|MadSci Network: Biochemistry|
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".
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