homogalacturonan

Homogalacturonan (HG) is a linear, unbranched polysaccharide composed of repeating α-D-galacturonic acid residues linked by α-(1→4) glycosidic bonds. It is the principal backbone of pectin, a group of plant cell-wall polysaccharides, and is widely distributed in the primary cell walls and middle lamella of vascular plants. In HG, each galacturonic acid unit can bear a methyl ester group at the C-6 carboxyl, resulting in varying degrees of esterification.

The degree of methyl esterification (DE) of HG influences its physical properties. High-DE HG forms gels in the presence of sugar and acid (high-methoxyl pectin), whereas low-DE HG can cross-link via Ca2+ ions to create gels through egg-box-type structures. HG may also be partially acetylated at O-2 and/or O-3 in some species. The esterification pattern is modified by plant pectin methylesterases during cell wall remodeling.

In plants, HG provides the main structural scaffold of pectin and contributes to wall porosity, hydration, and rigidity. It interacts with other pectic domains and with calcium-rich ions to affect cell adhesion and wall mechanics. During fruit ripening and tissue maceration, HG can be de-esterified and enzymatically degraded by polygalacturonases and pectate lyases, contributing to softening.

Analytically and industrially, HG serves as a model substrate for pectin-degrading enzymes. Its properties are central to understanding plant physiology and to applications in food science, where esterification status determines gelling behavior and texture.