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Pectin methylesterase, metal ions and plant cell-wall extension. Hydrolysis of pectin by plant cell-wall pectin methylesterase.

ABSTRACT: The hydrolysis of p-nitrophenyl acetate catalysed by pectin methylesterase is competitively inhibited by pectin and does not require metal ions to occur. The results suggest that the activastion by metal ions may be explained by assuming that they interact with the substrate rather than with the enzyme. With pectin used as substrate, metal ions are required in order to allow the hydrolysis to occur in the presence of pectin methylesterase. This is explained by the existence of 'blocks' of carboxy groups on pectin that may trap enzyme molecules and thus prevent the enzyme reaction occurring. Metal ions may interact with these negatively charged groups, thus allowing the enzyme to interact with the ester bonds to be cleaved. At high concentrations, however, metal ions inhibit the enzyme reaction. This is again understandable on the basis of the view that some carboxy groups must be adjacent to the ester bond to be cleaved in order to allow the reaction to proceed. Indeed, if these groups are blocked by metal ions, the enzyme reaction cannot occur, and this is the reason for the apparent inhibition of the reaction by high concentrations of metal ions. Methylene Blue, which may be bound to pectin, may replace metal ions in the 'activation' and 'inhibition' of the enzyme reaction. A kinetic model based on these results has been proposed and fits the kinetic data very well. All the available results favour the view that metal ions do not affect the reaction through a direct interaction with enzyme, but rather with pectin.

SUBMITTER: Nari J

PROVIDER: S-EPMC1151611 | biostudies-other | 1991 Oct

REPOSITORIES: biostudies-other

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Project description:BackgroundPectin methylesterases (PMEs) catalyze the demethylesterification of homogalacturonans in the cell wall; their activity is regulated in part by pectin methylesterase inhibitors (PMEIs). PME activity may result in either rigidification or loosening of the cell wall, depending on the mode of demethylesterification. The activity of PMEs in the middle lamella is expected to affect intrusive elongation of phloem fibers, and their adhesion to adjacent cells. Length and extractability of phloem fibers are qualities important for their industrial uses in textiles and composites. As only three flax PMEs had been previously described, we were motivated to characterize the PME and PMEI gene families of flax.ResultsWe identified 105 putative flax PMEs (LuPMEs) and 95 putative PMEIs (LuPMEIs) within the whole-genome assembly. We found experimental evidence for the transcription of 77/105 LuPMEs and 83/95 LuPMEIs, and surveyed the transcript abundance of these in 12 different tissues and stages of development. Six major monophyletic groups of LuPMEs could be defined based on the inferred relationships of flax genes and their presumed orthologs from other species. We searched the LuPMEs and LuPMEIs for conserved residues previously reported to be important for their tertiary structure and function. In the LuPMEs, the most highly conserved residues were catalytic residues while in the LuPMEIs, cysteines forming disulfude bridges between helices α2 and α3 were most highly conserved. In general, the conservation of critical residues was higher in the genes with evidence of transcript expression than in those for which no expression was detected.ConclusionsThe LuPMEs and LuPMEIs comprise large families with complex patterns of transcript expression and a wide range of physical characteristics. We observed that multiple PMEs and PMEIs are expressed in partially overlapping domains, indicative of several genes acting redundantly during most processes. The potential for functional redundancy was highlighted also by the phylogenetic analyses. We were able to identify a subset of PME and PMEIs that appeared particularly relevant to fiber development, which may provide a basis for the improvement of key traits in industrial feedstocks and a better understanding of the physiological roles of PMEs and PMEIs in general.

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Pectin methylesterase, metal ions and plant cell-wall extension. Hydrolysis of pectin by plant cell-wall pectin methylesterase.

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