Structure of PHB

The molecular structure of PHB/CapolyP channel complexes remains uncertain however, some assumptions can be made concerning the general organization of the complexes from the physical properties and sizes of the polymers, and the low... 

Figure 20. Diagrammatic representation of the PHB/polyP channel structure. The central cylinder represents the polyP helix, which contains pairs of closely spaced monovalent negative charges that provide a ladder of binding sites for Ca2+. A. The Ca(polyP) is surrounded and solvated by an exolipophilic-endopolarophilic helix of PHB.28 B. The Ca(polyP) is surrounded and solvated by a 0-sheet-like structure of PHB similar to the lamellar structure of solid PHB.28...

More recently, we succeeded in revealing the crystal structure of PHB depolymerase from Penicillium funiculosum (Hisano et al. 2006). The trimer substrate of (/ )-3-hydroxybutyrate with a planar zigzag conformation was perfectly bound in a crevice of the active site. This result suggests that the P-structure (planar zigzag conformation) is degraded faster than the a-structure (2/1 heUx conformation), which was obtained from the enzymatic degradation of P(3HB) fibers with two kinds of molecular conformations. These results indicate that the rate of enzymatic degradation can be controlled by the molecular conformations of polymers. 

The crystal structure of PHB is a orthorhombic form with unit cell parameters fl = 0.576 nm, = 1.320 nm, and c(fiber axis)=0.596 nm, and space group P2,2,2, (Alper et al. 1963 Okamura and Marchessault 1967). The conformational analysis by intermolecular energy calculation has indicated that the PHB molecule has a left-handed 2j helix conformation (Comibert and Marchessault 1972 Yokouchi et al. 1973 Bruckner et al. 1988). The crystal structure of random copolymers of 3HB and 3HV has been investigated extensively (Bloembergen et al. 1986 Kamiya et al. 1991 VanderHart et al. 1995). A structural characteristic of poly(3HB-co-3HV) is isodimorphism, i.e., cocrystallization, of the two monomer units. In contrast, the cocrystallization of 3HB with 3HH or (7 )-6-hydroxyhexanoate (6HH) does not occur. 

The crystalline structure of PHB is orthorhombic with dimensions of the basic crystalline cell a = 5.76 A, b = 13.20 A, c = 5.96 A. A partially planar zig-zag structure can be formed as a result of the mechanical uniaxial load from the amorphous phase between lamellae [117, 118]. PHB forms extremely thin lamellar crystals, with a thickness comprised between 4 and 7 nm and a prevailing size of 5 nm [98]. 

PHAs include a family of polyesters with different side groups and different numbers of carbon atoms in the repeating units (Fig. 7.4). The most studied PHAs are poly(3-hydroxybutyrate) (PHB) and its copolymer PHBV. The homopolymer PHB is a highly crystalline thermoplastic with a around 175°C. It possesses several physical properties, for example, T, r (15°C), crystallinity (80%), and tensile strength (40 MPa), similar to those of PR However, PHB is significantly more brittle than PP (strain at break 6% vs. 400%). With the introduction of 3-hydroxyvalerate (3HV) units to PHB, the regular structure of PHB is disrupted and therefore its crystallinity, crystallization rate, T, and T decrease as the content of HV... 

The fine powdered PHB was provided by Biomer Co (Krailling, Germany). The chemical structure of PHB structure is well established in the literature and the viscosity average molecular weight Mw = 2.5 x 10 was determined by intrinsic viscosimetry in chloroform solution. The granulated LDPE is commercial product of SAFCT Russian designated standard (15803-020) with following characteristics Mw = 2.5 xio and specific density 0.92 g/cm. ... 

Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]...

Figure 20.1 The chemical structures of the materials used in this study, i.e. the liquid crystal copolymer (PHB-PET), PEN and PET IV, intrinsic viscosity...

Fig. 4. The molecular structures of some aliphatic polyesters a) poly(glycolide), PGA b) poly(lactide), PLA c) poly(lactide-co-glycolide), PLGA d) poly(e-caprolactone), PCL e) poly(hydroxybutyrate), PHB and f) poly(hydroxybutyrate-co-hydroxyvalerate), P(HB-co-HV)...

Figure 4.2 Models of the PolyP-PHB channel structure as proposed by Seebach and co-workers (from Reusch, 1999a). (a) The tube-like arrangement of PHB helices, where neighbouring helices form a cylinder surrounding the Ca2+-PolyP complex (Seebach et al., 1994a, 1996). (b) The central cylinder represents the PolyP helix, which contains binding sites for Ca2+. The Ca2+-PolyP complex is surrounded by and solvated by an outer cylinder composed of a /i-like sheet of PHB (Das et al., 1997).

Figure 7.2. Chemical structures of poly-3-hydroxybutyrate (PHB A), PHB-co-3-hydroxyvalerate (PHB/V B), PHB-co-4-hydroxybutyrate (PHB/4HB C), Medium-chain-length polyhydroxyalkanoate (mcZ-PHA D), NODAX (E). Note The Rx side chain is generally composed of a straight chain aliphatic hydrocarbon from 3 to 11 carbons in length (mc/-PI IA) or 3, 5,7 or 9 carbons in length (NODAX ).

It may not be possible to resolve the structure of the complexes with certainty. In the Reusch model, the complexes have the liquidic properties of polymer electrolytes and this suggests a family of conformations rather than a single defined structure. In the Seebach model, several PHB molecules are involved in surrounding polyP. The individual PHB chains are free to adopt various positions in the phospholipid lattice hence, a well-defined structure is again unlikely. Nonetheless, further studies may help to decide between the two views of the arrangement of PHB molecules in the complexes and delineate the more probable conformations. 

The presence of PHB and polyP in a human calcium pump indicates that the role of these polymers in ion transport has been conserved, and further suggests that the polymers are likely constituents of other ion channels and pumps. Recently, the potassium channel of the Gram-positive soil bacterium, Streptomyces lividans (KcsA), attracted great interest as the first ion channel to have its structure analyzed... 

Simplicity of structure and ease of formation makes it reasonable to infer that polyP and PHB were components of early cells, possibly preceding RNA. PolyPs are prebiotic molecules, formed by condensation of phosphates in volcanic condensates and thermal vents at the bottom of the ocean.2,34,146 The synthesis of PHB is more demanding but requires only acetate and a reducing agent (see Figure 2B), both available in the primordial milieu. The conservation of these rudimentary homopoly-... 

The recent discoveries of PHB and polyP in a human calcium pump and bacterial potassium channel suggest that the naked PHB/polyP complexes found in bacteria are progenitors of protein ion transporters. The process by which protein channels and pumps may have evolved from PHB/polyP complexes is unknown however, one may surmise that over time proteins surrounded the complexes to support and regulate their activity. At first, the association may have been nonco-valent, but subsequently PHB may have become tethered to the protein by a covalent bond. By this view, many of the channels and pumps of prokaryotes and eukaryotes may be supramolecular structures in which protein, polyP, and PHB join together for efficient regulation of transmembrane ion transport. 

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