| |
[摘要]：The flavoprotein oxidase Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine and N-1-acetylspermine to yield spermidine and 3-aminopropanal or N-acetyl-3-aminopropanal. The kinetic mechanism of the enzyme has been determined with both substrates. The initial velocity patterns arc pingpong, consistent with reduction being kinetically irreversible. Reduction of Fms1 by either substrate is biphasic. The rate constant for the rapid phase varies with the substrate concentration, with limiting rates for reduction of the enzyme of 126 and 1410 s(-1) apparent K-d values of 24.3 and 484 mu M for spermine and N-1-acetylspermine, respectively. The rapid phase is followed by a concentration-independent phase that is slower than turnover. The reaction of the reduced enzyme with oxygen is monophasic, with a rate constant of 402 mM(-1) s(-1) with spermine at 25 degrees C and 204 mM(-1) s(-1) with N-1-acetylspermine at 4 degrees C and pH 9.0. This step is followed by rate-limiting product dissociation. The k(cat)/K-amine-pH profiles are bell-shaped, with an average pK(a) value of 9.3 with spermine and pK(a) values of 8.3 and 9.6 with N-1-acetylspermine. Both profiles are consistent with the active forms of substrates having two charged nitrogens. The pH profiles for the rate constant for flavin reduction show plc values of 8.3 and 7.2 for spermine and N-1-acetylspermine, respectively, for groups that must be unprotonated; these pK(a) values are assigned to the substrate N4. The k(cat)/K-O2 -pH profiles show pK(a) values of 7.5 for spermine and 6.8 for N-1-acetylspermine. With both substrates, the k(cat) value decreases when a single residue is protonated. |
|
