Keratins

By Jaclyn Udell and Izaak Earnhardt

Keratins are fibrous proteins which compose the structures and large portions of the cell compositions of living organisms. There are two primary groups of keratins, the alpha-keratins and the beta-keratins. While both fulfill similar roles, they differ slightly in structure, compositions and properties. The alpha-keratins are slightly basic or neutral and form a right-handed helical structure and the beta-keratins are slightly acidic and also form a right handed helical structure.

Composition and Structure

Keratins are composed of amino acids, primarily the amino acids glycine and alanine. Given the wide variety of keratins, both alpha and beta, no single chemical composition or monomer could be defined. There are over 20 Amino Acids that make up keratins. Amino Acids are the building blocks for proteins. Without them keratins would not be formed. Cysteine can account for up to 24% of the keratin structure. Despite their slight variance in composition, the keratins all share a common characteristic: their helical structure.

Individual alpha- and beta- keratin helices are the building blocks of the keratin structures that living organisms rely upon. The primary structures (amino acid chains) form the back bone of the secondary structure: the helix.
a-helix.jpg

Keratins, as fibrous proteins
, are elongated molecules in which the secondary structure forms the dominant structure. In their tertiary forms, they are mashed into large clumps held together with bridges to hold them together. When in those clumps, they form structures like the one shown below with myoglobin.
external image myoglobin.jpg


These proteins are what make up nails, hair, make up, hooves, and skin. They are essential to everything from fish, to human lives proving their depth and neccesity to organisms.
keratins are coiled.

Monomer Units

Glycine

external image glycine.jpg


Alanine Amino Acid

external image Amino-Acid-MCAT.png
Cysteine: relatively 24% of a keratin compound
external image cysteine.jpg

Bonding of the Composing Monomers
The amino acids are strung together forming dehydration synthesis. The hydroxyl group and hydrogen group break away from their respective amino acids to form an amino acid chain and a water molecule. the

Cross Linkage
Cross-linkage occurs with all the keratins. The cross-linkage occurs with hydrogen bonds formed between oxygen and hydrogen. It is this cross-linkage that allows layers upon layers of these chains to bond together in sheets, giving the structure their strength. It is those hydrogen bonds which hold the helix together and help to give it its shape.

external image fibroin.jpg

Properties
While the structures and compositions of keratins vary, the majority share a common set of properties.
Physical Properties:

  • water soluble
  • Hard structures
Chemical Properties:
  • Relatively non-reactive with oxygen
    • flammable if heat is applied
  • Reactive with acids

Roles and Uses
The uses of keratins vary greatly and their uses many times coincide with their characteristics. Beta-keratins tend to be harder and more rigid than alpha-keratins. Consequently, beta-keratins tend to make up harder structures like scales, feathers or claws while alpha-keratins tend to make up necessary cellular structures in mammals like hair (including furs and wools), skin and hooves. Beta-keratin structures are primarily composed of beta-sheets, but beta sheets can also be found in alpha-keratin structures. The quantity of the keratins and proportional occurrence of disulfide bridges in the overall structures can affect the rigidity of the substance. A greater quantity of keratin structures, for instance the epidermis, can result in a thicker, stronger layer and substance. Additionally, the more tightly bridged (with disulfide bridges) the keratin structure is, the more rigid it will be. That is why, hair, with fewer disulfide bridges, is less rigid than structures like fingernails and other structures that have significantly more disulfide bridges.


MLA Citations


Morrison, Thronton, Robert, and Robert Neilson Boyd. Organic Chemistry. Allyn and Bacon, Inc, 1969.

"Keratin." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 15 May 2008. <http://www.britannica.com/eb/article-9045143>.
Wikepedia Reference of Science Dail. "Keratin." Science Daily. 14 May. 2008. <http://www.sciencedaily.com/articles/k/keratin.htm>

Sandwalk: strolling with a Skeptical Biochemist. Larry Moran. Professor in the Department of Biochemistry at the University of Toronto. 15 May. 2008.

Photo."Fibrous Proteins." 15 May. 2008 <http://content.answers.com/main/content/img/McGrawHill/Encyclopedia/images.

Nelson D.L and Cox M.M. Photo. "Tertiary Structure of sperm whale myoglobin." 15 May. 2008 <http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit1/media/myoglobin.jpg>.<span style="FONT-SIZE: 14pt">

Photo. "Cysteine". 19 May. 2008 <http://www.benbest.com/nutrceut/cysteine.jpg>


Links

http://www.britannica.com/eb/article-9045143
http://www.sciencedaily.com/articles/k/keratin.htm
http://content.answers.com/main/content/img/McGrawHill/Encyclopedia/images

http://www.benbest.com/nutrceut/cysteine.jpg
<http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit1/media/myoglobin.jpg.>
sandwalk.blogspot.com/2008/03/helix.html