Enzymatic Regulation of the Radical Content of the Small Subunit of Escherichia coli Ribonucleotide Reductase Involving Reduction of Its Redox Centers

• Published in: The Journal of biological chemistry 1989-06, Vol.264 (16), p.9164-9170
• Main Authors: Fontecave, M, Eliasson, R, Reichard, P
• Format: Article
• Language: English
• Subjects: 550201 - Biochemistry- Tracer Techniques; AMIDES; AMINO ACIDS; Analytical, structural and metabolic biochemistry; ANIMALS; BACTERIA; Bacterial Proteins - isolation & purification; Bacterial Proteins - metabolism; BASIC BIOLOGICAL SCIENCES; BIOCHEMICAL REACTION KINETICS; Biological and medical sciences; BIOSYNTHESIS; CARBOXYLIC ACIDS; CATTLE; CHEMICAL COMPOSITION; Chemical Fractionation; CHEMICAL REACTIONS; DOMESTIC ANIMALS; ELEMENTS; ENZYMES; Enzymes and enzyme inhibitors; ESCHERICHIA COLI; Escherichia coli - enzymology; Escherichia coli - metabolism; Ferric Compounds - metabolism; Ferrous Compounds - metabolism; Ferrous Compounds - physiology; Flavins; FMN Reductase; FRACTIONATION; Free Radicals; Fundamental and applied biological sciences. Psychology; HETEROCYCLIC COMPOUNDS; HYDROXY ACIDS; HYDROXY COMPOUNDS; HYDROXYUREA; IRON COMPOUNDS; IRON ISOTOPES; Iron Radioisotopes; ISOALLOXAZINES; ISOTOPE APPLICATIONS; ISOTOPES; KINETICS; MAMMALS; MICROORGANISMS; NADH, NADPH Oxidoreductases - physiology; NONMETALS; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; ORGANIC OXYGEN COMPOUNDS; Oxidation-Reduction; OXIDOREDUCTASES; OXYGEN; PROTEINS; RADICALS; REACTION KINETICS; REAGENTS; REDOX REACTIONS; Ribonucleotide Reductases - isolation & purification; Ribonucleotide Reductases - metabolism; RUMINANTS; SEPARATION PROCESSES; SPECTROPHOTOMETRY; SYNTHESIS; TRACER TECHNIQUES; TRANSITION ELEMENT COMPOUNDS; TYROSINE; Tyrosine - metabolism; VERTEBRATES
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)60509-5

The active form of protein B2, a homodimeric subunit of Escherichia coli ribonucleotide reductase, contains a diferric iron center and a cationic free radical localized to tyrosine 122 of one of the two polypeptide chains. Hydroxyurea scavenges this radical but leaves the iron center intact. The resulting metB2 (earlier named B2/HU) is enzymatically inactive. Crude extracts of E. coli catalyze the interconversion of metB2 and B2. Radical introduction into metB2 requires a flavin reductase together with a second poorly defined protein fraction (“Fraction b”) as well as dioxygen, NAD(P)H, and a flavin (Fontecave, M., Eliasson, R., and Reichard, P. (1987) J. Biol. Chem. 262, 12325–12331). We now find that ferrous ions can substitute for Fraction b and that the diferric center of metB2 is reduced during anaerobic incubation of the system with reduced flavin and ferrous ions. Spectroscopic evidence and isotope experiments suggest an in situ reduction of the diferric to a diferrous center. Admission of oxygen then results in the instantaneous oxidation of tyrosine 122 to the cationic radical coupled to the reformation of the diferric center, giving enzymatically active B2. These data suggest that reduced diferrous B2 is an intermediate between metB2 and B2 during radical introduction. In addition, we find that anaerobic incubation of B2 with reduced flavin results in the loss of the tyrosyl radical and the formation of metB2. This reaction occurs in the absence of Fraction b or ferrous ions. Our experiments reconstitute with defined reagents the interconversion between metB2 and B2 observed earlier in the E. coli extract. The flavin reductase system catalyzes the interconversion in both directions with dioxygen as the critical factor deciding whether activation or inactivation of ribonucleotide reductase occurs.