Reaction intermediates and single turnover rate constants for the oxidation of heme by human heme oxygenase-1.
Heme oxygenase converts heme to biliverdin, iron, and CO in a reaction with two established intermediates, alpha-meso-hydroxyheme and verdoheme. Transient kinetic studies show that the conversion of Fe(3+)-heme to Fe(3+)-verdoheme is biphasic. Electron transfer to the heme (0.11 s(-1) at 4 degrees C and 0.49 s(-1) at 25 degrees C) followed by rapid O(2) binding yields the ferrous dioxy complex. Transfer of an electron (0.056 s(-1) at 4 degrees C and 0.21 s(-1) at 25 degrees C) to this complex triggers the formation of alpha-meso-hydroxyheme and its subsequent O(2)-dependent fragmentation to Fe(3+)-verdoheme. The conversion of Fe(3+)-verdoheme to Fe(3+)-biliverdin is also biphasic. Thus, reduction of Fe(3+) to Fe(2+)-verdoheme (0.15 s(-1) at 4 degrees C and 0.55 s(-1) at 25 degrees C) followed by O(2) binding and an electron transfer produces Fe(3+)-biliverdin (0.025 s(-1) at 4 degrees C and 0.10 s(-1) at 25 degrees C). The conversion of Fe(3+)-biliverdin to free biliverdin is triphasic. Reduction of Fe(3+)-biliverdin (0.035 s(-1) at 4 degrees C and 0.15 s(-1) at 25 degrees C), followed by rapid release of Fe(2+) (0.19 s(-1) at 4 degrees C and 0.39 s(-1) at 25 degrees C), yields the biliverdin-enzyme complex from which biliverdin slowly dissociates (0.007 s(-1) at 4 degrees C and 0.03 s(-1) at 25 degrees C). The rate of Fe(2+) release agrees with the rate of Fe(3+)-biliverdin reduction. Fe(2+) release clearly precedes biliverdin dissociation. In the absence of biliverdin reductase, biliverdin release is the rate-limiting step, but in its presence biliverdin release is accelerated and the overall rate of heme degradation is limited by the conversion of Fe(2+)-verdoheme to the Fe(3+)-biliverdin.
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Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco, California 94143-0446, USA.
Ortiz de Montellano
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D000818:Animals; D001664:Biliverdine / Q000378:metabolism*; D005290:Ferric Compounds / Q000378:metabolism; D006418:Heme / Q000031:analogs & derivatives* / Q000378:metabolism*; D006419:Heme Oxygenase (Decyclizing) / Q000378:metabolism*; D051547:Heme Oxygenase-1; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008565:Membrane Proteins; D008956:Models, Chemical; D051381:Rats; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013997:Time Factors
D005290:Ferric Compounds; D008565:Membrane Proteins; C099546:alpha-hydroxyheme; C099547:verdoheme; D006418:Heme; C496541:HMOX1 protein, human; D006419:Heme Oxygenase (Decyclizing); D051547:Heme Oxygenase-1; D001664:Biliverdine