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b-keratin



In  an previous post:John Jackson asked a series of questions regarding Beta
Keratin. I will try to answer each in tern with a few comments added:
-----Original Message-----
From: John V Jackson <johnvjackson@vorpent.screaming.net>
>
>A question not intimately connected with this but which I have always
wanted
>to ask Alan concerns keratin B.
>
>It is said that it contains more sulfur than the mammalian equivalent.

Technically, there is no exact equivalent. Birds, reptiles and mammals all
produce Alpha-keratins. But only reptiles and birds produce B-keratins. They
differ greatly in their structure and at both the proteins sequence and gene
levels. They do share some physical characteristics and all contain Cys, one
of the amino acids that contain S. The B-keratin that in feathers.
claws,beaks and scales of birds differs from that in the skin and scale of
reptiles. These differences caused me to use the term feather (or
phi)-keratin. They are made (in birds) by a family of genes specific to
birds, and although the primary monomers in feathers differ in size from
those in the other tissue, we know precisely that this is due to the
presence of a specific tri-peptide always located in the same place in the
sequence.

Because of the difference in the protein structure, A-keratins form an alpha
helix and the feather proteins a B-pleated sheets. Consequently they form
structurally different filaments (all keratins seem to have the capacity to
filament). There are structural consequences of this, but the filaments are
bound by s-s links in just about all cases.



>Sulfur sometimes provides extra cross-links between large polymer molecules
>and makes the substance stiffer
This is true. More exactly the links provide a type of crystallinity. This
is studies thru x-ray diffraction patterns. The pattern in fathers is the
most complex of any biological material.

Stiffness is not
>particularly useful in fur where flexibility and resistance to creasing is
>an advantage, but it obviously is in feathers.  Do we know if keratin B is
>intrinsically stiffer than the type mammals have?

That is not the typical comparison. There is,however, a small literature on
the mechanical properties of feathers (stiffness, bending etc). The
crystalline organization of the fibers does provide internal support in
feathers.
>
>It would be interesting to speculate (yes, still an essential part of
>science!) whether birds developed a stiffer version in the process of
>developing feathers for flying, or whether intrinsically stiffer
>integumentary fibres predisposed (oh!) birds for that style of flight,
where
>other vert groups used stretched skin.

All the variations in feather structure that have been investigated at this
level indicate that the filamenting process is independent of the a-a
sequence difference of the feather monomers (there are about 20 genes for
feather keratin that occur in tandem on the chromosome), and the various
combinations that form the filament. This is different from the situation in
A-keratin. There is no evidence that the internal structures of primary
feathers is stiffer than other feathers. Most of the stiffness results from
the interactions of the gross structure (e.g. barbules, etc).
>
>The question of differences in flight style (above) is not relevant as the
wings of bat for example would be made of A-keratin and the feathers are
B-keratin.

Hope this clears things a bit.

           Alan H. Brush
              92 High St.
       Mystic, CT 06355

   brush@uconn.cted.edu
          860-572-1717