Binding and protection of porphyrins by glutathione S-transferases of Zea mays L.
Glutathione S-transferases (GSTs) are multi-functional enzymes, known to conjugate xenobiotics and degrade peroxides. Herein, we report on the potential of four Zea mays GST isoforms (Zm GST I-I, Zm GST I-II, Zm GST II-II and Zm GST III-III) to act as binding and protection proteins. These isoforms bind protoporphyrin IX (PPIX), mesoporphyrin, coproporphyrin, uroporphyrin and Mg-protoporpyhrin, but do not form a glutathione conjugate. The binding is non-covalent and inhibits GSTs enzymatic activity, dependent on the type of the porphyrin and GST isoform tested. I(50) values are in the range of 1 to 10 microM for PPIX, the inhibition by mesoporphyrin and Mg-protoporphyrin (Mg-PPIX) is two to five times less. The mode of binding is non-competitive for the hydrophobic substrate and competitive for glutathione. Binding affinities (K(D) values) of the GST isoforms are between 0.3 and 0.8 microM for coproporphyrin and about 2 microM for mesoporphyrin.Zm GST III-III prevents the nonenzymatic autoxidation of protoporphyrinogen to the phytotoxic PPIX. Zm GST II-II can reduce the oxidative degradation of hemin. This points to a specific ligand role of distinct GST isoforms to protect tetrapyrroles in the plant cell.
Lehrstuhl für Physiologie und Biochemie der Pflanzen, Universität Konstanz, D-78457 Constance, Germany.
Barbara
B
Lederer
Peter
P
Böger
D005453:Fluorescence; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006427:Hemin / Q000378:metabolism; D010084:Oxidation-Reduction; D010940:Plant Proteins / Q000378:metabolism*; D011166:Porphyrins / Q000378:metabolism*; D011524:Protoporphyrins / Q000378:metabolism; D003313:Zea mays / Q000201:enzymology*
D010940:Plant Proteins; D011166:Porphyrins; D011524:Protoporphyrins; C011539:protoporphyrinogen; D006427:Hemin; D005982:Glutathione Transferase