Conversion of Methionine to Thiols by Lactococci, Lactobacilli, and Brevibacteria

Abstract

Methanethiol has been strongly associated with desirable Cheddar cheese flavor and can be formed from the degradation of methionine (Met) via a number of microbial enzymes. Methionine γ-lyase is thought to play a major role in the catabolism of Met and generation of methanethiol in several species of bacteria. Other enzymes that have been reported to be capable of producing methanethiol from Met in lactic acid bacteria include cystathionine β-lyase and cystathionine γ-lyase. The objective of this study was to determine the production, stability, and activities of the enzymes involved in methanethiol generation in bacteria associated with cheese making. Lactococci and lactobacilli were observed to contain high levels of enzymes that acted primarily on cystathionine. Enzyme activity was dependent on the concentration of sulfur amino acids in the growth medium. Met aminotransferase activity was detected in all of the lactic acid bacteria tested and α-ketoglutarate was used as the amino group acceptor. In Lactococcus lactis subsp. cremorisS2, Met aminotransferase was repressed with increasing concentrations of Met in the growth medium. While no Met aminotransferase activity was detected in Brevibacterium linens BL2, it possessed high levels of l-methionine γ-lyase that was induced by addition of Met to the growth medium. Met demethiolation activity at pH 5.2 with 4% NaCl was not detected in cell extracts but was detected in whole cells. These data suggest that Met degradation in Cheddar cheese will depend on the organism used in production, the amount of enzyme released during aging, and the amount of Met in the matrix.

Sulfur compound production by Geotrichum candidum from l-methionine: importance of the transamination step

“…This activity is also strongly correlated with methanethiol and its direct oxidation products. L-Methionine aminotransferase activity, although reported in several cheese-ripening microorganisms, is not ubiquitous [5], compared to the versatile enzyme L-methionine demethiolase [5,12,24]. For instance, L-methionine aminotransferase activity was reported in cellular extracts of the bacteria B. linens, M. luteus, C. glutamicum, but was not detected in Arthrobacter sp.…”

Flavour formation by lactic acid bacteria and biochemical flavour profiling of cheese products

“…Another important conversion route of amino acids is initiated by lyases ( Fig. 1(10)), like cystathionine b-lyase (EC 4.4.1.8), which is able to convert methionine to methanethiol [41][42][43]. Threonine aldolase (EC 4.1.2.5) ( Fig.…”

“…The homotetrameric enzyme with a M w of 170 kDa is PLP dependent and has a pH optimum of 7.5 [126]. The enzyme is also slightly active under cheese ripening conditions, but is also susceptible for proteolysis [41,126]. Amarita et al [133][134][135] studied the role of this lyase and found that disruption of the gene coding for this enzyme resulted in an almost complete abolishment of the production of volatile sulphur compounds in this strain, indicating the importance of this enzyme in the formation of sulphur compounds in B. linens.…”

Identification and characterization of a cystathionine β/γ-lyase from Lactococcus lactis ssp. cremoris MG1363

“…Sulfur compounds (particularly methanethiol and hydrogen sul¢de) are components of cheddar cheese £avor [6,8,9], and enzymes involved in the metabolism of sulfur containing amino acids may therefore have a major role in the formation of £avor compounds during cheese ripening [10,11]. Cystathionine lyase, which is involved in methionine biosynthesis [12], has been puri¢ed from a number of organisms, including Lactococcus lactis ssp.…”