Proteins that span hydrophobic lipid bolsters are often alpha-helical. why is an

Question

Proteins that span hydrophobic lipid bolsters are often alpha-helical. why is an alpha-helix particularly well suited to accomplish this, as supposed to, say, a beta-sheet or loop? Proteins that span hydrophobic lipid bolsters are often alpha-helical. why is an alpha-helix particularly well suited to accomplish this, as supposed to, say, a beta-sheet or loop? Proteins that span hydrophobic lipid bolsters are often alpha-helical. why is an alpha-helix particularly well suited to accomplish this, as supposed to, say, a beta-sheet or loop?

Explanation / Answer

Two types of proteins exist along with the membrane. They are peripheral or extrinsic, integral or intrinsic proteins. Membranes are bilayered lipid structures which give flexibility, rigidity and support for the cell and cell organelles.

Extrinsic proteins are present along and outside the lipid layer, and the intrinsic are embedded within the membrane. Peripheral membrane proteins are attached to the membranes by electrostatic and hydrogen-bonds with the head groups which are polar in nature. These interactions can be broken down by salting or changing the pH.

Intrinsic proteins are very difficult to extract and are not water soluble like peripheral proteins. Because these proteins interact with the non polar groups of the lipid bilayer, and hence require detergents for solubilisation.

Many of the integral proteins are made up of helices to span the bilayer. Because the alpha helices surfaces always face the hydrophobic groups in the membrane. And these integral proteins also need to interact with the hydrophobic side chains. As such the alpha helices cannot be disrupted easily by water and hence no protein unfolding. That’s the reason why most of the integral proteins are formed of alpha helices and majority of proton pumps are made up of alpha helices.

And also it is very difficult for any integral protein to get embedded into the lipid layer. It is due to the thermodynamic problem faced by the polypeptide chain. Only those protein molecules which have the hydrogen bonding potential will get embedded in the lipid bilayer. And alpha helix is the only structural component which fulfils the hydrogen binding capacity. Hence the integral proteins which span the hydrophobic groups of the lipid bilayer are always made of alpha helix.

References:

1. https://www.ncbi.nlm.nih.gov/books/NBK22495/

2. https://www.ncbi.nlm.nih.gov/books/NBK21570/

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