Does O2photoreduction occur within chloroplasts in vivo?

Abstract

Robinson, J. M. 1988. Does O2photoreduction occur within chloroplasts in vivo? ‐Physiol. Plant. 72: 666–680.This discussion reviews evidence supporting the hypothesis that within intact chloroplasts in vivo, molecular O2may serve as an alternative Hill oxidant (electron acceptor) on the reducing side of Photosystem I. Depending upon the availability of Hill oxidants such as NADP+and NO−2, there is the potential within intact plastids in vivo, for photolytically derived reducing equivalents to reduce O2to O−2and H2O2(the Mehler reaction). In chloroplasts of healthy tissues, the products of photosyn‐thetic O2reduction O−2and H2O2) are rapidly removed by superoxide dismutase (EC 1.15.1.1) and L‐ascorbate peroxidase (EC 1.11.1.11) to prevent toxicity. The presence of these two enzymes within chloroplasts in vivo reflects the potential for linear (non‐cyclic) photosynthetic electron transport systems to draw upon molecular O2as a terminal oxidant. In the intact plastid, O2may act as an electron acceptor in the place of any other physiological Hill oxidant, e.g., NADP+, NO−2, and, presumably, oxidized thioredoxin. Under aerobic, physiological conditions, photo reduced ferre‐doxin (Fdred), and/or reduced flavoprotein enzymes, e.g., ferredoxin:NADP+oxidoreductase (EC 1.18.1.2), can donate electrons to O2; this reductive reaction appears to be non‐enzymatic, but it is rapid. Stated from another viewpoint, O2may serve as a Hill oxidant to support some linear electron flow when reductant supplies are in excess of reductant demands. For example, there are nitrogen assimilatory sites in the chloroplast, i.e., ferredoxin‐nitrite reductase (NiR; EC 1.7.7.1) and glutamate synthase (ferredoxin) (GOGAT; EC 1.4.7.1), to which Fdredis allocated as reductant. Because NADH:nitrate reductase (NR; EC 1.6.6.1) is the rate limiting step of nitrogen assimilation, and, because NiR and GOGAT activities are in excess of NR activities by a factor of 2 or more, then an excess of unreacted Fdredcould accumulate. Alternatively, the allocated Fdredwould reduce the excess NiR and GOGAT sites, but the excess of reduced enzymes would not have substrates (e.g., NO−2, glutamine, and α‐ketoglutarate) with which to react. Therefore, if ‘excess’ NiR and GOGAT binding sites were not employed, the available excess Fdred, and/or the reduced NiR and GOGAT proteins, would be susceptible to oxidation by O2. The resulting O2photoreduction could account for nearly all of the observed in vivo Mehler type reactions. In vivo, apparent foliar O2photoreduction occurs simultaneously with maximal CO2photoassimilation, and, in high light, average rates have been determined by direct measurement to range from 10 to 40 μmol O2consumed (mg Chl)−1h−1. Therefore O2reduction would support a low rate of linear (non‐cyclic) electron flow which, in turn, could maintain a low, but significant rate of ATP production. However, there is not total agreement among researchers that the physiological role of O2is that of serving as an alternative Hill oxidant in order to recycle unutilized Fdredor other photoreduced proteins. Also, there continues to be considerable controversy on whether or not O2reduction supports significant photosynthetic phosphorylation. The total process of O2photoreduction, and its physiological role(s), requires much more study before absolute functions can be assigned to O2terminated, linear electron transport.SummaryMolecular O2possesses the physico‐chemical properties that permit this molecule to serve as an alternative Hill oxidant within chloroplasts in vivo. Additionally, the physical and physiological properties within the chloroplast in vivo favor the potential for O2to serve as an electron acceptor on the reducing side of Photosystem I. This may reflect an important ‘fail‐safe mechanism’ which prevents over‐reduction of linear photosynthetic electron transport chain proteins. This review has focused on the possibility that unutilized Fdredand/or other non‐utilized, reduced plastid enzymes (e.g., NiR) may be electron donors to O2. It is hypothesized that this oxidation ultimately would be reflected as an in vivo Mehler reaction. However, it remains for future studies to establish without doubt, that in vivo, photoreduced chloroplast enzyme proteins can utilize O2as a terminal electron acceptor.Further, that O2photoreduction supports a significant level of photophosphorylation in vivo remains to be firmly established. Certainly, considerable evidence, gained with experiments utilizing isolates of intact chlo‐roplasts and reconstituted chloroplast systems, supports the hypothesis that O2‐terminated linear electron transport has the potential to support high rates of ATP production. However, in vivo studies e.g., with intact leaf tissues, which actually quantitate the relationship between O2photoreduction and associated ATP production have not been conducted. These will be difficult experiments to perform, because, in vivo, it will be difficult to separate photosynthetic ATP production mediated by O2from ATP production mediated by those other, more predominant Hill oxidants (e.g., NADP+, NO−2). Also, it continues to be a possibility that it is cyclic, and not pseudocyclic photophosphorylation that provides additional ATP to support photosynthetic cell metabolism. To establish beyond doubt that an in vivo role of the Mehler reaction is that of supplying ‘additional ATP’, remains for considerable future study.