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Mapping Pectin Nano-Structure Following Esterase Modification, Influence of Structure on Functionality and Modeling Enzyme Action and Properties


Purify and characterize pectin methylesterase isoforms from papaya. Use them to modify a model homogalacturonan, introducing demethylated block structure which will be characterized for average block size, number of blocks per molecule and their intermolecular distribution within the population of molecules.<P>
These data will be used to mathematically model enzyme mode of action. Introduced structural features will be correlated to measured functional properties for predictive modeling of food product quality.

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NON-TECHNICAL SUMMARY: Organoleptic qualities of foods, primary determinates of consumer acceptability, are influenced by pectin, a complex structural polysaccharide found in plant cell walls and commercially extracted for use in many food products. Stabilization of milk proteins in acid dairy drinks, gelation in jams and jellies, cloud loss in fruit juices and texture firmness in fruits and vegetables are a few examples of product quality mediated by the pectin molecule's nano-structure. Our long-term goal is to develop an enzyme-based technology for precise pectin nano-structure engineering. This will enable pectin to be designed and tailored to possess novel functionality and fulfill well defined application-directed objectives. This technology would likely be transferable to other useful hydrocolloids. These goals will be accomplished by purifying and characterizing pectin modifying enzymes from a commercial papaya extract. The targeted pectin modifying enzyme(s) introduce charged blocks in linear regions of pectin. These enzymes will be used to modify a model pectin. The introduced nano-structural changes will be characterized, allowing for mathematical simulation of the enzyme mode of action, and correlated to functionality. Correlation of introduced structural nano-features to defined functional properties will allow for predictive modeling to produce pectin-based formulating agents, which have optimum and predictable physical properties.

<P>APPROACH: Pectin methylesterases (PME) present in commercial papaya fruit (Carica papaya) extracts, used industrially in the formulation of food ingredients for their inherent PME activity, will be purified with a multi-dimensional strategy incorporating affinity chromatography. The hypothesis that these enzymes can be used to rationally design functional polysaccharide food ingredients will be tested. They will be characterized for biochemical, physical and kinetic properties, and their ability to manipulate pectin nano-structure. Mass spectrometry and N-terminal amino acid sequencing will be used to establish identity tags and relatedness of individual PME forms, establishing a basis for subsequent cloning. A recently defined model homogalacturonan will be demethylated with purified papaya PME(s) to pectins with predetermined degrees of methylation. Introduced demethylated block sequence will be characterized by limited digestion with endo polygalacturonase to release demethylated fragments. Released oligomers of galacturonic acid will be visualized and quantified using HPAEC coupled to an evaporative light scattering detector, allowing for the estimation of average demethylated block size and number per pectin molecule. Capillary electrophoresis will be used to probe the intermolecular distribution of introduced demethylated blocks. These data will be used to mathematically model the enzyme mode of action/degree of processivity. Modified homogalacturonans will be submitted to functional testing (rheology, calcium sensitivity and suspension properties) to enable comparison of nanostructure features to functional properties. The relationship between block size/number and influence of reaction conditions on enzyme mode of action and pectin functionality will allow for predictive modeling of food product quality related to the listed variables.

Cameron, Randall
USDA - Agricultural Research Service
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