Pectates

In the first edition of this book this chapter was entitled "Antiparallel Beta Structures" but we have had to change this because an entirely unexpected structure, the p helix, was discovered in 1993. The p helix, which is not related to the numerous antiparallel p structures discussed so far, was first seen in the bacterial enzyme pectate lyase, the stmcture of which was determined by the group of Frances Jurnak at the University of California, Riverside. Subsequently several other protein structures have been found to contain p helices, including extracellular bacterial proteinases and the bacteriophage P22 tailspike protein. 

A more complex p helix is present in pectate lyase and the bacteriophage P22 tailspike protein. In these p helices each turn of the helix contains three short p strands, each with three to five residues, connected by loop regions. The p helix therefore comprises three parallel p sheets roughly arranged as the three sides of a prism. However, the cross-section of the p helix is not quite triangular because of the arrangement of the p sheets. Two of the sheets are... 

The number of helical turns in these structures is larger than those found so far in two-sheet p helices. The pectate lyase p helix consists of seven complete turns and is 34 A long and 17-27 A in diameter (Figure 5.30) while the p-helix part of the bacteriophage P22 tailspike protein has 13 complete turns. Both these proteins have other stmctural elements in addition to the P-helix moiety. The complete tailspike protein contains three intertwined, identical subunits each with the three-sheet p helix and is about 200 A long and 60 A wide. Six of these trimers are attached to each phage at the base of the icosahedral capsid. 

Figure 5.30 Schematic diagrams of the structure of the enzyme pectate lyase C, which has a three-sheet parallel P-helix topology.

Yoder, M.D., et al. New domain motifs the structure of pectate lyase C, a secreted plant virulence factor. Science 260 1503-1507, 1993. 

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

Fig. 15.—X-ray diffraction pattern from a polycrystalline and well-oriented fiber of sodium pectate (13), diagnostic of 3-fold helix symmetry.

Fig. 16.—Antiparallel packing arrangement of 3-fold sodium pectate (13) helices, (a) Stereo view of two unit cells roughly normal to the fcc-plane. The helix at the center (open bonds) is antiparallel to the two in the front (tilled bonds). Intrachain hydrogen bonds stabilize each helix. Sodium ions (crossed circles) and water molecules (open circles) connect adjacent helices, (b) A view of the unitcell contents down the t -axis highlights the ions and water molecules located between the helices.

Calcium hyaluronate, 376-377 Calcium pectate, 353 Calcium welan, structure, 432-434 Candida utilis, thiamine synthesis,... 

In our investigations, we also detected the sorption of isoPO from potato, Arabidopsis and wheat, by calcium pectate. Moreover, we observed the binding with calcium pectate of potato PO from the fraction of proteins ionically bound with cell walls. It is likely that the ability of some PO isoforms to bind with pectin ensures the spatial proximity of these enzymes to the sites of the initiation of lignin synthesis and that these "pectin-specific" isoforms take part in this process. 

Dunand C. Tognolli M. Overney S. et al. (2002) Identification and characterisation of Ca +pectate binding peroxidases in Arabidopsis thailiana // J. Plant Physiol. V. 159. P. 1165-1171. 

In contrast, observation of the c.d. with the addition of Ca(OH)2, as a function of d.p., demonstrated that terminal and central units react differently towards Ca ". This is illustrated in Fig. 27 for the dimer and the polymer. Again, the intensity of the c.d. band decreases as the polymer binds calcium and begins to gel. Results for both salt forms are attributed to a helix having a two-fold screw-symmetry, in analogy with calcium pectates. The gelling would then involve a multi-chain association, with crosslinking by the calcium ions to form an egg box structure. ... 

The viscosity of sodium pectinate was previously examined by Pals and Hermans [44]. In salt excess (NaCI 0.1 M), the viscosity of a pectate sodium form solution (ri) is related to the polymer concentration (c) and the molecular weight (or [rj] = KM ) by the relation ... 

The rheological behaviour in the range of LM pectin was analyzed and the sol-gel diagram established [59] for different stoichiometric ratios. In their paper, these authors determined the gel times for sodium pectate during calcium-induced gelation and the variation of the gel time with polymer concentration, stoichrometric ratio and temperature. 

At end, it is important to mention that calcium pectate gel beads were compared with calcium alginates gel beads for all entrapment uses [65, 66] in this work, the authors determined the pore size of the beads by size exclusion chromatography using dextran standards and other solutes. 

Figure 7. The egg-box structure for the junction zones of dilute calcium pectate gels two galacturonan chains in the twofold (2i) helical conformation with calcium ions (shaded circles) locked between them.

Figure 8. The Walkinshaw Amott model [46] for solid calcium pectate, with the galacturonan chains in the right-handed (ii) helical conformation.

Figure 9. The cable model for the structure of concentrated calcium pectate gels. Egg-box dimers link single-chains segments (top left) and are themselves ed together by larger aggregates of either egg-box or 3i helical chains (lower right)...

---