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Q: Biochemistry ( Answered 5 out of 5 stars,   1 Comment )
Subject: Biochemistry
Category: Science > Chemistry
Asked by: jackieblackie-ga
List Price: $2.00
Posted: 19 Aug 2002 03:45 PDT
Expires: 18 Sep 2002 03:45 PDT
Question ID: 56149
Explain how the formation and stabilisation of the structure of a
globular protein is similar to the formation of membrane lipid
Subject: Re: Biochemistry
Answered By: historybuff-ga on 19 Aug 2002 13:01 PDT
Rated:5 out of 5 stars
Note: Part of this answer related to globular protein appears in the
comments section below.

Both use the hydrophobic effect.  Lipids have one end that avoids
water and the other that doesn't.  When a bunch of lipids get
together, they line up with their water-avoiding ends together. 
Imagine a large field of corn, with rows and corn stalks.  Now mirror
that vertically, so that you have the identical scene repeated upside
down; half the corn is growing up and half the corn is growing down. 
That's how lipids arrange themselves into bilayers to form membranes. 
Children's Hospital of Oakland explains, "Often a happy solution for
phospholipids is a double layer with the hydrocarbon chains
sequestered away from the water in the interior of the bilayer , while
the polar phospholipid head groups maintain contact with the aqueous
phase."  They describe the shape of proteins by saying, "Similarly,
protein amino acid regions with hydrophobic side chains sequester away
from water while polar amino acid side chains are exposed to the
aqueous phase."
They also have a good diagram:

Another explanation says, "Each  represents a phospholipid. The
circle, or head, is the negatively charged phosphate group and the two
tails are the two highly hydrophobic fatty acid chains of the
phospholipid. Due to their thermodynamic properties they spontaneously
form a double layer in an aqueous (watery) environment."  Also has
good diagrams:

For a more technical explanation, see "General Principles of Membrane
Protein Folding and Stability" from the Steven White Laboratory at UC
Irvine.  The page also provides links more details on lipid layer

So to put it simply, globular proteins and lipid bilayers both depend
on the hydrophobic properties of their molecular components.  A
globular protein attains a 3D shape that is driven by the hydrophobic
properties of areas along its length, as they try to "hide" from the
aqueous surroundings.  Lipids line up in two mirror image planes
because it is the most efficient way to keep all the hydrophobic ends
away from the aqueous surroundings.

My answer here along with my comment in the comments section below
provides the complete answer.  I hope my visual examples have helped
and not offended in their simplicity.


jackieblackie-ga rated this answer:5 out of 5 stars
Great. Really helped. Very indepth and links given.

Subject: Re: Biochemistry
From: historybuff-ga on 19 Aug 2002 11:37 PDT
The similarity is polarity.  Lipid layers and the structure of
globular protein both use polarity and hydrophobic properties to
arrange themselves correctly.

"Regions of protein such as alpha-helix and beta-sheet combine to form
larger assemblies, either within one protein chain (tertiary
structure, as one large molecule folds back on itself) or between
chains (quaternary structure, where two or more molecules cluster
together). Particularly with enzymes, tertiary and quaternary
structure bring functional groups on one segment of a protein into
close proximity to functional groups on another segment. This
proximity is essential to the role of an enzyme as a catalyst.

The interactions which hold tertiary and quaternary structure together
are all non-covalent, and are much weaker than covalent bonds.
Hydrogen bonds are important here, as they are in secondary structure.
In addition, two other interactions are significant:

Hydrophobic "bonds"

Since water molecules cling together through strong hydrogen bonds,
non-polar regions of proteins are excluded from interaction with water
and thus are forced together in "hydrophobic" (water-fearing) regions.
They are effectively weakly "bonded" in this way. As you might expect,
the amino acids in hydrophobic regions are mostly those whose R groups
are hydrocarbon groups."

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