Reconsidering Pectin Structure

Pectins are known for their gelling and stabilising properties in jams and jellies. These properties are associated with galacturonic acid–rich homogalacturonan (HG) that is abundant in food grade pectin, a pectin isolated from apple and lemon primary cell walls. Perhaps, because of the large body of research on food grade pectin or perhaps because seminal studies of plant cell wall structure were done using walls from suspension cultures that provided a source of homogenous cells, the view of pectin as composed largely of HG is pervasive. Yet, as discussed in this chapter, this view is true only for certain types of cells and tissues; in others, rhamnogalacturonan I (RG-I) is the most abundant pectin. Equally accepted is the view of pectin as a family of three pectic polysaccharides: HG, RG-I and rhamnogalacturonan II (RG-II). Determining the structures of these pectic glycans has been vital for the field, but by limiting our view of pectin to these we may have constrained our thinking. The goal of this chapter is to evaluate the structural data that led to our current concepts of pectin structure and role in wall architecture, and to consider those data in the context of more recent biosynthesis results, indicating that pectin is a collection of distinct heteroglycans and glycoconjugates. Understanding the unique and overlapping structures and covalent and non-covalent associations of these different pectic glycopolymers is essential to understand pectin function in the cell wall and plant, and in pectin-containing food, materials and medicinal and other bioproducts.