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Title: CRITICAL ANALYSIS OF THE EXTINCTION COEFFICIENT AND MIDPOINT POTENTIAL OF CHLOROPLAST CYTOCHROME F

Author
item METZGER, SABINE - UNIV OF ILLINOIS URBANA
item CRAMER, WILLIAM - PURDUE UNIVERSITY
item WHITMARSH, CLIFFORD

Submitted to: Biochimica et Biophysica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plants depend on photosynthesis to provide the energy needed to make food and fiber. The photosynthetic process is driven by light and depends on proteins, which act as molecular machines. Many of these proteins contain iron atoms that act as electron carriers, thereby transporting energy. Plants have a critical need for iron in building the photosynthetic machinery. We have isolated an iron containing protein known as cytochrome f that is necessary for all photosynthetic organisms and determined its electrical properties. This work will contribute directly to determine how iron proteins contribute to the photosynthetic performance of plants. Understanding these molecular steps in photosynthesis is essential for developing genetically engineered plants designed to perform well under a wide range of environmental conditions.

Technical Abstract: In oxygenic photosynthesis the cytochrome bf complex links electron transport between photosystem II and photosystem I. The largest subunit of the complex is cytochrome f which is anchored in the membrane by a transmembrane alpha helix located near the carboxyl end. Here we report the extinction coefficient and pH behavior of the midpoint redox potential of the soluble domain of turnip cytochrome f. We found the extinction coefficient for turnip cytochrome f to be significantly higher than previously published values. For reasons discussed in the text, we believe earlier determinations underestimated the extinction coefficient. We found the midpoint potential of soluble turnip cytochrome f to be 362 mV at pH 7.5. The midpoint potential was pH-independent from 5.0 to 8.5, and pH-dependent from pH 8.5 to 10.5 with a pK on the oxidized form near 9. We found that some samples of purified cytochrome f were modified after being stored at -20# C, so that 30 percent of the heme exhibited a midpoint potential of -165 mV (pH 7.5) and a shift in the peak of the alpha-band absorption spectrum from 554 nm to 552 nm.