固氮酶催化的放H2反应

MECHANISM OF H2 EVOLUTION BY NITROGENASE

以不同来源的纯化固氮酶定量地研究了在不同 pN_2(N_2分压)条件下,催化 N_2还原和 H_2释放的反应。并对两者之间的关系进行了动力学分析,得出了固氮酶催化 N_2还原和 H_2释放反应的化学计量式:N_2+(8+(8Km(N_2)/[pN_2]))(H^++e^-)→2NH_3+(1+(4Km(N_2)/[pN_2]))H_2从而提出了双位点放 H_2的模式,并合理地解释了在正常条件下,每还原1摩尔 N_2总是放出大于1摩尔 H_2,和 H_2是 N_2还原反应的竞争性抑制剂,而 N_2却是放 H_2反应的非竞争性抑制剂等难以理解的问题。

A modified stoichiometric equation for biological nitrogen fixstion,N_2+(H^++e^-)→2NH_3+H_2,is proposed based uponquantitative study on N_2 reduction and H_2 evolution by purified nitrogenase(fromAzotobacter vinelandii,Klebsiella pneumoniae,Clostridium pasteurianum)under optimalconditions and by kinetic analysis of the interrelationship between these two reactions.In this equation,the H_2/N_2 ratio is not a constant as the current stoichiometricequation,N_2+8(H^++e^-)→2NH_3+H_2,has shown.The amount of H_2 evolved duringnitrogen fixation is variable,it is directly proportional to the apparent Km(N_2)of thenitrogenase(kin N_2 for Av N_2ase is 12.5 kPa,for Kp N_2ase is 13.5 kPa,for Cp N_2aes is4. 8 kPa)and inversely proportional to pN_2.When pN_2=0,the equation changes to2H^++2e^-→H_2;if pN_2=∞,it changes to N_2+8(H^++e^-)→2NH_3+H^2.A double-sitemodel for H_2 evolution catalyzed by nitrogenase is suggested.One site catalyzes H_2evolution accompaning N_2 reduction,and one H_2 evolved per N_2 reduced.This reaction isinhibited by CO or H_2.The other site only catalyzes H_2 evolution which is inhibited byN_2 but not inhibited by CO or H_2,and the amount of H_2 evolved is equal to 4Km(N_2)/[pN_2].It is easy to explain,by this model,why more than one H_2 evolved per N_2reduced and why N_2 is an uncompetitive inhibitor for H_2 evolution,whereas H_2 is acompetitive inhibitor for N_2 reduction.