Homogalacturonans Substituted galacturonans

Homogalacturonans Substituted galacturonans ; Homogalacturonan (HG) is a linear chain of 1,4-linked α-D-galactosyluronic acid residues in which some of the carboxyl groups are methyl esterified (see the figure on the right). HG with a high degree of methyl esterification is referred to as "pectin" whereas HG with a low degree of methyl esterification is "pectic acid". HGs may, depending on the plant source also be partially O-acetylated.

HG may account for up to 60% of the pectin in a primary wall. Many of the properties and biological functions of HG are believed to be determined by ionic interactions. Increases in the degree of methyl esterification of HG have been implicated in increased cell separation. Enzymes that fragment HG (endo and exo polygalacturonases and lyases) are secreted by many plant pathogens. The oligogalacturonide fragments generated have been shown to induce several plant defence responses

The sequencing of the Arabidopsis genome has revealed the existence of at least 52 genes encoding putative polygalacturonases. However, there is little know about the function or specificities of these enzymes.

Studies with transgenic plants that over-express a specific endopolygalacturonase (EPG) or have had the levels of endogenous EPG reduced have begun to provide information about the function of HG. For example, neither the supression or over-expression of EPG in tomato fruits affected the fruit ripening process but did alter tissue texture and pectin solubility. In contrast transgenic apples containing additional copies of a fruit-specific EPG exhibited premature leaf shedding, reduced cell adhesion, brittle leaves, and had malformed stomata. It is likely that HG together with other pectic polysaccharide have numerous roles in plant growth and development.

Unusual ester cross links in pectin

The existence of esters that form between the carboxyl group of a 4-linked GalpA residue and a hydroxyl group of another glycosyl residue has been hypothesized. The presence of non-methyl galacturonosyl esters has been implied from chemical analysis and FT-IR spectroscopy. The trans-esterification of pectin has been reported to be catalyzed by pectin methyl esterases. However, no fragments of HG containining unusual esters have been isolated and structurally characterized.