GENERAL MICROBIOLOGY - T.P. Pirog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.6. INCOMPLETE OXIDATION

12.6.3. Bacterial Biosynthesis of Amino Acids

In 1957, the Japanese researcher S. Kinoshita isolated Corynebacterium glutamicum, thereby ushering in a new era in the industrial application of

incomplete oxidation processes. This bacterium serves as a producer of L-glutamic acid. Glucose Catabolism proceeds via The Glycolytic Pathway, with glutamate subsequently formed through citrate and 2-oxoglutarate. The production of glutamate depends on the accumulation of 2-oxoglutarate, which is caused by the absence of 2-oxoglutarate dehydrogenase in these Bacteria. In the absence of ammonium ions in the medium, 2-oxoglutarate is excreted. Pyruvate carboxylation acts as the anaplerotic reaction supplying oxaloacetate to the TCA cycle. Strains of Corynebacterium glutamicum and Brevibacterium divaricatum excrete L-glutamic acid under specific physiological conditions, namely when biotin is limited in the medium. The biotin concentration (1-5 mcg/L) in the environment is a decisive factor for glutamate synthesis. At this biotin concentration, Structural and functional changes occur within The Cell membrane, increasing its permeability to glutamic acid and leading to the release of The amino acid from the Cells. A similar effect is exhibited by certain Antibiotics and Surfactants. Mutants lacking 2-oxoglutarate dehydrogenase synthesize up to 50 g/L of glutamic acid in a biotin-limited medium.

Proline belongs to the glutamic acid family and is synthesized via ATP-dependent reduction of the latter. The Use of prototrophic mutants of Corynebacterium glutamicum makes it possible to obtain over 25 g/L of proline. Cultivating such mutants requires media with an elevated biotin content (1000 mcg/L instead of 5 mcg/L, as used for glutamic acid production). Under these conditions, glutamic acid is retained within the cells and converted into proline.

Auxotrophic mutants (Histidine-, Methionine-, leucine-, and isoleucine-dependent) of bacteria belonging to the genus Brevibacterium synthesize up to 20-25 g/L of proline. In such mutants, the activities of Key Enzymes in the respective pathways are inhibited or repressed, generating an excess of both carbon precursors and ATP within the cell, which in turn enhances The activity of glutamate kinase—the initial enzyme in proline Biosynthesis.

The production of Other Amino Acids involves alternative auxotrophic mutants of Corynebacterium glutamicum as well as species from the genus Brevibacterium. For instance, homoserine-requiring mutants function as producers of Lysine. Other mutants of Corynebacterium glutamicum, alongside Representatives of the families Enterobacteriaceae and Pseudomonaceae, serve as producers of homoserine, valine, Tryptophan, Tyrosine, and various other amino acids.