Serine pathways

More recently, Pearce and Heydeman suggested non-oxidative removal of ethylene glycol units as acetaldehyde by a membrane-bound, oxygen-sensitive enzyme of a novel type, i.e., diethylene glycol lyase (18). Schoberl suggested that PEG was catabolized by Ci step, liberating formate which was metabolized by a serine pathway (19). 

Figure 17-15 One of the serine pathways for assimilation of one-carbon compounds.

Figure 2. Aerobic catabolism of methylated sulfides (adapted from Kelly, 1988). 1) DMSO reductase (Hyphomicrobium sp.) 2) DMDS reductase (Thiobacillus sp. 3) trimethylsulfonium-tetrahydrofolate methyltransferase (Pseudomonas sp.) 4) DMS monooxygenase 5) methanethiol oxidase 6) sulfide oxidizing enzymes 7) catalase 8) formaldehyde dehydrogenase 9) formate dehydrogenase 10) Calvin cycle for CO2 assimilation (Thiobacillus sp.) 11) serine pathway for carbon assimilation (Hyphomicrobium sp.).

Dedysh S. N., Liesack W., Khmelenina V. N., Suzina N. E., Trotsenko Y. A., Semrau J. D., Bares A. M., Panikov N. S., and Tiedje J. M. (2000) Methylocella palustris gen. nov., sp. nov., a new methane-oxidizing acidophihc bacterium from peat bogs representing a novel sub-type of serine pathway methanotrophs. Int. J. Syst. Evol. Microbiol. 50, 955—969. 

Dimethyldisulfide is degraded by autotrophic sulfur bacteria with the formation of sulfate and C02 which then enters the Ben-son-Calvin cycle (Smith and Kelly 1988). On the other hand, dimethyl sulfide and dimethyl sulfoxide are degraded by a strain of Hyphomicrobium sp. by pathways involving the formation from both carbon atoms of formaldehyde which subsequently enters the serine pathway (Suylen et al. 1986) (Figure 6.121). The key enzyme is methyl mercaptan oxidase which converts methyl sulfide into formaldehyde, sulfide, and peroxide (Suylen et al. 1987). A strain... 

The remaining entries in Table 5 are involved in the production and consumption of methane. Methylococcus capsulatus, Methylosinus trichosporium, and the isolate designated as cel 1923 are methylotrophic bacteria. Such organisms assimilate methane (and, in many cases, other Ci compounds, such as methanol, formaldehyde, formic acid, methyl amines, and methyl sulfides) using either a pathway that yields carbohydrates (the ribulose monophosphate or RuMP pathway) or one which yields acetyl-CoA after the Ci unit is initially added to glycine to yield serine (thus called the serine pathway). The methyl carbon in that acetyl-CoA derives from the Ci substrate but the carboxyl carbon derives from CO2. Variations are possible within the serine pathway. 

Energetically independent carbon conversion efficiency (CCE) in %. Serine pathway. 

The methanotrophs have broadly been classified into two groups, type I and type II, on the basis of the carbon assimilation pathways in the metabolism of methane and on the arrangement of intracytoplasmic membranes. Two different metabolic strategies are utilized for the assimilation of carbon from formaldehyde formed in the course of the methane oxidation to carbon dioxide (Fig. 1). The type I bacteria involves the ribulose monophosphate pathway while the type II bacteria involves the glycine serine pathway. Recently, the Type I group has been further subdivided (type X), to accommodate strains... 

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