The intermediary acceptor

The intermediary acceptor I is thought to be a complex involving a BPh and one of the monomeric BChls (18,30-34) This view is based on the observation that the reduction of I causes absorption changes in regions of the spectrum attributable to BPh and also in regions normally ascribed to BChl It has been proposed from nanosecond studies on reaction centers having Q reduced that P" is a thermal mixture of P BPh and P BChl , with about 60% of the added electron density on the BPh at room temperature (33) Our recent kinetic measurements (22) and low-temperature picosecond photodichroism stu-... 

Recent picosecond absorbance difference spectroscopy [58] and pigment extraction studies [59] have shown that the intermediary acceptor in green sulfur bacteria is a (probably monomeric) BChl c molecule. As yet no EPR data are available. In H. chlorum the intermediate is probably also a BChl c-like molecule [60]. 

We ascribe the field-induced quenching observed when both Q and the oxygen evolving apparatus were in the reduced state to reversed electron transport from Q to the intermediary acceptor, Pheophytin. The fluorescence is low when Phe is in the reduced state (Klimov et al., 1977), probably due to excitation transfer towards Phe, followed by rapid internal conversion to the ground state (if the electron is actually pushed on towards P680, the same quenching mechanism is expected). By lack of a positive charge to recombine with, the electron returns to Q as soon as the membrane potential decreases. 

If the synthesis starts from glucose molecules, then the initial step is the transfer of glucose residues from UDP-glucose onto an intermediary acceptor-dolichol phosphate (membrane-bound polyprenol phosphate). Dolichol phosphate assists in the synthesis of an... 

Carbon acts as the electron donor for denitrification. The availability of carbon often limits denitrification in anaerobic microsites in non-submerged soils. As a result, the reaction does not go to completion and the intermediaries NO2 and N2O accumulate. Completion of the reaction may also be hindered by low pH. But under uniformly anaerobic conditions NO3 as electron acceptor is more likely to be limiting than carbon as electron donor because NO3 is not regenerated. 

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

The intermediary enolates react also with Michael acceptors. Scheme 12 shows the nitro-alkene trapping process that gives C(6)-derivatized PGs including antiulcer 6-nitro-PGE, and 6-oxo-PGE, (22). 

The intermediary cofactor bound acetyl anion equivalent can be transferred to an aldehyde acceptor, e.g. to acetaldehyde already produced during regular catalytic reaction in which optically active 3-hydroxy-2-butanone (acetoin, an important aroma constituent) is formed. Interestingly, PDCs from different sources differ in stereoselectivity [443] acetoin (I )-143 is obtained using brewer s yeast PDC (ee 28-54%) [444,445] while the enantiomeric (S)-143 is produced preferentially by PDC from wheat germ (ee 16-34%) [446] or from Z. mobilis (ee 24-29%) [445], When glyoxylate 14 (instead of 2) is subjected to decarboxylation in the presence of acetaldehyde, optically active lactaldehyde... 

Several years ago optical spectroscopy on PS II particles provided evidence that before PQ Pheo a functions as an earlier acceptor, with -610 mV [68]. By photoaccumulation it was established that the reduced intermediary acceptor has an EPR signal characteristic of monomeric Pheo a g value 2.0033 0.0003, AB = 12.6 0.3 G) [48,69]. ENDOR work established a good agreement between methyl hyperfine splittings of Pheo" in vivo and monomeric Pheo" in vitro [70]. 

Fig. 5. EPR spectra associated with the reduced intermediary acceptor i [BO"] formed in the reaction-center/cytochrome compiex and measured at different temperatures and microwave-power levels (A, B and C). (D) Redox titration of the I signal. Samples poised at indicated redox potentials and then illuminated for 3 m before being cooled to 7 K and assayed. See text for details. Figure source Tiede, Prince and Dutton (1976) EPR and optical spectroscopic properties of the electron carrier intermediate between the reaction center bacteriochlorophylls and the primary acceptor in Chromatium vinosum. Biochim Biophys Acta. 449 455, 460.

D. Picosecond Spectroscopic Measurement of the Intermediary Electron Acceptor (I) of... 

By analogy to the intermediary electron acceptor in purple bacteria, the absorbance change seen at 670 run in Fig. 5 (D) was tentatively assigned to a BO c molecule. 

In this chapter, we will look at how charge separation takes place in PS-II reaction centers after photoexcitation and at the properties of the PS-II primary electron donor P680. In the following chapter we will discuss the so-called stable primary electron acceptor and the secondary electron acceptor Qb. This will be followed by a discussion of the intermediary electron acceptor, the species that actually accepts the electrons from the photoexcited primary donor P680. We adopt this sequence of presentation because the reduction ofQ was experimentally more readily observed than that of and was quite naturally the first experimentally observed acceptor in the course of photosystem-II research. 

W Klimov, E Dolan, ER Shaw and B Ke (1980) Interaction between the intermediary electron acceptor (pheo-phytin) and a possible plastoquinone-iron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA 77 7227-7231... 

Fig. 1. (A) Model of the photosystem-ll reaction center showing the location of

Fig. 4. (A) EPR spectra of TSF lla particles poised at -450 mV and after 90-s illumination at 295 or 220 K and measured at two different microwave powers. (B) shows effect of microwave power (P) on the amplitude of the photoinduced narrow (singlet) and doublet EPR signals at 7 K, Figure source Klimov, Dolan and Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 98,99 and Klimov, Dolan, Shaw and Ke (1980) Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-lron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA. 77 7228...

Fig. 1. Location of the intermediary electron acceptor FeS-X (a [4Fe 4S] cluster) in the reaction center of photosystem I (A) and in the sequence of electron acceptors (with the years of their discovery shown) (B).

In spite ofthe inherent difficulties cited above in sorting out spectral differences among the three iron-sulfur centers in the electron-transfer chain of photosystem I, two research groups" ° " have independently measured minute differences in the optical spectra ofthe two types of iron-sulfur centers, and used these differences to identify FeS-X as the intermediary electron acceptor located between A and FeS-A/B, and to confirm the electron-transfer sequence from FeS-X to FeS-A/B. We will first review the attempt made by Liineberg, Fromme, Jekow and Schlodder" to identify FeS-X as the intermediary electron carrier located between A] andFeS-A/B. 

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