The magnesium chelation step in chlorophyll biosynthesis

Technical Report · Wed Oct 31 23:00:00 EST 1990

DOI:https://doi.org/10.2172/6074407· OSTI ID:6074407

Weinstein, J

In photosynthetic organisms, the biogenesis of energy transducing membranes requires the coordinate synthesis of prosthetic groups, proteins, and various lipids. Two of the major prosthetic groups, chlorophyll and heme, share a common biosynthetic pathway that diverges at the point of metal insertion into protoporphyrin IX (Proto). Insertion of iron leads to the formation of hemes, while insertion of magnesium is the first step unique to chlorophyll formation. This project is directed toward identifying the enzyme(s) responsible for magnesium chelation and elucidating the mechanism which regulates the flux of precursors through the branch point enzymes in isolated chloroplasts. Using intact chloroplasts from greening cucumber cotyledons, we have confirmed the ATP requirement for Mg-Proto formation. Use of non-hydrolyzable ATP analogs, uncouplers and ionophores has led to the conclusions that ATP hydrolysis is necessary, but that this hydrolysis is not linked to the requirement for membrane intactness by transmembrane ion gradients or electrical potentials. The enzyme(s) are flexible with respect to the porphyrin substrate specificity, accepting porphyrins with -vinyl, -ethyl, or -H substituents at the 2 and 4 positions. The activity increases approximately four-fold during greening. Possible physiological feedback inhibitors such as heme, protochlorophyllide, and chlorophyllide had no specific effect on the activity. The activity has now been assayed in barely, corn and peas, with the system from peas almost ten-fold more active than the cucumber system. Work is continuing in pea chloroplasts with the development of a continuous assay and investigation of the feasibility of characterizing an active, organelle-free preparation. 6 figs.

• Research Organization: Clemson Univ., SC (USA). Dept. of Biological Sciences
• Sponsoring Organization: DOE/ER
• DOE Contract Number: FG09-89ER13989
• OSTI ID: 6074407
• Report Number(s): DOE/ER/13989-2; ON: DE91006619
• Country of Publication: United States
• Language: English

Related Subjects

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59 BASIC BIOLOGICAL SCIENCES
ACID ANHYDRASES
ALKALINE EARTH METALS
ATP-ASE
BARLEY
BIOLOGICAL PATHWAYS
BIOSYNTHESIS
CARBOXYLIC ACIDS
CELL CONSTITUENTS
CEREALS
CHELATING AGENTS
CHLOROPHYLL
CHLOROPLASTS
CUCUMBERS
DOCUMENT TYPES
ELEMENTS
ENZYMES
FOOD
GRAMINEAE
HEME
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROLASES
LEGUMINOSAE
LILIOPSIDA
MAGNESIUM
MAGNOLIOPHYTA
MAGNOLIOPSIDA
MAIZE
MEMBRANES
METALS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOSPHOHYDROLASES
PHOTOSYNTHETIC MEMBRANES
PHYTOCHROMES
PIGMENTS
PISUM
PLANTS
PORPHYRINS
PROGRESS REPORT
PROTEINS
PROTOPORPHYRINS
SYNTHESIS
VEGETABLES