Cell-free enzymatic conversion of spent coffee grounds into the platform chemical lactic acid

Dominik Kopp, Robert D. Willows, Anwar Sunna*

*Corresponding author for this work

Research output: Contribution to journalArticle

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Abstract

The coffee industry produces over 10 billion kg beans per year and generates high amounts of different waste products. Spent coffee grounds (SCG) are an industrially underutilized waste resource, which is rich in the polysaccharide galactomannan, a polysaccharide consisting of a mannose backbone with galactose side groups. Here, we present a cell-free reaction cascade for the conversion of mannose, the most abundant sugar in SCG, into L-lactic acid. The enzymatic conversion is based on a so far unknown oxidative mannose metabolism from Thermoplasma acidophilum and uses a previously characterized mannonate dehydratase to convert mannose into lactic acid via 4 enzymatic reactions. In comparison to known in vivo metabolisms the bioconversion is free of phosphorylated intermediates and cofactors. Assessment of enzymes, adjustment of enzyme loadings, substrate and cofactor concentrations, and buffer ionic strength allowed the identification of crucial reaction parameters and bottlenecks. Moreover, reactions with isotope labeled mannose enabled the monitoring of pathway intermediates and revealed a reverse flux in the conversion process. Finally, 4.4 ± 0.1 mM lactic acid was produced from 14.57 ± 0.7 mM SCG-derived mannose. While the conversion efficiency of the process can be further improved by enzyme engineering, the reaction demonstrates the first multi-enzyme cascade for the bioconversion of SCG.
Original languageEnglish
Article number389
Pages (from-to)1-13
Number of pages13
JournalFrontiers in Bioengineering and Biotechnology
Volume7
DOIs
Publication statusPublished - 3 Dec 2019

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Bibliographical note

Copyright the Author(s) 2019. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

Keywords

  • enzyme biocatalysis
  • mannose
  • cell-free
  • spent coffee grounds
  • lactic acid
  • NMR flux analysis
  • metabolic engineering
  • waste

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