Hirozo Oh-oka

The 8 kDa polypeptide in photosystem I is a probable candidate of an iron-sulfur center protein coded by the chloroplast gene frxA

The N‐terminal sequence of the 8 kDa polypeptide isolated from spinach photosystem I (PS I) particles was determined by a gas‐phase sequencer. The sequence showed the characteristic distribution of cysteine residues in the bacterial‐type ferredoxins and was highly homologous to that deduced from the chloroplast gene frxA of liverwort, Marchantia polymorpha. It is strongly suggested that the 8 kDa polypeptide has to be an apoprotein of one of the iron‐sulfur center proteins in PS I particles.

Complete amino acid sequence of 33 kDa protein isolated from spinach photosystem II particles

The amino acid sequence of the 33 kDa protein isolated as an extrinsic protein from spinach photosystem II particles was determined by using solid‐phase sequencing and conventional procedures. The 33 kDa protein was found to be composed of 248 amino acid residues, to lack histidine and to have a molecular mass of 26 663 Da, which was considerably smaller than the value deduced from SDS‐polycrylamide gel electrophoresis. The sequence of the 33 kDa protein was compared with those of the bacterial Superoxide dismutases (SOD) with Mn atoms at an active site. A part of the sequence of the 33 kDa protein was similar to a region in Mn‐SODs from Bacillus stearothermophilus and Escherichia coli, which was expected to be the Mn‐binding site.

The major carotenoid in all known species of heliobacteria is the C30 carotenoid 4,4'-diaponeurosporene, not neurosporene.

Abstract The carotenoids of five species of heliobacteria (Heliobacillus mobilis, Heliophilum fasciatum, Heliobacterium chlorum, Heliobacterium modesticaldum, and Heliobacterium gestii) were examined by spectroscopic methods, and the C30 carotene 4,4′-diaponeurosporene was found to be the dominant pigment; heliobacteria were previously thought to contain the C40 carotenoid neurosporene. In addition, trace amounts of the C30 diapocarotenes diapolycopene, diapo-ζ-carotene, diapophytofluene, and diapophytoene were also found. Up to now, diapocarotenes have been found in only three species of chemoorganotrophic bacteria, but not in phototropic organisms. Furthermore, the esterifying alcohol of bacteriochlorophyll g from all known species of heliobacteria was determined to be farnesol (C15) instead of the usual phytol (C20). Heliobacteria may be unable to produce geranylgeranyol (C20).

Two molecules of cytochrome c function as the electron donors to P840 in the reaction center complex isolated from a green sulfur bacterium, Chlorobium tepidum

A photoactive reaction center complex was isolated from a thermophilic green sulfur bacterium, Chlorobium tepidum under anaerobic conditions. The electron transfer occurred from heme c to the photo‐oxidized reaction center chlorophyll, P840+, with a half time ( ) of 110 or 340 μs at 24 or 12°C, respectively. Optical measurements under multiflash excitations indicated that two hemes function as the immediate electron donors to P840+. SDS‐PAGE analysis of the RC complex in combination with the N‐terminal amino acid sequence analyses revealed five subunit bands; a core protein (65 kDa), the light harvesting bacteriochlorophyll a protein (41 kDa), a protein with 2[4Fe‐4S] clusters (31 kDa), monoheme cytochrome c (22 kDa), and a 18‐kDa protein whose function is unknown. The reaction center complex, thus, contains two molecules of cytochrome c per P840.

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll 663, is chlorophyll a esterified with Δ 2,6-phytadienol

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll (BChl) 663, was isolated at high purity by an improved purification procedure from a crude reaction center complex, and the molecular structure was determined by fast atom bombardment mass spectroscopy (FAB-mass), 1H- and 13C-NMR spectrometry, double quantum filtered correlation spectroscopy (DQF-COSY), heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) spectral measurements. BChl 663 was 2.0 mass units smaller than plant Chl a. The NMR spectra showed that the macrocycle was identical to that of Chl a. In the esterifying alcohol, a singlet P71 signal was observed at the high-field side of the singlet P31 signal in BChl 663, while a doublet peak of P71 overlapped that of P111 in Chl a. A signal of P7-proton, seen in Chl a, was lacking, and the P6-proton appeared as a triplet signal near the triplet P2-proton signal in BChl 663. These results indicate the presence in BChl 663 of a C=C double bond between P6 and P7 in addition to that between P2 and P3. The structure of BChl 663 was hence concluded to be Chl a esterified with 2,6-phytadienol instead of phytol. In addition to BChl 663, two molecules of the 132-epimer of BChl a, BChl a′, were found to be present per reaction center, which may constitute the primary electron donor.

Type 1 Reaction Center of Photosynthetic Heliobacteria

The reaction center (RC) of heliobacteria contains iron–sulfur centers as terminal electron acceptors, analogous to those of green sulfur bacteria as well as photosystem I in cyanobacteria and higher plants. Therefore, they all belong to the so‐called type 1 RCs, in contrast to the type 2 RCs of purple bacteria and photosystem II containing quinone molecules. Although the architecture of the heliobacterial RC as a protein complex is still unknown, it forms a homodimer made up of two identical PshA core proteins, where two symmetrical electron transfer pathways along the C2 axis are assumed to be equally functional. Electrons are considered to be transferred from membrane‐bound cytochrome c (PetJ) to a special pair P800, a chlorophyll a‐like molecule A0, (a quinone molecule A1) and a [4Fe–4S] center FX and, finally, to 2[4Fe–4S] centers FA/FB. No definite evidence has been obtained for the presence of functional quinone acceptor A1. An additional interesting point is that the electron transfer reaction from cytochrome c to P800 proceeds in a collisional mode. It is highly dependent on the temperature, ion strength and/or viscosity in a reaction medium, suggesting that a heme‐binding moiety fluctuates in an aqueous phase with its amino‐terminus anchored to membranes.

