【摘要】 棕色固氮菌中电子载体Fld直接向固氮酶铁蛋白传递电子。Fld_ox至Fld_R是双电子二步还原反应,极谱半波电位分别为-210、-550 mV。Fld_ox至Fld_SR的中点电位为-280 mV,Fld_SR至Fld_R为-500mV。铁蛋白中点电位为-256mV,加MgATP后为-390 mV。Fld_R与铁蛋白ox组成的电池电动势为244mV,电子传递可自发进行,反应的J△G^o为-23KJ/摩尔,铁蛋白被Fld_R还原的K_a=1.3×120⁴,加入MgATP后△G^o为-10.6KJ/摩尔,K_a=72。因此,未加入MgATP时电子传递反应更易进行。更多还原

【Abstract】 Azotobacter vinelandii flavodoxin （Fld） whcih can serve as electron donorfor nitrogenase was isolated, purified and characterized. The absorption spectraof purified Fld showed peaks at 450 and 600 nm in semireduced state(Fld_SR), at 375and 455 nm in oxidized state (Fld_OX), but gave no peaks in reduced state （Fld_R）.The ratio of A_450nm/A_375nm of purified Fld_ox is 1.45. The reduction of Fldfrom Fld_OX to Fld_R is a two-step reaction, each step being a one-electron process.The half-wave potentials of the two steps estimated by polarography are-210 mVand-580 mV respectively.The midpoint redox potentials (E^0’) measured by coulometry at fixed potentialsand calculated by Nernst’s Equation of Fld_ox to Fld_SR and Fld_SR to Fld_R are-280mV and-500 mV respectively. E^0’ values of Fe protein and Fe protein plusMgATP have been-256 mV and-390mV, respectively. Fld_R transfers electronto Fe-protein directly, which has been confirmed by acetylene reduction activity,but Fld_SR cannot. The ΔG⁰ of electron transfer reaction from Fld_R to Fe protein is-23 KJmol^-1, and equilibrium constant K_a is 1.3×10⁴. However, ΔG⁰ became-10.6 KJ mol^-1 and K_a became 72,when MgATP was added. The results suggestthat electron transfer from Fld_R to Fe protein is easier when MgATP is absent.更多还原