1.D.24 The Marine Sponge Polytheonamide B (pThB) Family
Polytheonamide B, from the marine sponge, Theonella swinhoei, forms a β-helix that is stable in membranes (Oiki et al. 1997; Hamada et al. 2005). It forms channels with cation selectivity: H+ > Cs+ > Rb+ > K+ > Na+ (Oiki et al. 1997). Voltage-dependent transitions between brief openings and long closures were observed (Iwamoto et al. 2010; Matsuoka et al. 2011). It has been totally synthesized, and structural permutations have been designed (Ducho 2010; Inoue et al. 2010; Inoue 2011). Derivatives have been characterized with respect to their channel activities (Itoh et al. 2012; Shinohara et al. 2012). The vectorial insertion of polytheonamide B into the membrane has been studied (Kalathingal et al. 2021). This family had previously been described under TC#s 1.D.33 and 1.D.105.
Polytheonamide B (pTB), a highly cytotoxic polypeptide, is one of the most unusual nonribosomal peptides of sponge origin. pTB is a linear 48-residue peptide with alternating D- and L-amino acids and contains a total of eight types of nonproteinogenic amino acids. Hamada et al. 2010 determined the three-dimensional structure of pTB by NMR spectroscopy, structure calculation, and energy minimization. pTB adopts a single right-handed β(6.3)-helical structure in a 1:1 mixture of methanol/chloroform with a length of approximately 45 Å and a hydrophilic pore of ca. 4 Å inner diameter. These features indicate that pTB molecules form transmembrane channels that permeate monovalent cations as gramicidin A channels do. The strong cytotoxicity of pTB can be ascribed to its ability to form single molecule channels through biological membranes.
A cytotoxic peptide, polytheonamide B (pTB or pThB), from a symbiotic bacterium in the marine sponge, Theonella swinhoei, has been examined for the cytotoxic spectrum and specific activity to mammalian cells (Iwamoto et al., 2010). pTB is composed of alternative D- and L-amino acid residues throughout the 48-mer peptide. This suggests the formation of a β-helix similar to gramicidin channels. Planar bilayer experiments revealed that pTB forms monovalent cation-selective channels, being compatible with the inner pore diameter of 4Å for a β-helical structure. pTB penetrated vectorially into the membrane, formed a channel by means of a single molecule, and remained in the membrane. These functional properties may account for specific cytotoxic activity (Iwamoto et al., 2010). Functional analysis of synthetic substructures of polytheonamide B have been reported (Matsuoka et al., 2011).
pTB has more than half of its residues posttranslationally modified. Epimerization reactions result in alternating L- and D-amino acids that allow the peptide to adopt a helical conformation. Other posttranslational modifications include side chain hydroxylations and C- and N-methylations. N-methylations appear to be crucial for stability in a polar environment. They are the driving force for the formation of stable side chain hydrogen-bond chains that act as an 'exoskeleton' (Renevey and Riniker 2016).
Polytheonamide B (PThB) is a unique peptide natural product with an extremely complex structure, a channel-forming ability in vitro, and an extremely potent cytotoxicity (Hayata et al. 2018). The 48-mer sequence of PThB consists of alternating d,l-amino acids and possesses an array of sterically bulky beta-tetrasubstituted and hydrogen bond forming residues. These unusual structural features are believed to drive it to fold into a 4.5 nm long tube, form a transmembrane ion channel in plasma membranes, and exert cytotoxicity. Synthetic PThB was utilized for analysis of its cellular behavior. Reflecting its ion-channel function, its addition to cells rapidly diminished the potential across the plasma membrane, and it was also internalized into the cells, accumulating in acidic lysosomes, neutralizing the lysosomal pH gradient. Thus, it is capable of exerting two functions upon causing apoptotic cell death of mammalian cells: It induces free cation transport across the plasma as well as lysosomal membranes (Hayata et al. 2018).
The generalized reaction catalyzed by pTB is:
Monovalent cation (in) ⇌ Monovalent cation (out)