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The recombinant alpha subunit of glutamate synthase: spectroscopic and catalytic properties.
Biochemistry. 1998 Feb 17; 37(7):1828-38.B

Abstract

As part of our studies of Azospirillum brasilense glutamate synthase, a complex iron-sulfur flavoprotein, we have overproduced the two enzyme subunits separately in Escherichia coli. The beta subunit (53.2 kDa) was demonstrated to contain the site of NADPH oxidation of glutamate synthase and the FAD cofactor, which was identified as Flavin 1 of glutamate synthase, the flavin located at the site of NADPH oxidation. We now report the overproduction of the glutamate synthase alpha subunit (162 kDa), which is purified to homogeneity in a stable form. This subunit contains FMN as the flavin cofactor which exhibits the properties of Flavin 2 of glutamate synthase: reactivity with sulfite to yield a flavin-N(5)-sulfite addition product (Kd = 2.6 +/- 0.22 mM), lack of reactivity with NADPH, reduction by L-glutamate, and reoxidation by 2-oxoglutarate and glutamine. Thus, FMN is the flavin located at the site of reduction of the iminoglutarate formed on the addition of glutamine amide group to the C(2) carbon of 2-oxoglutarate. The glutamate synthase alpha subunit contains the [3Fe-4S] cluster of glutamate synthase, as shown by low-temperature EPR spectroscopy experiments. The glutamate synthase alpha subunit catalyzes the synthesis of glutamate from L-glutamine and 2-oxoglutarate, provided that a reducing system (dithionite and methyl viologen) is present. The FMN moiety but not the [3Fe-4S] cluster of the subunit appears to participate in this reaction. Furthermore, the isolated alpha subunit of glutamate synthase exhibits a glutaminase activity, which is absent in the glutamate synthase holoenzyme. These findings support a model for glutamate synthase according to which the enzymes prepared from various sources share a common glutamate synthase function (the alpha subunit of the bacterial enzyme, or its homologous polypeptide forming the ferredoxin-dependent plant enzyme) but differ for the chosen electron donor. The pyridine nucleotide-dependent forms of the enzyme have recruited a FAD-dependent oxidoreductase (the bacterial beta subunit) to mediate electron transfer from the NAD(P)H substrate to the glutamate synthase polypeptide. However, it appears that the presence of the enzyme beta subunit and/or of the additional iron-sulfur clusters (Centers II and III) of the bacterial glutamate synthase is required for communication between Center I (the [3Fe-4S] center) and the FMN moiety within the alpha subunit, and for ensuring coupling of glutamine hydrolysis to the transfer of the released ammonia molecule to 2-oxoglutarate in the holoenzyme.

Authors+Show Affiliations

Dipartimento di Fisiologia e Biochimica Generali, Università degli Studi di Milano, Italy. mav@imiucca.csi.unimi.itNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

9485308

Citation

Vanoni, M A., et al. "The Recombinant Alpha Subunit of Glutamate Synthase: Spectroscopic and Catalytic Properties." Biochemistry, vol. 37, no. 7, 1998, pp. 1828-38.
Vanoni MA, Fischer F, Ravasio S, et al. The recombinant alpha subunit of glutamate synthase: spectroscopic and catalytic properties. Biochemistry. 1998;37(7):1828-38.
Vanoni, M. A., Fischer, F., Ravasio, S., Verzotti, E., Edmondson, D. E., Hagen, W. R., Zanetti, G., & Curti, B. (1998). The recombinant alpha subunit of glutamate synthase: spectroscopic and catalytic properties. Biochemistry, 37(7), 1828-38.
Vanoni MA, et al. The Recombinant Alpha Subunit of Glutamate Synthase: Spectroscopic and Catalytic Properties. Biochemistry. 1998 Feb 17;37(7):1828-38. PubMed PMID: 9485308.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - The recombinant alpha subunit of glutamate synthase: spectroscopic and catalytic properties. AU - Vanoni,M A, AU - Fischer,F, AU - Ravasio,S, AU - Verzotti,E, AU - Edmondson,D E, AU - Hagen,W R, AU - Zanetti,G, AU - Curti,B, PY - 1998/3/4/pubmed PY - 1998/3/4/medline PY - 1998/3/4/entrez SP - 1828 EP - 38 JF - Biochemistry JO - Biochemistry VL - 37 IS - 7 N2 - As part of our studies of Azospirillum brasilense glutamate synthase, a complex iron-sulfur flavoprotein, we have overproduced the two enzyme subunits separately in Escherichia coli. The beta subunit (53.2 kDa) was demonstrated to contain the site of NADPH oxidation of glutamate synthase and the FAD cofactor, which was identified as Flavin 1 of glutamate synthase, the flavin located at the site of NADPH oxidation. We now report the overproduction of the glutamate synthase alpha subunit (162 kDa), which is purified to homogeneity in a stable form. This subunit contains FMN as the flavin cofactor which exhibits the properties of Flavin 2 of glutamate synthase: reactivity with sulfite to yield a flavin-N(5)-sulfite addition product (Kd = 2.6 +/- 0.22 mM), lack of reactivity with NADPH, reduction by L-glutamate, and reoxidation by 2-oxoglutarate and glutamine. Thus, FMN is the flavin located at the site of reduction of the iminoglutarate formed on the addition of glutamine amide group to the C(2) carbon of 2-oxoglutarate. The glutamate synthase alpha subunit contains the [3Fe-4S] cluster of glutamate synthase, as shown by low-temperature EPR spectroscopy experiments. The glutamate synthase alpha subunit catalyzes the synthesis of glutamate from L-glutamine and 2-oxoglutarate, provided that a reducing system (dithionite and methyl viologen) is present. The FMN moiety but not the [3Fe-4S] cluster of the subunit appears to participate in this reaction. Furthermore, the isolated alpha subunit of glutamate synthase exhibits a glutaminase activity, which is absent in the glutamate synthase holoenzyme. These findings support a model for glutamate synthase according to which the enzymes prepared from various sources share a common glutamate synthase function (the alpha subunit of the bacterial enzyme, or its homologous polypeptide forming the ferredoxin-dependent plant enzyme) but differ for the chosen electron donor. The pyridine nucleotide-dependent forms of the enzyme have recruited a FAD-dependent oxidoreductase (the bacterial beta subunit) to mediate electron transfer from the NAD(P)H substrate to the glutamate synthase polypeptide. However, it appears that the presence of the enzyme beta subunit and/or of the additional iron-sulfur clusters (Centers II and III) of the bacterial glutamate synthase is required for communication between Center I (the [3Fe-4S] center) and the FMN moiety within the alpha subunit, and for ensuring coupling of glutamine hydrolysis to the transfer of the released ammonia molecule to 2-oxoglutarate in the holoenzyme. SN - 0006-2960 UR - https://www.unboundmedicine.com/medline/citation/9485308/The_recombinant_alpha_subunit_of_glutamate_synthase:_spectroscopic_and_catalytic_properties_ L2 - https://doi.org/10.1021/bi972342w DB - PRIME DP - Unbound Medicine ER -