3.4.1
Peptitergents
The substitution of amphipathic peptides to apolipoprotein A to assemble lipoprotein particles was
studied in the 1980s by Jere Segrest and collaborators (Anantharamaiah et al. 1985, 1990; Chung et al.
1985), but it seems that it is only in the 1990s that their application as substitutes to detergents for
stabilizing MPs in aqueous solutions was first reported. Robert A. Stroud and his colleagues
synthesized by solid-phase organic chemistry 24-residue peptides, which they called “peptitergents,”
designed to fold in water as laterally amphipathic helices (Table 3.2, Line 1). Peptitergent PD 1 was
shown by circular dichroism and X-ray crystallography to indeed fold into highly water-soluble αhelices, which, upon crystallization, assembled into antiparallel helix bundles (Schafmeister et al.
1993). PD 1 was added to purified detergent solutions of two α-helical MPs, BR (in nonyl-glucoside
solution) and rhodopsin (in lauryldimethylamine oxide solution; LDAO), and a β-barrel one, PhoE
porin (in OG), while diluting the detergent to <1/20 of its CMC. Under such conditions, PhoE
precipitated completely, whereas ~85% of BR and ~60% of rhodopsin remained in solution under
their native form (as ascertained from their UV-visible spectra) over a period of 2 days. Subsequent
studies showed that PD 1 was efficient neither at directly extracting MPs from their native environment
(Soomets et al. 1997; Bavec et al. 1999) nor at keeping rat cerebral cortical Na
+ /K
+
-exchanging
ATPase water-soluble (Soomets et al. 1997). An original variation on the original strategy has been
described, in which PD 1 was substituted, by genetic engineering, to the native TM hydrophobic helix
of a P450 cytochrome, yielding a water-soluble enzyme suitable for
1 H-NMR investigations (Schoch
et al. 2003).
Peptitergents were initially developed with the view of applying them to MP crystallization
(Schafmeister et al. 1993). This was a long shot, however, because the formation of well-ordered
crystals of MP/peptitergent complexes would require that all complexes have strictly the same
arrangement. No such crystals have been reported, and the study of peptitergents seems to have
stopped.
3.4.2
Lipopeptide Detergents
Peptitergents provided a starting point for the development by Gilbert G. Privé and colleagues of a next
generation of peptide-based surfactants called lipopeptide detergents (LPDs; Table 3.2, Line 2). In
LPDs, each 24-residue peptide is endowed with two fatty acyl chains, 12 to 20 carbon long, one bound
to the N- and the other to the C-terminus (McGregor et al. 2003) (Fig. 3.16A). The rationale is that the
acyl chains ought to preferentially interact with the hydrophobic face of the α-helix formed by the
peptide, providing a flexible, softer, more versatile surface for associating with the TM surface of target
MPs. An additional potential bonus is that the structure of LPDs is expected to favor a MP/LPD
arrangement in which the acyl chains align more or less with the protein’s TM axis (Fig. 3.16C), much
as lipid acyl chains do in membranes (Chap. 1). In aqueous solutions, LPDs do form α-helices, which,
according to both MD simulations (Kelly et al. 2005) and crystallographic data (Ho et al. 2008),
assemble into bundles in which the alkyl chains run roughly parallel to helix axes and form the
hydrophobic core of the bundle (Fig. 3.16B). LPDs can solubilize lipid vesicles (McGregor et al.
2003). They thus seem more detersive than PDs, but whether they can directly extract MPs from
biological membranes has not been reported. Because they are expensive to produce, their
recommended use is, rather, to trap and stabilize MPs that have been solubilized and purified in
detergent solution. This has been demonstrated for a variety of MPs from both the α-helical and
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3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions
Peptitergents
The substitution of amphipathic peptides to apolipoprotein A to assemble lipoprotein particles was
studied in the 1980s by Jere Segrest and collaborators (Anantharamaiah et al. 1985, 1990; Chung et al.
1985), but it seems that it is only in the 1990s that their application as substitutes to detergents for
stabilizing MPs in aqueous solutions was first reported. Robert A. Stroud and his colleagues
synthesized by solid-phase organic chemistry 24-residue peptides, which they called “peptitergents,”
designed to fold in water as laterally amphipathic helices (Table 3.2, Line 1). Peptitergent PD 1 was
shown by circular dichroism and X-ray crystallography to indeed fold into highly water-soluble αhelices, which, upon crystallization, assembled into antiparallel helix bundles (Schafmeister et al.
1993). PD 1 was added to purified detergent solutions of two α-helical MPs, BR (in nonyl-glucoside
solution) and rhodopsin (in lauryldimethylamine oxide solution; LDAO), and a β-barrel one, PhoE
porin (in OG), while diluting the detergent to <1/20 of its CMC. Under such conditions, PhoE
precipitated completely, whereas ~85% of BR and ~60% of rhodopsin remained in solution under
their native form (as ascertained from their UV-visible spectra) over a period of 2 days. Subsequent
studies showed that PD 1 was efficient neither at directly extracting MPs from their native environment
(Soomets et al. 1997; Bavec et al. 1999) nor at keeping rat cerebral cortical Na
+ /K
+
-exchanging
ATPase water-soluble (Soomets et al. 1997). An original variation on the original strategy has been
described, in which PD 1 was substituted, by genetic engineering, to the native TM hydrophobic helix
of a P450 cytochrome, yielding a water-soluble enzyme suitable for
1 H-NMR investigations (Schoch
et al. 2003).
Peptitergents were initially developed with the view of applying them to MP crystallization
(Schafmeister et al. 1993). This was a long shot, however, because the formation of well-ordered
crystals of MP/peptitergent complexes would require that all complexes have strictly the same
arrangement. No such crystals have been reported, and the study of peptitergents seems to have
stopped.
3.4.2
Lipopeptide Detergents
Peptitergents provided a starting point for the development by Gilbert G. Privé and colleagues of a next
generation of peptide-based surfactants called lipopeptide detergents (LPDs; Table 3.2, Line 2). In
LPDs, each 24-residue peptide is endowed with two fatty acyl chains, 12 to 20 carbon long, one bound
to the N- and the other to the C-terminus (McGregor et al. 2003) (Fig. 3.16A). The rationale is that the
acyl chains ought to preferentially interact with the hydrophobic face of the α-helix formed by the
peptide, providing a flexible, softer, more versatile surface for associating with the TM surface of target
MPs. An additional potential bonus is that the structure of LPDs is expected to favor a MP/LPD
arrangement in which the acyl chains align more or less with the protein’s TM axis (Fig. 3.16C), much
as lipid acyl chains do in membranes (Chap. 1). In aqueous solutions, LPDs do form α-helices, which,
according to both MD simulations (Kelly et al. 2005) and crystallographic data (Ho et al. 2008),
assemble into bundles in which the alkyl chains run roughly parallel to helix axes and form the
hydrophobic core of the bundle (Fig. 3.16B). LPDs can solubilize lipid vesicles (McGregor et al.
2003). They thus seem more detersive than PDs, but whether they can directly extract MPs from
biological membranes has not been reported. Because they are expensive to produce, their
recommended use is, rather, to trap and stabilize MPs that have been solubilized and purified in
detergent solution. This has been demonstrated for a variety of MPs from both the α-helical and
120
3 Alternatives to Detergents for Handling Membrane Proteins in Aqueous Solutions