Pectate lyase

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. 

Ennnia pectate lyase differences revealed by action pattern analyses... 

Bartling, S., Wegener, C. and Olsen, O. (1995). Synergism between Erwinia pectate lyase isoenzymes that depolymerize both pectate and pectin. Microbiology 141,873-881. 

Wegener, C., Bartling, S., Olsen, O., Thomsen, K.K., Bahlow, R. and von Wettstein, D. Differences in cell wall degradation patterns by Erwinia carotovora pectate lyase isoenzymes. Submitted to Protoplasma. 

Lietzke, S.E., Yoder, M.D., Keen, N.T. and Jurnak, F. (1994) The three-dimensional structure of pectate lyase E, a plant virulence factor from Erwinia chrysanthemi. Plant Physiol Q6, 849-862. 

Functional Implications of the Three-Dimensional Structures of Pectate Lyases... 

Erwinia chrysanthemi synthesizes and secretes a large number of pectinases. The major pectinases include a pectin methylesterase PemA and five isoenzymes of endo-pectate lyases PelA, PelB, PelC, PelD and PelE. In addition, secondary pectinases were identified a pectin methylesterase PemB, two endo-pectate lyases PelL and PelZ, an exo-pectate lyase PelX and an exopolygalacturonase, PehX. The regulation of pectinase synthesis is very complex and dependent on many environmental conditions. It is induced by pectin catabolic products and affected by growth phase, catabolite repression, osmolarity, iron or oxygen starvation... 

Three regulators were identified by genetic analysis. The main repressor, KdgR, controls the transcription of pectinase genes, the intracellular catabolic pathway and the secretion machinery. The PecS repressor controls the production of pectate lyases and cellulases, the secretion machinery and the biosynthesis of a blue pigment. PecT acts as a repressor of the production of some pectate lyases. Other proteins are involved in the regulation of pectinase s5mthesis but their role is not well characterized. 

The major endo-pectate lyases PelA, PelB, PelC, PelD and PelB... 

Structural analysis of the two pectate lyases PelC and PelE (5, 6), demonstrated that these proteins fold in a large heHx of parallel P strands. A stack of asparagine residues parallel to the helix probably plays a role in the stabUity of this structure. Identification of the structurally conserved amino adds lead to a reaHgnment of the protein sequences (7). In addition to Erwinia extracellular pectate lyases, the multiple aHgnment indudes the Bacillus subtilis pectate lyase, Aspergillus tdger and E. carotovora pectin lyases and plant proteins. 

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