https://ucanr.edu/datastoreFiles/608-836.pdf

Pectin methylesterases: cell wall enzymes with important roles in plant physiology.

New insights into pectin methylesterase structure and function.

Modulation of the Degree and Pattern of Methyl-esterification of Pectic Homogalacturonan in Plant Cell Walls IMPLICATIONS FOR PECTIN METHYL ESTERASE ACTION, MATRIX PROPERTIES, AND CELL ADHESION

SUMMARY AND PERSPECTIVE The enlargement of plant cells involves the coordinate con- trol of wall synthesis and expansion, solute and water trans- port, membrane synthesis, Golgi secretion, ion transport, and many other processes. In this review, I have focused on the wall because it is the major control point for cell enlarge- ment. Some of the key processes that may be involved in wall enlargement are summarized in Figure 4. I offer the following speculative picture as a tentative working model for the control of wall expansion. The primary wall is initially secreted and assembled in a form that is me- chanically tough yet has “hot spots” where expansin can weaken microfibril-matrix adhesion. Expansin activity, which is modulated both by secretion of the protein to the wall and by changes in the pH and redox potential of the wall, in- duces the stress relaxation and polymer creep needed for wall enlargement and water uptake by the cell. By altering Synthesis & secretion secretion of wall polysaccharides and proteins

/pdf/relaxation-in-a-high-stress-environment-the-molecular-bases-58ft23tgcu.pdf

Relaxation in a high-stress environment: the molecular bases of extensible cell walls and cell enlargement.

Growing plant cells are shaped by an extensible wall that is a complex amalgam of cellulose microfibrils bonded noncovalently to a matrix of hemicelluloses, pectins, and structural proteins. Cellulose is synthesized by complexes in the plasma membrane and is extruded as a self-assembling microfibril, whereas the matrix polymers are secreted by the Golgi apparatus and become integrated into the wall network by poorly understood mechanisms. The growing wall is under high tensile stress from cell turgor and is able to enlarge by a combination of stress relaxation and polymer creep. A pH-dependent mechanism of wall loosening, known as acid growth, is characteristic of growing walls and is mediated by a group of unusual wall proteins called expansins. Expansins appear to disrupt the noncovalent bonding of matrix hemicelluloses to the microfibril, thereby allowing the wall to yield to the mechanical forces generated by cell turgor. Other wall enzymes, such as (1-->4) beta-glucanases and pectinases, may make the wall more responsive to expansin-mediated wall creep whereas pectin methylesterases and peroxidases may alter the wall so as to make it resistant to expansin-mediated creep.

/pdf/assembly-and-enlargement-of-the-primary-cell-wall-in-plants-sf5r4sdsxs.pdf

Assembly and enlargement of the primary cell wall in plants

The study of pectin methylesterase and wall-loosening enzyme activities in situ, as well as the estimation of the electrostatic potential of the cell wall, suggest a coherent picture of the role played by metal ions and pH in cell-wall extension. Cell-wall growth brings about a decrease of local proton concentration because the electrostatic potential difference (delta psi) of the wall decreases. This in turn activates pectin methylesterase, which restores the initial delta psi value. This process is amplified by the attraction of metal ions in the polyanionic cell-wall matrix. The amplification process is basically due to the release of enzyme molecules that were initially bound to 'blocks' of carboxy groups. This increase of metal-ion concentration also results in the activation of wall-loosening enzymes. Moreover, the apparent 'inhibition' of pectin methylesterase by high salt concentrations may be considered as a device which prevents the electrostatic potential from becoming too high.

/pdf/pectin-methylesterase-metal-ions-and-plant-cell-wall-15bup7u073.pdf

Pectin methylesterase, metal ions and plant cell-wall extension. The role of metal ions in plant cell-wall extension.

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

/pdf/pectin-methylesterase-metal-ions-and-plant-cell-wall-4bqffkeijv.pdf

J Ricard

Papers

Pectin methylesterase, metal ions and plant cell-wall extension. The role of metal ions in plant cell-wall extension.

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