Re: What makes the glycosidic links rigid in cellulose as opposed to starch?

Dear Bryan,

I'm an organic chemist (certainly not a biochemist), so I will be giving 
you an answer which reflects what I have learned in reading and teaching 
about carbohydrates in the organic chemistry courses.

There seem to be several aspects to your question:

1. So that I am sure that "we" are beginning at the same beginning, I'll 
cover the topic of 1-4 linkages.  I presume that you know about the chair 
conformation of the six-membered ring.  Haworth drawings do not do justice 
to what we think is the true shape of the six-membered ring with its 
equitorial and axial bonds.  The 4-carbon of one glucose unit is connected 
by a bond that is always equitorial FROM that 4 carbon on glucose.  If it 
were not equitorial (but axial) we would be dealing with a galactose unit 
not glucose.  This equitorial oxygen is then connected to the 1-carbon of 
another glucose unit.  Because the 1-carbon is in a hemiacetal group, the 
connection can be either axial or equitorial TO the 1-carbon.  If the 
connection TO the 1-carbon is axial on the 1-carbon, then we have what is 
called an alpha linkage; if the connection is equitorial TO the 1-carbon, 
then the linkage is termed beta.  Haworth formulas do not present an 
accurate picture here.

These different linkages present different topologies or shapes in these 
regions.  It is my understanding that starch can be easily hydrolyzed 
(taken apart with water) at the 1-4 linkage (which starch has) because the 
amylase enzyme is shaped properly to fit the shape of the 1-4 alpha 
linkage (lock and key description of the enzyme/substrate specificity).  
Amylase can thus promote the hydrolysis.  This enzyme cannot similarly fit 
the beta linkage (different shape) present in cellulose in order to assist 
in its hydrolysis.

2. Cellulose is not branched; it is composed of long chains of glucose 
units all held together by 1-4 beta linkages.  Starch on the other hand is 
more complicated, because it can be composed of amylose and amylopectin.  
Amylose is mostly unbranched.  Amtylopectin is more branched.  The 
branches occur because of 1-6 linkages.  I understand that amylopectin is 
harder to digest because of the branching.  This is again the result of 
the fact that the shape at the 1-6 linkage is inappropriate for a good fit 
to amylase.  Amylase can assist with the hydrolysis at the 1-4 alpha links 
in amylopectin but can't assist in the hydrolysis at the 1-6 branching 
points.  I know that glycogen is branched and easily hydrolyzed to glucose,
but I do not know what the details are of that process*.

So far what I have said is more about shape than "rigidity" of bonds. In 
any case, I would not say that because starch is more highly branched it 
is more easily degraded.  In fact, it is the unbranched part that is most 
easily degraded

3. There is an element of rigidity of the molecules that does come into 
play.  Because of the beta linkage in cellulose, there is possible some 
intramolecular hydrogen bonding that seems to keep adjacent glucose units 
aligned along the same line and gives the LARGE cellulose molecules a 
linear character that might be rather rigid.  They can then easily pack 
against one another and hydrogen bond to one another to form cellulose 
fibers.  This tight attraction tends to keep cellulose from dissolving in 
water.  This is another minor part of the reason why digestion of 
cellulose would be hard for us.

On the other hand, the alpha linkage of amylose allows for more 
flexibility.  Amylose is not well ordered to be crystalline or fibrous, 
but it is amorphous.  Amylose dissolves in water because the 
intermolecular attractions are weaker.  Each molecule seems to form a 
helix in water solution (some intramolecular hydrogen bonding here).  It 
is theoried that the dark color of iodine with starch comes from the 
iodine fitting snugly inside the starch helix in solution.

The branching in amylopectin seems to prevent it from taking up this nice 
aggangement in water, and thus it is not as soluble.

For more basic information on all of this, you can consult most any 
current undergraduate organic chemistry text.  For more information on the 
advanced aspects consult "Biochemistry" by Geoffrey Zubay.  I have the 
second edition (copyright 1988) and this material is found on pages 138 to 
144.  I hope that the same material would be in any later editions.

Dr. Jerry Franzen
Chemisry Department
Thomas More College
333 Thomas More Parkway
Crestview Hills, KY 41017

*Admin note:
The alpha 1-6 linkage of glycogen and starches is broken down by 
"debranching
enzyme".