Light-independent isomerization of bacteriochlorophyll g to chlorophyll a catalyzed by weak acid in vitro

Abstract Rapid conversion of bacteriochlorophyll g (BChl g) to chlorophyll a (Chl a) was observed in acetone on addition of acid in the dark. The product, Chl a esterified with farnesol (Chl aF), was identified by liquid chromatography and fast atom bombardment mass spectrometry. Acid-catalyzed formation of 81-OH-Chl aF, a primary electron acceptor in the heliobacterial reaction center, was also observed in diethyl ether in the dark. These results suggest that acid-catalyzed isomerization is a candidate for the chemical evolution of BChl g to the more stable Chl a and that 81-OH-Chl aF can easily be synthesized from BChl g under weakly acidic conditions in the dark.

Function of a PscD Subunit in a Homodimeric Reaction Center Complex of the Photosynthetic Green Sulfur Bacterium Chlorobium tepidum Studied by Insertional Gene Inactivation REGULATION OF ENERGY TRANSFER AND FERREDOXIN-MEDIATED NADP+ REDUCTION ON THE CYTOPLASMIC SIDE

The PscD subunit in the homodimeric “type I” photosynthetic reaction center (RC) complex of the green sulfur bacterium Chlorobium tepidum was disrupted by insertional mutagenesis of its relevant pscD gene. This is the first report on the use of the direct mutagenic approach into the RC-related genes in green sulfur bacteria. The RC complex of C. tepidum is supposed to form a homodimer of two identical PscA subunits together with three other subunits: PscB (FA/FB-containing protein), PscC (cytochrome cz), and PscD. PscD shows a relatively low but significant similarity in its amino acid sequence to PsaD in the photosystem I of plants and cyanobacteria. We studied the biochemical and spectroscopic properties of a mutant lacking PscD in order to elucidate its unknown function. 1) The RC complex isolated from the mutant cells showed no band corresponding to PscD on SDS-PAGE analysis. 2) The growth rate of the PscD-less mutant was slower than that of the wild-type cells at low light intensities. 3) Time-resolved fluorescence spectra at 77 K revealed prolonged decay times of the fluorescence from bacteriochlorophyll c on the antenna chlorosome and from bacteriochlorophyll a on the Fenna-Matthews-Olson antenna protein in the mutant cells. The loss of PscD led to a much slower energy transfer from the antenna pigments to the special pair bacteriochlorophyll a (P840). 4) The mutant strain exhibited slightly less activity of ferredoxin-mediated NADP+ photoreduction compared with that in the wild-type strain. The extent of suppression, however, was less significant than that reported in the PsaD-less mutants of cyanobacterial photosystem I. The evolutionary relationship between PscD and PsaD was also discussed based on a structural homology modeling of the former.

An overview on chlorophylls and quinones in the photosystem I-type reaction centers

Minor but key chlorophylls (Chls) and quinones in photosystem (PS) I-type reaction centers (RCs) are overviewed in regard to their molecular structures. In the PS I-type RCs, the prime-type chlorophylls, namely, bacteriochlorophyll (BChl) a′ in green sulfur bacteria, BChl g′ in heliobacteria, Chl a′ in Chl a-type PS I, and Chl d′ in Chl d-type PS I, function as the special pairs, either as homodimers, (BChl a′)2 and (BChl g′)2 in anoxygenic organisms, or heterodimers, Chl a/a′ and Chl d/d′ in oxygenic photosynthesis. Conversions of BChl g to Chl a and Chl a to Chl d take place spontaneously under mild condition in vitro. The primary electron acceptors, A0, are Chl a-derivatives even in anoxygenic PS I-type RCs. The secondary electron acceptors are naphthoquinones, whereas the side chains may have been modified after the birth of cyanobacteria, leading to succession from menaquinone to phylloquinone in oxygenic PS I.

Novel carotenoid glucoside esters from alkaliphilic heliobacteria

Pigments of three species of alkaliphilic heliobacteria of the genus Heliorestis, H. daurensis, H. baculata and an undescribed species Heliorestis strain HH, were identified using spectroscopic methods. In these species, bacteriochlorophyll g esterified with farnesol was present, as for other heliobacteria. The carotenoids consisted of 4,4′-diaponeurosporene, also found in other heliobacteria, plus the novel pigments OH-diaponeurosporene glucoside esters (C16:0 and C16:1). In addition, trace amounts of biosynthetic intermediates, OH-diaponeurosporene and OH-diaponeurosporene glucoside, were found. Trace amounts of a carotenoid with 20 carbon atoms, 8,8′-diapo-ζ-carotene, were also found in these species as well as in the non-alkaliphilic heliobacteria. The non-alkaliphilic species Heliophilum fasciatum also contained trace amounts of the two OH-diaponeurosporene glucoside esters. The results are used to predict the pathway of carotenoid biosynthesis in heliobacteria.