Biochemistry: Alpha - k eratin is a very common structural protein that makes up
ID: 162958 • Letter: B
Question
Biochemistry: Alpha - k eratin is a very common structural protein that makes up our nails, hair, and in other species , makes up their horns and claws. The basis of the - keratin structure is a coiled coil.
a) What is the handedness of the individual - helixes , and how does that handedness compare to the overall handedness of the coiled - coil ?
b) What pattern distinguishes coiled - coil sequences, and which positions in that pattern are important for stabilizing the coiled - coil structure?
c) What predominant type of non - covalent interaction holds the coils together?
Explanation / Answer
What is the handedness of the individual - helixes , and how does that handedness compare to the overall handedness of the coiled - coil ?
Handedness of the individual – helixes is Right handed, when it is viewed along the helix axis and the handedness of alpha keratin is RIGHT-handed and the two helices are found to be twisted around each other in a LEFT-handed manner.
What pattern distinguishes coiled - coil sequences, and which positions in that pattern are important for stabilizing the coiled - coil structure?
Here in this coiled coils, we can see repeated pattern, hxxhcxc, of hydrophobic (h) and charged (c) amino-acid residues, which forms the heptad repeat. And this is found to be labelled as abcdefg ( a and d- hydrophobic positions and these are found to be occupied by isoleucine, leucine, or valine).
a and d positions must be occupied by hydrophobic amino acids (e.g. leucine, valine, or isoleucine). This will helps in stabilizing helix dimerization through hydrophobic and van der Waals interactions.
Residues e and g will always be charged (e.g. glutamate or lysine), so that it can form interhelical electrostatic interactions. There should be opposite charge in heterodimers in order to stabilize their interaction.
Three positions (b, c, and f) should be filled by hydrophilic amino acid so that they can form helical surfaces that are exposed to the solvent.
What predominant type of non - covalent interaction holds the coils together?
The two strands in a coiled-coil are found to be held together by hydrophobic interaction. On disruption of these bonds, these coils will dissociate from each other.
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