Chapter 9: Primary Metabolites: Acids & Amino Acids

Primary Metabolites: Acids & Amino Acids begins by exploring the fermentation of citric acid by the fungus Aspergillus niger, detailing the critical physiological parameters—such as low pH, high sugar concentrations, and manganese limitation—required to alter metabolic flux through glycolysis and the citric acid cycle for maximum yield. The discussion transitions to amino acid fermentation, highlighting the mass production of L-glutamate by Corynebacterium glutamicum. The summary explains the historical significance of monosodium glutamate (MSG) and the biochemical mechanisms involving biotin starvation and membrane permeability that trigger glutamate secretion, likely as an osmoregulatory response. A significant portion of the text is dedicated to the regulation of amino acid biosynthesis, contrasting the sophisticated control systems in Escherichia coli, such as feedback inhibition, repression, and attenuation (illustrated by the trp operon), with the looser regulation found in industrial soil bacteria. Strategies for strain improvement are examined in depth, including the isolation of auxotrophic mutants via penicillin selection to accumulate biosynthetic intermediates like ornithine. Furthermore, the chapter details the development of regulatory mutants through the use of toxic amino acid analogs, using the overproduction of proline and histidine by Serratia marcescens and lysine by C. glutamicum as primary case studies. The text also covers the complexities of managing branched pathways, specifically the aspartate family, and the application of metabolic engineering and metabolic flux analysis using 13C-NMR to optimize carbon flow. Finally, the chapter reviews alternative production methods utilizing immobilized enzymes and whole cells, providing industrial examples of converting fumarate to L-aspartic acid via aspartase, as well as the enzymatic synthesis of alanine and lysine.