Pectin-Degrading Enzymes: Polygalacturonase and Pectin Lyase

Abstract

Pectic substances are polysaccharides rich in galacturonic acid. They are particularly abundant in the middle lamellae of plant tissues where they hold leaf cells together. During decomposition of leaf litter in streams, fungal activity softens leaf tissue and may release individual cells (maceration). The enzymes degrading pectins are thought to be responsible for this softening or macerating activity. This chapter describes methods for determining the activity of two classes of enzymes that depolymerize pectin. Polygalacturonases hydrolyze the glycosidic bonds between subunits. Activities are determined by incubating leaf discs with polygalacturonic acid and measuring the increase in concentration of galacturonic acid in a spectrophotometric assay using dinitrosalicylic acid as the reagent. Pectin lyases cleave the glycosidic bonds between subunits by β-elimination. Activities are determined by incubating leaf discs with pectin and measuring the increases in double bonds in an assay involving thiobarbituric acid solution. Polygalacturonases exhibit pH optima near 5 and are more active in softwater streams, whereas pectin lyases exhibit optima near pH 8 and are more active in hardwater streams. These enzyme assays have demonstrated the extraordinary importance of fungal leaf maceration in the decomposition of litter in streams.