reactio n centre complex; F e - S x , bound to th e core complex p ro tein s P S I - A
a n d P S I - B , an d F e - S ^ and F e - S g housed on a p erip h erally bound
polypeptide, P S I - C . The stru ctu re and function of th ese cen tres h a s been
extensively researched and is discussed below.
Iron-Sulphur Centre Fe-Sx
Chem ical reduction of Fe-S centres A an d B followed by low te m p e ra tu re illum ination gives rise to an o th er photoreduced E P R signal coupled to th e photo-oxidation of P700. This com ponent h a s g v alu es of 2.04, 1.88, an d 1.78 an d an EPR line shape consistent w ith identification as a modified iro n -su lp h u r centre (Evans et al., 1975). The E PR sp ectru m of Fe-Sx is bro ad er th a n would be expected of a n iro n -su lp h u r cen tre and it h a s a n u n u su a l tem p eratu re dependence b u t th e re is no evidence to suggest th a t it is liganded by an y th in g o th er th a n cysteine resid u es (see section 1.6.1). F lash induced difference spectra, achieved u n d e r sim ilar conditions to E PR experim ents, confirm th e identification of th is com ponent as a n iro n -su lp h u r centre (Koike and K atoh, 1982; P a r r e tt et a l , 1989).
D eterm in atio n of th e precise chemical n a tu re of Fe-Sx b a s proven difficult w ith experim ental evidence indicative of b o th [2Fe-2S] a n d [4Fe- 4S] clusters. These anom alies are due largely to th e close proxim ity of Fe- Sx to iro n -su lp h u r centres A a n d B w hich re su lts in inconsistencies of d a ta in te rp re ta tio n . However, early M ossbauer experim ents identified Fe-Sx to be a 4Fe-4S iro n -su lp h u r centre (E. H. E vans et al., 1981). F u rth e r
M ossbauer studies, using core complex photosystem -I devoid of its two te rm in a l iro n -su lp h u r centres, confirmed the findings of E. H. E v an s et al. (1981) d em o n stratin g th a t both oxidised and reduced sp ectra show ed isom er shifts typical of those expected in a 4Fe-4S centre (P etro u leas et at., 1989).
EXAFS stu d ies on photosystem -I particles containing Fe-Sy^\g and
Fe-Sx proved am biguous indicating Fe-Sx to be e ith er 2 X [2Fe-2S] clu sters or a d isto rted 4Fe-4S clu ster (M cDermott et at., 1988). However, experim ents on Fe-S^xe depleted particles h ad sim u lated sp ectra m ost ch aracteristic of a 4Fe-4S clu ster (M cDermott et at., 1989) a n d i t is now g enerally agreed th a t Fe-Sx is a four iron centre.
P700W e-Sx back reaction kinetics (Sauer et al.j 1979; Golbeck an d C ornelius, 1986), m idpoint potential (Cham orovsky a n d C am m ack, 1982; P a r r e tt et al., 1989), photo-accum ulation (Evans et a l.y 1975) a n d position on th e core complex all suggest th a t it is an in te rm e d iate redox com ponent situ a te d betw een Aj an d Fe-S^xB i^^ ^ system of lin e a r electron tra n s p o rt from P700 to th e term in a l iron-sulphur centres. Two p ath w ay s of electron flow have been postulated: a lin ear p a th from X->B->A or a b ran ch ed system of delivery to eith er Fe-S A or B (See section 1.6.6 ), b u t in rea lity th e role of Fe-Sx i^ down stream reduction of iro n -su lp h u r c en tres h a s not been directly dem o n strated a t physiologically rele v an t te m p e ra tu re s.
Iron-Sulphur Centres Fe-S^xB
spectroscopy as a n oxidised-m inus-reduced absorption b a n d w ith a bleach cen tred a t 430 nm . Kinetic m easu rem en ts of the 430 nm optical tra n s ie n t show ed th a t it back reacted w ith P700^ w ith a of 30 m s and a m idpoint p o te n tia l of -520 mV w as m easured (H iyam a an d Ke, 1971; Ke, 1972). T hese in itia l findings lead w orkers to believe th a t th is redox com ponent, labelled P430, w as an iron-sulphur centre and th e electron acceptor involved in p rim ary charge separation w ith P700. A t aro u n d th e sam e tim e M alkin an d B earden (1971) d em onstrated low te m p e ra tu re photoreduction of a bound redox com ponent giving a n E P R signal w ith g v alu es of 2.05, 1.94, an d 1.86 characteristic of a n iro n -su lp h u r centre. S u b seq u en t E PR studies identified a fu rth e r iro n -su lp h u r centre w ith g v alu es of 2.07, 1.92, an d 1.89 (B earden and M alkin, 1972; E v an s et al., 1972). It was proposed th a t these two sets of signals rep re se n te d two m agnetically in te rac tin g iron su lp h u r centres existing on a single polypeptide w ith a n interactive E PR spectra a t g = 2.05, 1.94, 1.92 an d 1.89 (E vans et al., 1974). S au er et al. (1977) confirm ed th is show ing th a t P430 could hold two electron equivalents on successive flashes. T he iron- su lp h u r centres w ithin th e acceptor complex were labelled Fe-S^ a n d Fe-Sg. Redox m easu rem en t of Fe-S A an d B placed th e m idpoint p o ten tials a t - 530 mV an d -580 mV respectively (Ke et al., 1973; E v an s et al., 1974).
Identification of Fe-S^ and Fe-Sg as 4Fe-4S cen tres cam e from E PR (C am m ack an d E vans, 1975), EXAFS (M cD erm ott et al., 1988), an d M ossbauer stu d ies (Evans et al., 1979). F u rth erm o re, sequence stu d ies on th e 9 kD a gene product of psaC , th e polypeptide m ost likely to house th e
Fe-S^/B centre (ref. section 1.6 .2), show a d istrib u tio n of cysteine c h aracteristic of those found in bacterial 2 X [4Fe-4S] ferredoxins (Oh-oka et al., 1988ab).
Fe-SA\B pl&ys a n essential role in forw ard electron tra n s fe r to soluble ferredoxin b u t th e exact pathw ay of tra n sfe r from F e - S x to ferredoxin
th ro u g h th e Fe-S^xB complex is uncertain.