Abstract:
:Mg-chelatase catalyzes the first step unique to the chlorophyll branch of tetrapyrrole biosynthesis, namely the insertion of Mg into protoporphyrin IX (Proto). Mg-chelatase was assayed in intact chloroplasts from semi-green cucumber (Cucumis sativus, cv Sumter) cotyledons. In the presence of Proto and MgATP, enzyme activity was linear for 50 minutes. Plastid intactness was directly related to (and necessary for) Mg-chelatase activity. Uncouplers and ionophores did not inhibit Mg-Chelatase in the presence of ATP. The nonhydrolyzable ATP analogs, beta,gamma-methylene ATP and adenylylimidodiphosphate, could not sustain Mg-chelatase activity alone and were inhibitory in the presence of ATP (I(50) 10 and 3 millimolar, respectively). Mg-chelatase was also inhibited by N-ethylmaleimide (I(50), 50 micromolar) and the metal ion chelators 2,2'-dipyridyl and 1, 10 phenanthroline (but not to the same degree by their nonchelating analogs). In addition to Proto, the following porphyrins acted as Mg-chelatase substrates, giving comparable specific activities: deuteroporphyrin, mesoporphyrin, 2-ethyl, 4-vinyl Proto and 2-vinyl, 4-ethyl Proto. Mg-chelatase activity and freely exchangeable heme levels increased steadily with greening, reaching a maximum and leveling off after 15 hours in the light. Exogenous protochlorophyllide, chlorophyllide, heme, and Mg-Proto had no measurable effect on Mg-chelatase activity. The potent ferrochelatase inhibitors, N-methylmesoporphyrin and N-methylprotoporphyrin, inhibited Mg-chelatase at micromolar concentrations.
journal_name
Plant Physioljournal_title
Plant physiologyauthors
Walker CJ,Weinstein JDdoi
10.1104/pp.95.4.1189subject
Has Abstractpub_date
1991-04-01 00:00:00pages
1189-96issue
4eissn
0032-0889issn
1532-2548journal_volume
95pub_type
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