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308 Dehydrogenases

different pathways: (i) elimination reactions catalysed by amino acid lyases that produce different alcohols, and (ii) transamination reactions leading to different 2-oxoacids. The latter is the major pathway in LAB. The resulting 2-oxoacids are not responsible for flavour production but are then transformed to aldehydes, alcohols, carboxylic acids, hydroxy acids or methanethiol by additional steps that may be catalysed by the 2-oxoacid-producing LAB or by the inoculated starter LABs [134, 135]. Figure 5 shows the transaminations and further reactions leading to aroma compounds. It appears that the different flavours produced by different LAB strains depend on the proportion of the different 2-oxoacid produced and,

**Figure 5.** Main amino acid catabolic pathways in LAB leading to aroma compounds (boxed). Doted

Although lactococci have high aminotransferase activity, only very low and slow amino acid degradation occurs. Cheeses are rich in amino acids and peptides but the concentration of 2- OG is low. Supplementation of several types of cheeses with 2-OG led to a decrease in the levels of amino acids important for aroma development, which indicated that catabolism of these amino acids had been enhanced [137, 138], and sensory analyses indicated that addition of 2-OG resulted in changes in creamy character, aroma intensity and fruity notes. These results clearly established that 2-OG availability is the limiting factor for the transamination reactions that convert aminoacids to aroma compounds and prompted

Ocurrence of GDH in starter and non-starter LAB is heterogeneous, as just a few LAB strains show GDH activity. Some strains of the starter *Lactococcus lactis* have GDH activity but it is coded in a plasmid (pGDH442) rather than in its chromosome [139]. However, these GDH+ strains are glutamate auxotrophs and cannot synthesize glutamate because of 2-OG limitation. The oxidative TCA cycle in these strains is interrupted at the isocitrate dehydrogenase step [140], thus blocking the major source of 2-OG for most bacteria. Because

therefore, on the relative aminotransferases activities [136].

arrows represent chemical reactions.

scientist to look for sources of 2-OG.

Eduardo Santero\* , Ines Canosa and Fernando Govantes *Centro Andaluz de Biología del Desarrollo, CSIC-Universidad Pablo de Olavide-Junta de Andalucía, Seville, Spain* 

Ana B. Hervás *Department of Microbiology, Faculty for Biology. University of Freiburg, Freiburg, Germany* 
