Biology Reference
In-Depth Information
voila
e
the
a
-helix was born! A single span
a
-helix integral protein, glycophorin a, was dis-
cussed above. More common than the single span proteins are the multiple span
a
-helix
proteins. Of particular importance is a very large family of proteins that have 7
membrane-spanning
a
-helices. These proteins have been referred to as the 'magnificent
seven' due to their biochemical importance
[21]
. Examples of proteins containing the 7
a
-helix motif include the large family of G-protein-coupled receptors (GPCR) including
the visual receptor rhodopsin and the olfactory receptor, and a variety of channels
including voltage gated potassium channels, mechanosensitive channels, aquaporin, chlo-
ride channels, and polysaccharide transporters. Included in the G-protein coupled 7
a
-helix
family are receptors for most of the important hormones and signaling molecules in man
including
g
-aminobutyric acid (GABA), adenosine, bradykinin, opioid peptides, somato-
statin, vasopressin, dopamine, epinephrine, histamine, glucagons, acetylcholine, serotonin,
prostaglandins, platelet activating factor, leukotrienes, calcitonin and follicle stimulating
hormone (FSH). The 7
a
-helix motif is so important that it has been estimated that more
than half of all commercial pharmaceuticals are modulators of the GPCR.
The best studied of the 7
-helix motif proteins is bacteriorhodopsin, a product of the salt
loving archaea
Halobacterium
[22]
. Under anaerobic conditions this bacteria switches its
metabolism to produce enormous quantities of bacteriorhodopsin, a 7
a
-helix trans-
membrane protein that accumulates in two-dimensional crystalline patches known as
a 'purple membrane'. The 'purple membrane' may occupy up to 50% of the cell surface.
This highly unusual feature allows for the easy isolation and crystallization of bacteriorho-
dopsin. Bacteriorhodopsin's function is bioenergetic. Under anaerobic conditions, it captures
light to generate a trans-membrane proton gradient that is used to drive ATP synthesis. The
light absorbing entity is a retinal connected by a Schiff base to a lysine buried deep in the
membrane hydrophobic interior.
The crystallographic structure of bacteriorhodopsin has been determined to 1.55
˚
resolu-
tion
[23]
. The protein was shown to have seven trans-membrane
a
-helices accounting for 80%
of the protein's mass. The helices are connected by short, extra-membrane, non-helical loops
(
Figure 6.8a and 6.8
b)
[25]
. In agreement with the Kyte Doolittle hydropathy plot analysis
(
Figure 6.8a and 6.8
b)
[24]
, each
a
-helix is composed of about 20 amino acids, the vast
majority of which are hydrophobic. However, there are several amino acids found in the
a
-helices that would not be expected. These include lysine-216 that binds to the retinal, 2
prolines that normally terminate
a
-helices, as well as the charged amino acids lysine, argi-
nine, aspartic acid and glutamic acid. Since each of the 7
a
-helices has at least one of these
unfavorable amino acids, application of a Kyte Doolittle hydropathy plot
without
employing
a rolling average to smooth the curves would not detect any 20 or more amino acid
a
-helix
spans in the protein. The 7
a
-helices are clustered together and are oriented a little off perpen-
dicular to the membrane plane. Since this structural motif is typical of the entire family of
GPCRs, bacteriorhodopsin was then used to model the other 7
a
-helix family members until
some additional crystallography data finally became available in 2007.
a
TYPE III. MULTIPLE TRANS-MEMBRANE SPANS BY
-BARRELS
The second major type of trans-membrane structure is the
b
-barrel
[26]
. This structure is
stabilized inside the membrane by the same hydrophobic forces that stabilize
a
-helix integral
proteins and, furthermore, the protein's folding is facilitated by water-soluble chaperones.
b
