Quantitative Determination of the Distribution between Two Photosystem II-Mediated Oxidation Reactions in Oscillatoria chalybea: A Comparison between Water Oxidation and Hydrogen Peroxide Decomposition

M ass sp ec tro m etric analysis reveals th a t oxygen ev o lu tio n m easu red as th e co n seq u en ce o f sh o rt s a tu ra tin g light flashes in th y lak o id p re p a ra tio n s o f the filam en to u s cy an o b a c te riu m O scilla toria chalybea consists o f tw o p o rtio n s , one com in g from p h o to sy n th e tic w a te r sp littin g an d o n e co m in g from H 20 2 deco m p o sitio n . T h is H 20 2 d eco m p o sitio n is p h o to sy stem II-m ed ia ted an d it is the S -sta te system w hich oxidizes H 20 2 to give p ro to n s an d oxygen. W a te r is n e ith e r in te rm ed ia te n o r seem s it to be the o rig in o f the reac tio n . A t th e h igh oxygen p a r tia l p ressu re o f n o rm al a ir H 20 2 p ro d u c tio n an d its d eco m p o sitio n exceeds m an y fo ld the H 20 sp littin g reac tio n . H 20 2 p ro d u c tio n seem s to com e fro m p h o to sy stem II b u t is n o t neces­ sarily p ro d u c e d o n th e accep to r side o f ph o to sy stem II in the sense o f a M eh ler type reac tio n . F ro m th e re ac tio n ra te an d the observed labeling d ensity , it is in ferred th a t p ro d u c tio n an d d e co m p o sitio n m u st tak e p lace w ith in the sam e reac tio n site w hich m ig h t be fo r b o th , the p ro d u c tio n a n d d eco m p o sitio n , th e S -sta te system . In th is sense, H 20 2 m ig h t be th e p ro d u c t o f th e S -s ta te system an d seem s som ehow assoc ia ted w ith S2 o r S3. T h u s, if oxygen e v o lu tio n is m easu red as th e co n sequence o f sh o rt sa tu ra tin g ligh t flashes in an a m b ien t a tm o sp h e re o f 21% oxygen , m ass sp ec tro m etry reveals a flash p a tte rn w hich bears th e oxygen label o f the a m b ie n t a tm o sp h e re an d w hich has n o t m uch in co m m o n w ith an usua l K o k sequence. Such a p a tte rn m ig h t s ta r t in the first th ree flashes m ore o r less as a K ok p a tte rn w ou ld , b u t is th en in the p a tte rn p o rtio n usually ch arac te ris tic for stead y -s ta te oxygen ev o lu tio n c h arac te rized by a p e rio d ic ity o f tw o in d ica tin g th a t H 20 2 d eco m p o sitio n requ ires on ly tw o ligh t q u a n ta . A t high oxygen p a r tia l p ressu re (e.g. in 21% l60 2) b o th reac tio n p o rtio n s can be q u a n tita tiv e ly d e te r­ m in ed by lab e lin g th e assay w ith H 2180 an d m easu rin g th e evolved ( l60 : : l80 ) / l80 2 ra tio w hich is rep re sen ta tiv e fo r w a ter sp litting . T he m easu red ev o lu tio n o f l60 2 (m ass 32) rep re sen ts H 20 2 d e co m p o sitio n , if th e IA0 2 p o r tio n ca lcu la ted from the m easu red m ass 34/36 ra tio is su b tra c t­ ed. A t low oxygen p a rtia l p ressu re th e H 20 2 fo rm in g an d d eco m p o sin g reac tio n is largely su p ­ p ressed an d oxygen ev o lu tio n from w ater-sp littin g p revails . A s a h y p o th esis , H 20 2 p ro d u c tio n a n d its d e co m p o sitio n m igh t be a defective p e rfo rm an ce o f p h o to sy stem II a t h igh a m b ien t oxygen p a r tia l p ressu re in these cy an o b ac te ria , p e rh ap s d u e to the p rin c ip a l absence o f tw o o f the ex trin sic p ep tid es from p h o to sy stem II.