Crystal structure of the multidomain pectin methylesterase PmeC5 from <i>Butyrivibrio fibrisolvens</i> D1^T

Pectin is a dynamic and complex polysaccharide that forms a substantial proportion of the primary plant cell wall and middle lamella of forage ingested by grazing ruminants. Pectin methylesterases (PMEs) are enzymes that belongs to the carbohydrate esterase family 8 (CE8) and catalyze the demethylesterification of pectin, a key polysaccharide in cell walls. In this study, the catalytic domain of a pectin methylesterase (PME) gene, was cloned from the rumen bacterium <i>Butyrivibrio fibrisolvens</i> D1^T and the recombinant enzyme was expressed in <i>Escherichia coli</i>. Here we present the crystal structure of the catalytic domain of PmeC5 determined to a resolution of 1.33 Å with the potential substrate binding domain defined. PmeC5 is a large modular enzyme that is characteristic of rumen <i>B. </i><i>fibrisolvens</i> megaplasmids and plays a central role in degrading plant cell wall components and releasing methanol in the rumen environment. Such secreted PMEs are significant contributors to plant fiber digestion as well as methane production, making them worthwhile targets for both methane mitigation strategies and livestock productivity enhancement.