Photosynthetic Oxygen Evolution and the Water Splitting Enzyme

Abstract

A brief review is given of the present knowledge of the mechanism by which higher plants and algae bring about the photooxidation of water. Emphasis is placed on the role of manganese in this process and a summary is given of recent efforts from several laboratories to isolate the “water‐splitting enzyme”. Experimental evidence is presented that, in contrast to earlier beliefs, the Mn not removed by washing chloroplast thylakoids with high concentrations of Tris (hydroxymethyl) amino methane could be involved in O2 evolution. This manganese is equivalent to about 2 Mn per photosystem two reaction centre. Also presented are details of the isolation of a heme‐protein from spinach chloroplasts which may be intimately associated with the water splitting process. This protein has a native molecular weight of approximately 232 k Daltons and has characteristics similar to those of a catalase type enzyme. The possibility of involvement of peroxide formation during the water splitting process is discussed.

The mechanism of photosynthetic water oxidation

“…However, two of the four manganese ions (or atoms) were found to be different with regard to their magnetic interaction with the oxidized donor, Z °x [45]. Based on the fact that partial release of manganese ions (or atoms) after Tris-washing of inside-out thylakoids did not prevent reconstitution of the oxygen-evolving capacity, only two manganese atoms (or ions) were inferred to be directly involved in water oxidation [46]. Similar conclusions were made after quantitative measurements of the manganese content of PS II particles [47].…”

Modification of oxygen evolving center by Tris-washing

“…16,18,23,[25][26][29][30][31]. Considering that Tris-washed chloroplasts could be reactivated by DCPIP'H2-treatment alone [12,17,[22][23][24][25][26][27][28][29][30][31], the 02-evolving center should still have kept necessary Mn in it even after the Tris-washing. This variety in Mn content would be reflecting the toughness of thylakoid membrane of each investigators' preparations against the damaging in Triswashing.…”

“…The inactivated chloroplasts and particles could be reactivated by Mn2+-addition [20][21], DCPIP.H2-treatment [12,17,[22][23][24][25][26][27][28][29][30][31] and photo-reactivation [26-27, 29, 31-36]. 0.8 M Tris-buffer at pH 8.0 was usually used for the inactivation, however, 0.2 M buffers at pH 8.0 [38] and at pH i0.0 [59] could also be used.…”