Interaction Site for Soluble Cytochromes on the Tetraheme Cytochrome Subunit Bound to the Bacterial Photosynthetic Reaction Center Mapped by Site-Directed Mutagenesis

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• 作者：Artur Osyczka ; Kenji V. P. Nagashima ; Satoshi Sogabe ; Kunio Miki ; Makoto Yoshida ; Keizo Shimada ; and Katsumi Matsuura
• 刊名：Biochemistry
• 出版年：1998
• 出版时间：August 25, 1998
• 年：1998
• 卷：37
• 期：34
• 页码：11732 - 11744
• 全文大小：176K
• 年卷期：v.37,no.34(August 25, 1998)
• ISSN：1520-4995

文摘

The crystallographic structure of the Blastochloris (formerly called Rhodopseudomonas) viridistetraheme cytochrome subunit bound to the photosynthetic reaction center (RC) suggests that all fourhemes are located close enough to the surface of the protein to accept electrons from soluble cytochromec₂. To identify experimentally the site of this reaction we prepared site-directed mutants of Rubrivivaxgelatinosus RCs with surface charge substitutions in the bound cytochrome subunit and studied the kineticsof their reduction by soluble cytochromes (mitochondrial horse cytochrome c, Blc. viridis cytochrome c₂,and Rvi. gelatinosus cytochrome c₈). In comparison with the wild-type, the mutants E79K (glutamate-79substituted by lysine), E93K (glutamate-93 substituted by lysine), and E85K (glutamate-85 substituted bylysine) located near the solvent-exposed edge of low-potential heme 1, the fourth heme from the specialpair of bacteriochlorophyll, exhibited decreased second-order rate constants for the reaction between thetetraheme subunit and the soluble cytochromes. Double charge substitutions in this region: E79K/E85K(glutamate-79 and -85 both replaced by lysine) and E93K/E85K (glutamate-93 and -85 both replaced bylysine) appeared to show an additive inhibitory effect. Mutations in other charged regions did not alterthe kinetics of electron transfer between bound and soluble cytochromes. In light of the available structuralinformation on Blc. viridis RC, these results indicate that the cluster of acidic residues immediatelysurrounding the distal heme 1 of the RC-bound tetraheme subunit forms an electrostatically favorablebinding site for soluble cytochromes. Thus, all four hemes in the subunit seem to be directly involved inthe electron transfer toward the photo-oxidized special pair of bacteriochlorophyll. On the basis of thesefindings, a model is proposed for the hypothetical cytochrome c₂-RC transient complex for Blc. viridis.