Serine-glyoxylate aminotranferases from methanotrophs using different C1-assimilation pathways
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The indicator enzyme of the serine pathway of assimilation of reduced C1 compounds, serine-glyoxylate aminotransferase (Sga), has been purified from three methane-oxidizing bacteria, Methylomicrobium alcaliphilum 20Z, Methylosinus trichosporium OB3b and Methylococcus capsulatus Bath. The native enzymes were shown to be dimeric (80 kDa, strain 20Z), tetrameric (~ 170 kDa, strain OB3b) or trimeric (~ 120 kDa, strain Bath). Sga from the three methanotrophs catalyse the pyridoxal phosphate-dependent transfer of an amino group from serine to glyoxylate and pyruvate; the enzymes from strains 20Z and Bath also transfer an amino group from serine to α-ketoglutarate and from alanine to glyoxylate. No other significant differences between the Sga from the three methanotrophs were found. The three methanotrophic Sga have their highest catalytic efficiencies in the reaction between glyoxylate and serine, which is in agreement with their function to provide circulation of the serine assimilation pathway.The disruption of the sga gene in Mm. alcaliphilum resulted in retardation of growth rate of the mutant cells and in a prolonged lag-phase after passaging from methane to methanol. In addition, the growth of the mutant strain is accompanied by formaldehyde accumulation in the culture liquid. Hence, Sga is important in the serine cycle of type I methanotrophs and this pathway could be related to the removal of excess formaldehyde and/or energy regulation.
KeywordsC1-assimilation Methanotrophic bacteria Serine cycle Serine-glyoxylate aminotransferase
The authors are grateful to all members of the Organization for Methanotroph Genome Analysis for collaboration (OMeGA), the U.S. Department of Energy Joint Genome Institute and Genoscope for the access to methanotrophic genomes for comparative analyses. This work was supported by Russian Foundation for Basic Research# 17-04-01113-a.
SY But designed the experiments, coordinated the study, carried out the mutant generation. SV Egorova carried out purification and characterization of the enzymes, and cultures growth measurements. SY But, VN Khmelenina and YA Trotsenko contributed to data analysis and manuscript preparation. All authors reviewed and approved the final manuscript.
Conflict of interest
The authors declare that they have no conflict of interest.
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