These tools will no longer be maintained as of December 31, 2024. Archived website can be found here. PubMed4Hh GitHub repository can be found here. Contact NLM Customer Service if you have questions.


Pubmed for Handhelds

PUBMED FOR HANDHELDS

Search MEDLINE/PubMed

  • Title: Comparison of the processes involved in reduction by the substrate for two homologous flavocytochromes b2 from different species of yeast.
    Author: Capeillère-Blandin C, Barber MJ, Bray RC.
    Journal: Biochem J; 1986 Sep 15; 238(3):745-56. PubMed ID: 3026360.
    Abstract:
    A detailed study of the electron exchanges involved between FMN and haem b2 groups within flavocytochrome b2 of yeast Hansenula anomala (H-enzyme) was performed. The results were compared with those for the homologous enzyme of yeast Saccharomyces cerevisiae (Sx-enzyme) re-investigated at 5 degrees C. The mid-point reduction potentials of FMN and haem were determined by two complementary methods: potentiometric titration with substrate, L-lactate, in the presence of dye mediators with quantification of the reduced species performed by spectrophotometry at suitable wavelengths; anaerobic titration of the enzyme by its substrate by monitoring the e.p.r. signals of the semiquinone and Fe3+ species. Values of Em,7 = -19, -23 and -45 V were determined respectively from the data for the three redox systems Ho/Hr, Fo/Fsq and Fsq/Fr in the H-enzyme instead of +6, -44 and -57 mV respectively in the Sx-enzyme [Capeillère-Blandin, Bray, Iwatsubo & Labeyrie (1975) Eur. J. Biochem. 54, 549-566]. Parallel e.p.r rapid-freezing and absorbance stopped-flow studies allowed determination of the time courses of the various redox species during their reduction by L-lactate. The flavin and the haem reduction time courses were biphasic. In the initial fast phase the reduction of flavin monitored by absorbance measurements is accomplished with a rate constant kF = 360 s-1. The reduction of the haem lags the reduction of flavin with a rate constant kH = 170 s-1. The appearance of flavin free radical is slower than the reduction in flavin absorbance and occurs with a rate constant close to that of the reduction of the haem. At saturating L-lactate concentration the initial rapid phase (up to 15 ms) involved in the overall turnover can be adequately simulated with a two-step reaction scheme. The main difference between the enzymes lies especially at the level of the first step of electron exchange between bound lactate and flavin, which for the H-enzyme is no longer the rate-limiting step in the haem reduction and becomes 8-fold faster than in the Sx-enzyme. Consequently in the H-enzyme for the following step, the intramolecular transfer from flavin hydroquinone to oxidized haem, a reliable evaluation of the rate constants becomes possible. Preliminary values are k+2 = 380 s-1 and k-2 = 120 s-1 at 5 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)
    [Abstract] [Full Text] [Related] [New Search]