MadSci Network: Biochemistry

Re: What is the structure of chitin and how does it help its function?

Date: Thu Dec 15 18:27:50 2005
Posted By: Matt Kinseth, Grad student, Division of Biological Sciences, UCSD
Area of science: Biochemistry
ID: 1134190373.Bc

Hi Elizabeth,

Chitin is a long complex polysaccharide made up of repeating units of the disaccharide acetylglucosamine. This is similar to cellulose (which is made up of glucose) but chitin has different chemical groups coming off the sugar molecules, therefore giving chitin its own unique function.

Like all macromolecules, the structure and function all rely on the initial chemical sequence. If we look at the sugar groups, we start out with simple sugars, like glucose. Glucose is a six carbon ring with oxygens and hydrogens. Disaccharides have two simple sugars linked to each other, like sucrose. Polysaccharides are usually three or more. Chitin is a large polysaccharide but the individual sugar group has a chemical group called an acetylamino. As I said before, chitin is a repeating disaccharide of acetyglucoamine. Breaking this word down you see gluco (which stands for glucose), acetyl (which is a particular chemical group, as below:

the other word is amine, which is a nitrogen (-NH). The acetyl and amine group come together to make up acetylamino that is connected to the second carbon the glucose instead of the hydroxyl group (-OH).

My simple drawing of the acetylamino group:

sugar-N-C=O <--hydrogen bond formed with hydrogen on adjacent sugar group
      | |
      H CH3
It is this group which gives chitin a different property than other large polysaccharides.

If you consider a single polysaccharide like a string, then the structure chitin makes would be like putting strings side by side. What holds the "strings" or chitin molecules together are weak interactions called hygrogen bonds. This comes from the chemical properties of the acetylglucoamine, in particular, the acetylamino group that comes off the sugar group. If this part of the molecule was removed, there would be no way to link the "strings" together to form such a hard and stable structure. Although the hydrogen bonds are weak interactions based on weak positive and negative attractions, there are millions and billions of them that create such a strong force that is very hard to break.

How many acetylglucosamines a single chitin macromolecule has isn't really known as these molecules are regulated by enzymes of the cell. This means there are proteins in the cell responsible for making and breaking chitin. All of this depends on the cellular function and development of the organism that uses chitin.

Functionally, chitin is one of the major components of the cell wall of fungi (yeast) and also makes up the exoskeleton of insects and arthopods.

Consider the exoskeleton of crabs for instance. This large polysaccharide, along with other chemicals, is resonsible for the crabs' defense. It acts as a barrier to all internal organs. If you've ever eaten crab then you know how hard this shell can be to break.

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