13345-95-4Relevant articles and documents
Studies on the reaction between reduced riboflavin and selenocystine
Dereven'kov, Ilia A.,Makarov, Sergei V.,Molodtsov, Pavel A.,Makarova, Anna S.
, p. 146 - 153 (2020/09/21)
Selenocysteine (Sec) is a crucial component of mammalian thioredoxin reductase (TrxR) where it serves as a nucleophile for disulfide bond rupture in thioredoxin (Trx). Generation of the reduced state of Sec in TrxR requires consecutive two electron transfer steps, namely: (i) from NADPH to flavin adenine dinucleotide, (ii) from reduced flavin to the disulfide bond Cys59-S-S-Cys64, and finally (iii) from Cys59 and Cys64 to the selenosulfide bond Cys497-S-Se-Sec498. In this work, we studied the reaction between reduced riboflavin (RibH2) and selenocystine (Sec-Sec), an oxidized form of Sec. The interaction between RibH2 and Sec-Sec proceeded relatively slowly in comparison with its reverse reaction, that is, reduction of riboflavin (Rib) by Sec. The rate constant for the reaction between RibH2 and Sec-Sec was (7.9?±?0.1)?×?10?2?M?1 s?1 (pH 7.0, 25.0°C). The reaction between Rib and Sec proceeded via two steps, namely, a rapid reversible binding of Rib to Sec having a protonated selenol group to form a Sec-Rib complex, followed by nucleophilic attack of Sec-Rib by a second Sec molecule harboring a deprotonated selenol group. The equilibrium constant for the overall reduction process of Rib by Sec is (1.2?±?0.1)?×?106?M?1 (25.0°C). The finding that the interaction of RibH2 with oxidized selenol is reversible with its equilibrium favored toward the reverse reaction provides an additional explanation for the exceptional mechanism of the mammalian Trx/TrxR system involving transient reduction of a disulfide bond.
Mechanistic implications of a linear free-energy correlation of rate constants for the reduction of active- and Met-R2 forms of E. coli ribonucleotide reductase with eight organic radicals
Dobbing, A. Mark,Borman, Christopher D.,Twitchett, Mark B.,Leese, David N.,Salmon, G. Arthur,Sykes, A. Geoffrey
, p. 2206 - 2212 (2007/10/03)
Cross-reaction rate constants k12 (22 °C) at pH 7.0 have been determined for the reduction of Fe(III)2 and tyrosyl-radical-containing active-R2 from E. coli ribonucleotide reductase with eight organic radicals (OR), e.g., MV·+ from methyl viologen. The more reactive OR's were generated in situ using pulse radiolysis (PR) techniques, and other OR's were generated by prior reduction of the parent with dithionite, followed by stopped-flow (SF) studies. In both procedures it was necessary to include consideration of doubly-reduced parent forms. Values of k12 are in the range 109 to 104 M-1 s-1 and reduction potentials E1/(o) for the OR vary from -0.446 to +0.l94 V. Samples of E. coli active-R2 also have an Fe2(III) met-R2 component (with no Tyr·), which in the present work was close to 40%. From separate experiments met-R2 gave similar k12 rate constants (on average 66% bigger) to those for active-R2, suggesting that reduction of the Fe2/(III) center is the common rate-limiting step. A single Marcus free-energy plot of log k12 - 0.5 log f vs - E1/(o)/0.059 describes all the data, and the slope of 0.54 is in satisfactory agreement with the theoretical value of 0.50. It is concluded that the ratelimiting step involves electron transfer. In addition, the intercept at -E1/(o)/0.059 = 0 is 5.94, where values of the reduction potential and self-exchange rate constant for met-R2 contribute to this value. To maintain electroneutrality at the ~10 A buried active site H+ uptake is also required. For both e- and H+ transfer the conserved pathway Trp-48, Asp-237, His-118 to Fe(A) is a possible candidate requiring further examination.
Artificial Redox Enzymes. Part 3. Structure and Properties
Ye, Hongping,Rong, Ding,Tong, Weida,D'Souza, Valerian T.
, p. 2071 - 2076 (2007/10/02)
The structural and catalytic properties of models of flavoenzymes in which flavin is covalently attached to the catalytically important secondary side of 2--α-cyclodextrin and 2--β-cyclodextrin as well as the primary side 6-(10-N-isoalloxazinylmethyl)-β-cyclodextrin are reported.
