Directing carbon flow though catechol and protocatechuate pathways for increased yield in biological lignin upgrading

Lignin is an energy-rich polymer that accounts for approximately 15-30% of biomass but is underutilized in biomass upgrading due to its heterogeneity and recalcitrance. The degradation pathways used by microorganisms capable of metabolizing lignin-derived aromatics have been a focus of research for several decades. Here we examine the ortho and meta cleavage pathways that degrade the central intermediates catechol and protocatechuate for entry into the TCA cycle as they relate to lignin upgrading. Depending upon the pathway, carbon enters the TCA cycle in the form of different molecules and, thus, different pathways may be more suitable for some upgrading strategies than others. We demonstrate that directing carbon flow by manipulating these degradation pathways does indeed affect the yield of products of interest, thus offering versatile approaches for the valorization of lignin.