Methionine catabolism

The subsequent cleavage of cystathionine to yield cysteine, a-ketobutyrate and NH4+ is catalyzed by y-cystathionase, a pyridoxal-phosphate-containing enzyme. This transsulfura-tion pathway is one of the routes used for methionine catabolism. 

Glutamine transaminase from bovine liver, one of the enzymes involved in methionine catabolism, utilizes SeMet as well as methionine (Blazon et al., 1994). However, with some enzymes, differences in the reaction rates for SeMet and Met have been observed. For example, SeMet is a better substrate than Met for the a,7-elimination by L-methionine 7-lyase of Pseudomonas putida (Esaki et al., 1979). The adenosyltransferase from rat liver reacts with L(+)-SeMet at 51% of the rate with L(+)-Met, and with the corresponding d(—) isomers at only 13 and 10% of the rate of L-Met (Pan and Tarver, 1967). The adenosyl transferase from yeast, on the other hand, is more active with SeMet than with Met (Mudd and Cantoni, 1957). This enzyme produces the... 

Pripis-Nicolau, L., de Revel, G., Bertrand, A., Lonvaud-Eunel, A. (2004). Methionine catabolism and production of volatile sulphur compounds by Oenococcus oeni. J. Appl. Microbiol., 96,... 

FIGURE a.27 Pathway for methionine cataboLsm and cysteine synthesis. Methionine is the source of the sulfur atom of cysteine. Serine is the source of the carbon skeleton of serine. In methionine catabolism, the carbon skeleton of methionine is converted to propionyl-CoA, which eventually enters the Krebs cycle at the point of succinyl-CoA. BCAA dehydrogenase catalyzes the oxidation of a ketobutyrate to propionyXloA-... 

The concept of sparing of one nutrient by another was introduced earlier, where it was demonstrated that dietary carbohydrate can spare protein. Similarly, cysteine can spare methionine and tyrosine can spare phenylalanine. A certain proportion of dietary methionine is converted to cysteine. Mediionine normally supplies part of the body s needs for cysteine. With cysteine-free diets, methionine can supply all of the body s needs for cysteine. The methionine catabolic pathway that leads to cysteine production is shown in Figure 8.27. Only the sulfur atom of methionine appears in the molecule of cysteine serine supplies the carbon skeleton of cysteine. a-Ketobutyrate is a byproduct of the pathway. a-Ketobutyrate is further degraded to propionyl-CoA by BCKA dehydrogenase or pyruvate dehydrogenase. Propionyl-CoA is then converted to succinyl-CoA, an intermediate of the Krebs cycle. 

L-Homoserine is found in many tissues as a intermediate in amino acid metabolism, including threonine, isoleucine, and methionine. Catabolism of aspartate to homoserine is shown here. The biosynthetic pathway from homoserine to methionine is shown in Figure 21.6. 

Trimble KC, Molloy AM, Scott JM, Weir DG. The effect of ethanol on one-carbon metabolism increased methionine catabolism and lipotrope methyl-group wastage. 

Sulfur compounds are generated by sulfur amino acid catabolism and are potent odorants that contribute flavor to many fermented foods. Methionine catabolism produces various volatile sulfur compounds (VSCs) such as H S, methanethiol, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS) (Fernandez et al. 2000). The enzymes in LAB strains from raw goats milk cheeses crucial for VSC formation from L-methionine have very diverse enzyme capabilities. 

Hanniffy, S.B., Pelaez, C., Martinez-Bartolome, M.A., et al. (2009) Key enzymes involved in methionine catabolism by cheese lactic acid bacteria. Int J Food Microbiol 135,223-230. 

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