This review examines microbial esterases that degrade plant cell wall polysaccharides by removing ester-linked side chains and cross-linkages. Plants protect themselves through cell walls containing complex polysaccharides—hemicelluloses, pectins, and lignin—decorated with acetyl, methoxyl, and feruloyl groups that resist enzymatic attack. Microbes have evolved specialized esterases to remove these modifications, enabling access to main-chain-degrading enzymes and facilitating nutrient acquisition or pathogenic infection. The three main classes are cinnamoyl esterases (removing feruloyl groups and cross-links), acetylesterases (removing acetyl groups), and pectin methylesterases (removing methyl esters). These enzymes work synergistically with other polysaccharide-degrading enzymes and have industrial applications in food processing, paper de-inking, and animal feed improvement. The review discusses enzyme specificity, catalytic mechanisms involving serine and histidine residues, and substrate requirements. Despite over 20 published gene sequences, esterases show limited sequence conservation and don't group into neat families like other cell-wall-degrading enzymes, though conserved motifs like GXSXG appear in some lipases and esterases.

Key findings

• Microbial esterases remove ester-linked substituents from plant cell wall polysaccharides, enabling pathogenic and saprophytic microbes to access and degrade polysaccharide backbones
• Ferulic acid cross-links between polysaccharide chains, formed during plant growth cessation, are critical for cell wall strength and are targets for microbial cinnamoyl esterases
• Plant-cell-wall esterases demonstrate strong synergistic relationships with main-chain-degrading enzymes like xylanases and polygalacturonases, with activity enhancement by orders of magnitude
• Multiple distinct esterases are required per organism to hydrolyze all ester bonds in plant cell walls, likely reflecting specificity for different substrates and positions
• Industrial applications include pectin gelation, juice clarification, paper de-inking, and feed digestibility improvement through de-esterification