Modified ligands to F(A) and F(B) in photosystem I. Proposed chemical rescue of a [4Fe-4S] cluster with an external thiolate in alanine, glycine, and serine mutants of PsaC

Yean Sung Jung, Ilya R. Vassiliev, Fengyu Qiao, Fan Yang, Donald Ashley Bryant, John H. Golbeck

Research output: Contribution to journalArticle

25 Scopus citations

Abstract

The F(B) and F(A) electron acceptors in Photosystem I (PS I) are [4Fe- 4S] clusters ligated by cysteines provided by PsaC. In a previous study (Mehari, T., Qiao, F., Scott, M. P., Nellis, D., Zhao, J., Bryant, D., and Golbeck, J. H. (1995) J. Biol. Chem. 270, 28108-28117), we showed that when cysteines 14 and 51 were replaced with serine or alanine, the free proteins contained a S = 1/2, [4Fe-4S] cluster at the unmodified site and a mixed population of S = 1/2, [3Fe-4S] and S = 3/2, [4Fe-4S] clusters at the modified site. We show here that these mutant PsaC proteins can be rebound to P700-F(X) cores, resulting in fully functional PSI complexes. The low temperature EPR spectra of the C14X(PsaC)·PS I complexes (where X = S, A, or G) show the photoreduction of a wild-type F(A) cluster and a modified F(B)' cluster, the latter with g values of 2.115, 1.899, and 1.852 and linewidths of 110, 70, and 85 MHz. Since neither alanine nor glycine contains a suitable side group, an external thiolate provided by β-mercaptoethanol has likely been recruited to supply the requisite ligand to the [4Fe-4S] cluster. The EPR spectrum of the C51S(PsaC)·PS I complex differs from that of the C51A(PsaC)·PS I or C51G(PsaC)·PS I complexes by the presence of an additional set of resonances, which may be derived from the serine oxygen- ligated cluster. In all other mutant PSI complexes, a wild-type spin-coupled interaction spectrum appears when F(A) and F(B) are simultaneously reduced. Single turnover flash studies indicate ~50% efficient electron transfer to F(A)/F(B) in the C14S(PsaC)·PS I, C51S(PsaC)·PS I, C14G(PsaC)·PS I, and C51G(PsaC)·PS I mutants and less than 40% in the C14A(PsaC)·PS I and C51A(PsaC)·PS I mutants, compared with ~76% in the PS I core reconstructed with wild-type PsaC. These data are consistent with the measurements of the rates of cytochrome c6-NADP+ reductase activity, indicating lower rates in the alanine mutants. It is proposed that the chemical rescue of a [4Fe-4S] cluster with a recruited external thiolate at the modified site allows the mutant PsaC proteins to rebind to PSI and to function in forward electron transfer.

Original languageEnglish (US)
Pages (from-to)31135-31144
Number of pages10
JournalJournal of Biological Chemistry
Volume271
Issue number49
DOIs
StatePublished - Jan 1 1996

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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