Chloride Requirement for Oxygen Evolution in Photosynthesis

Abstract

Dedicated to O t t o W a r b u r g on the occasion of his 80th birthdayThe role of chloride in photosynthetic oxygen evolution was reinvestigated by determining the effect of this ion on photochemical reactions of chloroplasts in which oxygen either is or is not produced. The chloroplasts used were isolated from normal spinach leaves. The level of chloride in the reaction mixture was controlled by washing the isolated chloroplasts and by avoiding a chloride contamination from the water and chemicals used. Chloride was found to be essential for each of the photochemical reactions of chloroplasts in which oxygen is produced. These included (a) photo reduction of TPN, (b) photophosphorylation of the noncyclic type in which TPN or ferricyanide reduction is coupled with ATP formation and (c) photophosphorylation of the aerobic, "pseudocyclic" type in which oxygen production occurs but is masked by an equal oxygen consumption. No chloride requirement was found for the anaerobic, cyclic photophosphorylation in which oxygen is not produced. These results support the view that chloride is an essential cofactor for oxygen evolution in photosynthesis.W a r b u r g 1 discovered that chloride was essential for oxygen evolution by isolated chloroplasts in the presence of the nonphysiological hydrogen (elec tron) acceptors, benzoquinone and ferricyanide. This discovery was confirmed by A r n o n and W h a t l e y 2 and G o r h a m and C l e n d e n n in g 3 but they had little inclination to accept W a r b u r g 's conclusion 1 that chloride served as a coenzyme for oxygen evolu tion in photosynthesis because there was no evidence at the time that chloride was essential for the growth of intact plants; hence, it seemed unlikely that chloride would be required for oxygen evolution in vivo. Moreover, chloride, a common constituent of leaves, could be completely replaced in the in vitro system by bromide 1~3, an anion of doubtful physio logical importance to land plants. A r n o n and W h a t l e y 2 suggested, therefore, that the role of chloride was to protect the photochemical activity of chloro plasts in vitro against inactivation by light. G o r h a m and C l e n d e n n in g 3 drew attention to the effects of

Metabolic Effects on Ion Fluxes in Nitella Translucens I. Active Influxes

“…This association of chloride uptake with the second light reaction is interesting in view of the fact that it is this particular part of photosynthesis for which CI-(or Br-) is an essential cofactor (Bove et al 1963). This suggests some capacity for at least temporary chemical association of chloride with some component of the photosynthetic chain, and this may have relevance for the active uptake process.…”

A manganese‐chloride cluster as the functional centre of the O2 evolving enzyme in photosynthetic systems

“…Our model is based on and accommodates 4 well-established experimental observations: (i) the photo-oxidation of water to molecular 02 proceeds via the successive storage of four oxidising equivalents [7]; (ii) the 4 protons released for every molecule of 02 liberated during the charge accumulation occur with a stoichiometry sequence of 1, 0, 1, 2 over the successive S-states 181; (iii) 4 manganese atoms per active centre appear to be optimal [9, lo]; (iv) chloride is a necessary cofactor [ 11,121. The multiple electron transfers required, the Clinvolvement, the charging aspect of the process and the multiplicity of Mn ions all imply a manganese cluster with bridging Cl-between the Mn centres as a principal unit in the oxygen evolving system. The proposition that Cl-bridges between Mn ions was put forward years ago [13] and has been 'rediscovered' in a recent publication [14]. The present authors had the same notion arising from the demand for Cl-in the mangrove photosystem [12].…”

“…The multiple electron transfers required, the Clinvolvement, the charging aspect of the process and the multiplicity of Mn ions all imply a manganese cluster with bridging Cl-between the Mn centres as a principal unit in the oxygen evolving system. The proposition that Cl-bridges between Mn ions was put forward years ago [13] and has been 'rediscovered' in a recent publication [14]. …”

Studies on the Energy-coupling Sites of Photophosphorylation

“…However, both of these methods are not quite satisfactory in dealing with critical experiments on photophosphorylation, as briefly mentioned earlier in this paper. Well coupled chloroplasts are rather resistant to C1--removal treatment (7) which is also known to inhibit water oxidation (22). Repeated washings with Cl-free media combined with room-temperature treatment did severely suppress the Hill reaction, but their secondary effects were quite appreciable.…”