrose is used as a transport molecule in plants to move carbohydrate energy to cells where they can hydrolyze it into glucose and fructose which are then used to generate ATP for cell work or they are used to synthesize other molecules such as starch or cellulose.

fig16
Figure 16: Synthesis of Sucrose in Plants
(Ref: Modified by D. DeWitt)
(Click on image to visit originating website.)
Wellll... you asked for it!

As you can see in Figure 16, the real world of sucrose synthesis in plants is more complex.  The diagram shows how the cytoplasm (lower area) and the chloroplast (upper area) interact in the production of sucrose and starch.  The basic cytosolic reactions are shown below:
  1. Uridine-triphosphate (UTP) + Glucose-1-phosphate (G1P) ---> UDP-glucose + Pyrophosphate (PPi)
  2. UDP-glucose + PPi + Fructose-6-phosphate (F6P) ---> UDP + sucrose-6(F)-phosphate (S6P)
  3. Sucrose-6(F)-phosphate (S6P)  + H2O ---> PPi + Sucrose
Some of the structures can be seen in Figure 17.

Note that the addition of fructose-6P to UDP-glucose removes a H from the -OH hanging below carbon 5.  However, that H attaches to O-UDP to make HO-UDP which is shown as UDP.  The bottom line is that water does not show up  because the -OH and -H normally produced are added to other molecules.

As you can see, mother nature does not combine glucose and fructose quite in the way as is taught in introductory biochemistry of carbohydrates.  Life has evolved in a much more complex way. 





What does UDP look like and how does it carry glucose?

fig 17
Figure 17: Synthesis of Sucrose in Plants
(Click on image to visit originating website.)

When we studied nucleic acids you learned about mono-, di- and triphosphate nucleotides which are used in various tasks within cells such as nucleic acid synthesis (dAMP, UMP, etc), or energy currency (ATP) or messengers (cAMP).  Another function of nucleotides is to carry molecules in biosynthetic or degradation pathways.  In this case, Uridine-triphosphate (UTP) + Glucose-1-phosphate (G1P) ---> UDP-glucose + Pyrophosphate (PP i).  So do not fear UDP-glucose.  It is a nice molecule that plants use to synthesize sucrose and cellulose.  Animals use to process glycogen too!  As shown in Figure 16, ADP-glucose is used in starch synthesis however.

fig 18
Figure 18: Uridine Diphosphate Glucose
(Click on image to visit originating website.)


Return to Top