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Effect of iron on activity of soybean multi‐subunit acetyl‐coenzyme A carboxylase
Author(s) -
Plank David W.,
Gengenbach Burle G.,
Gronwald John W.
Publication year - 2001
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1034/j.1399-3054.2001.1120206.x
Subject(s) - pyruvate carboxylase , biochemistry , size exclusion chromatography , protein subunit , acetyl coa carboxylase , chloroplast , enzyme , incubation , chemistry , biology , chromatography , gene
Multi‐subunit acetyl‐coenzyme A carboxylase (MS‐ACCase; EC 6.4.1.2) isolated from soybean chloroplasts is a labile enzyme that loses activity during purification. We found that incubating the chloroplast stromal fraction under anaerobic conditions or in the presence of 5 m M FeSO 4 stimulated ACCase (acetyl‐CoA→malonyl‐CoA) and carboxyltransferase (malonyl‐CoA→acetyl‐CoA) activity. Fe‐stimulation of activity was associated with 59 Fe binding to a stromal protein fraction. ACCase and carboxyltransferase activities measured in the stromal protein fraction containing bound 59 Fe were 2‐fold and 6‐fold greater, respectively, than the control (stromal fraction not pretreated with FeSO 4 ). Superose 6 gel filtration chromatography indicated 59 Fe comigrated with stromal protein of approximately 180 kDa that exhibited carboxyltransferase activity, but lacked ACCase activity. Anion exchange (Mono‐Q) chromatography of the Superose 6 fraction yielded a protein peak that was enriched in carboxyltransferase activity and contained protein‐bound 59 Fe. Denaturing gels of the Mono‐Q fraction indicated that the 180‐kDa protein was composed of a 56‐kDa subunit that was bound by an antibody raised against a synthetic β ‐carboxyltransferase ( β ‐CTase) peptide. Incubation of the Mono‐Q carboxyltransferase fraction with increasing concentrations of iron at a fixed substrate concentration resulted in increased initial velocities that fit well to a single rectangular three parameter hyperbola (v=v o +V max [FeSO 4 ]/K m +[FeSO 4 ]) consistent with iron functioning as a bound activator of catalysis. UV/Vis spectroscopy of the partially purified fraction before and after iron incubation yielded spectra consistent with a protein‐bound metal cluster. These results suggest that the β ‐CTase subunit of MS‐ACCase in soybean chloroplasts is an iron‐containing enzyme, which may in part explain its labile nature.