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The system concerned with reduction of the pyridine nucleotide coenzymes depends on two proteins: a non-heme iron protein associated closely with the photochemical reaction center (Photosystem I) and a flavoprotein that functions as a reductase. The non-heme iron protein, previously known as the “methemoglobin reducing factor (MRF),” as “photosynthetic pyridine nucleotide reductase (PPNR),” and as the “TPN reducing factor (TRF),” is recognized as a member of a class of proteins now called “ferredoxin (Fd).” The FAD-containing flavoprotein known now as “Fd:NADP oxidoreductase (FNR)” was known previously as “TPNH diaphorase,” as “pyridine nucleotide transhydrogenase,” and as “TPN reductase.” An abbreviated review of the investigations leading to the isolation and characterization of these two proteins that have extended over a number of years and in a number of different laboratories is presented herein. This has unavoidably led to a synonymy and I describe here my personal recollection of how this came about.

The discovery of bound iron–sulfur proteins in chloroplast membranes was made in 1971 using EPR spectroscopy. This chapter describes the events leading up to that discovery and the conclusions drawn concerning the structure and function of these proteins.

This article presents a brief historic account of the discovery of the electron acceptor P430 of green-plant Photosystem I (PS I) by optical spectroscopy. The spectro-kinetic and redox properties of P430 are described. A brief description of the recently-discovered P430 analogs, C-P430, in green-sulfur bacteria (Kusumoto et al., 1995) and its spectrokinetic properties are also included.

This chapter summarizes some of the results obtained by the authors, in a collaborative work done in 1976–1977, about the Photosystem I electron acceptors. The results are placed in a historical perspective and discussed in the light of the present knowledge on the Photosystem I reaction center.

Green plant photosystem I (PS I) not only binds a chlorophyll -binding, membrane-intrinsic antenna complex (LHCI) that is associated with the PS I core complex under almost all physiological conditions, but it can also transiently bind the major chlorophyll -binding light-harvesting complex (LHCII), when the light conditions favor excitation of photosystem II (PS II) and the photosynthetic apparatus is in the so-called state 2. Recently, a low-resolution structure was obtained of a PS I–LHCII supercomplex from . We describe here some of the structural features of this transient complex, and discuss the role of small PS I subunits that are involved in the binding of LHCII.We also discuss structural features of the PS I complex of the green algae , which has a larger LHCI antenna and shows a more pronounced difference between state 1 and state 2.

Photosystem I is a large membrane protein complex that catalyzes the first step of oxygenic photosynthesis. It can be regarded as a solar energy converter that captures the light from the sun through a large core-antenna system of chlorophylls and carotenoids and transfers the energy into the center of the complex, where the energy is used to catalyze the light-driven transmembrane electron transfer from plastocyanin to ferredoxin.

The recently determined structure of plant photosystem I (PS I) provides the first relatively high-resolution structural model of a supercomplex containing a reaction center and its peripheral antenna. The peripheral antenna of PS I (LHCI) is composed of four gene products (Lhca1–4) that are unique among the chlorophyll a/b binding proteins in their pronounced long-wavelength absorbance and their assembly into dimers. We describe some of the structural features responsible for the unique properties of LHCI and its interaction with the reaction center. The possible architecture of the docking sites for plastocyanin, ferredoxin, ferredoxin:NADP reductase, and LHCII are discussed.

The X-ray crystal structure of cyanobacterial Photosystem I (PS I) has been solved to atomic resolution (PDB entry 1JB0). It provides a structural model for the subunits PsaC, PsaD, and PsaE that comprise the stromal ridge of PS I. Independently, the three-dimensional solution structures of unbound, recombinant PsaC (PDB entry 1K0T) and PsaE (PDB entries 1PSF, 1QP2, and 1GXI) have been solved using NMR spectroscopy.

The IsiA protein accumulates in many cyanobacteria under conditions of iron starvation. It is a Chl -binding protein, which is closely related to the six-transmembrane α-helical antenna family typified by CP43 of PS II. One of its functions is to provide an efficient accessory light harvesting system for Photosystem I (PS I) by forming a ring of 18 IsiA subunits around the trimeric reaction center core. This response is probably to compensate for a drop in the PS I level relative to Photosystem II (PS II) and, the level of phycobiliproteins, in response to iron deficiency. A similar accessory light harvesting system for PS I has been shown to occur in cyanobacteria that do not contain phycobiliproteins, often termed prochlorophytes. This light harvesting system is composed of Pcb proteins that are closely related to IsiA but bind Chl as well as Chl . Unlike IsiA, Pcb proteins can also provide accessory light harvesting systems for PS II. Some cyanobacteria contain phycobiliproteins and Pcb proteins, where both are able to function as light harvesting systems. In one case the Pcb protein seems to bind only Chl () while in another they bind Chl (). Sequence analysis indicates that the IsiA/Pcb proteins have very similar pigment binding sites to those of CP43 and to a lesser extent to the other reaction center proteins of this family and have the capacity to accommodate several different forms of Chl. The six-transmembrane α-helical Chl-binding proteins, of which IsiA and Pcb are members, seem to have evolved from a basic evolutionary building block providing both internal and accessory light harvesting systems for a wide range of photosynthetic organisms.

In this chapter we summarize the results of reports published since 1979 on the Chl -binding light-harvesting complex of PS I. In the first part of this chapter, we review the results that led to our current knowledge of the biochemical properties of the individual gene products constituting LHCI, in particular, the presence and distribution of red-shifted spectral forms and the idea that LHCI, different from LHCII, is organized into heterodimeric complexes.