Further characterization of the magnesium chelatase in isolated developing cucumber chloroplasts
- Clemson Univ., SC (USA)
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{sub 50} 10 and 3 millimolar, respectively). Mg-chelatase was also inhibited by N-ethylmaleimide (I{sub 50}, 50 micromolar) and the metal ion chelators 2,2{prime}-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.
- DOE Contract Number:
- FG09-89ER13989
- OSTI ID:
- 5267448
- Journal Information:
- Plant Physiology; (United States), Journal Name: Plant Physiology; (United States) Vol. 95:4; ISSN 0032-0889; ISSN PLPHA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
59 BASIC BIOLOGICAL SCIENCES
ALKALINE EARTH METALS
ATP
BIOLOGICAL PATHWAYS
BIOSYNTHESIS
CARBOXYLIC ACIDS
CELL CONSTITUENTS
CHELATING AGENTS
CHLOROPHYLL
CHLOROPLASTS
CUCUMBERS
ELEMENTS
ENZYME ACTIVITY
FOOD
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
MAGNESIUM
MAGNOLIOPHYTA
MAGNOLIOPSIDA
METALS
NUCLEOTIDES
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHYTOCHROMES
PIGMENTS
PLANTS
PORPHYRINS
PROTEINS
PROTOPORPHYRINS
SUBSTRATES
SYNTHESIS
VEGETABLES