32 TIBS - February 1980 Open Question but again reduced pyridine nucleotides and What is the physiological role of ATP are consumed. It has been claimed [7] that the metabolism of glycolate fuels the photorespiration? production of mitochondrial ATP. This refers to the conversion of glycine to serine U. Heber and G. H. Krause in the mitochondria that yields NADH. hz vitro, NADH can be oxidized in the respiratory chain. In vivo, however, reduc- There is scarcely any cell that is metaboli- competes with CO2 at the active sites of ing equivalents must be reinvested for the cally more flexible than the mesophyll cell ribulose bisphosphate (RuBP) carboxyl- reduction of hydroxypyruvate and refixa- of plant leaves. Upon illumination, ase. Instead of being carboxylated, RuBP is tion of NH~ liberated during the formation respiratory oxygen uptake is rapidly oxygenated to form phosphoglycerate and of serine (Fig. 1). On balance, those reduc- replaced by photosynthetic oxygen evolu- phosphoglycolate. The latter is hydrolysed ing equivalents are therefore unavailable tion which may be up to 100 times faster to glycolate. for ATP production. Apparently the than oxygen uptake in the dark. Often, A peculiar feature of photorespiration is ammonia is not incorporated directly into mitochrondrial respiration, which gives that, in contrast to dark respiration, it does glutamate by mitochondrial glutamate rise to oxygen uptake, is suppressed in the not conserve but consumes energy [6]. This dehydrogenase, but rather is ref'Lxed by light. However, in the presence of atmos- becomes apparent from the energy balance glutamine synthetase in the cytosol or pheric concentrations of oxygen, most of the metabolic cycles depicted in Fig. 1. chloroplasts to form glutamine [8]. In the plants exhibit another type of oxidative The formation of glycolate requires energy chloroplasts, the glutamine reacts with sugar degradation that is not observed in in the form of ATP and reducing equiv- 2-oxoglutarate to glutamate. The latter the dark and is therefore called photo- alents, since it utilizes photosynthetic pathway requires, in addition to reducing respiration [1]. This process has unusual intermediates. Isolated chloroplasts can equivalents, one ATP to re-assimilate one properties. It proceeds at appreciable rates oxidize triosephosphate (TP) via RuBP molecule of NH~. The balance of the inte- only at high oxygen tensions and can there- and the Calvin cycle completely to glyco- grated cycles in the scheme in Fig. 1 shows fore be suppressed by a reduction in the late and inorganic phosphate [3]. The that evolution of 1 mol CO2 by photorespi- partial pressure of oxygen. In an atmos- energy requirement of the oxidation pro- ration re.quires about twice as much energy phere containing 1-2% oxygen, almost no cess can be derived from a summation of as assimilation of 1 mol CO, in the Calvin photorespiration is observed. the individual reactions involved, yielding: cycle [6]. It is noteworthy that the ratio of The biochemical pathway of photores- reducing equivalents consumed to ATP 2 TW- + 3 O2 + 3 NADPH + 6 ATW- piration has been elucidated mainly by consumed is the same or almost the same in + 6 OH--~ Tolbert and co-workers [2]. In brief, glyco- photorespiratory carbohydrate oxidation 3 glycolate - + 8 Pi ~- + 3 NADP ~ late is formed in the chloroplasts at the as in photosynthetic carbohydrate forma- + 6 ADP 3- + H20 expense of sugar-phosphate intermediates tion. Clearly, energy-dependent oxidation of the Calvin cycle. After export from the Details of the conversion of glycolate to instead of formation of carbohydrates must chloroplasts, the glycolate undergoes a phosphoglycerate are not entirely clear, reduce photosynthetic yield. series of reactions taking place in peroxi- somes, mitochondria and finally again in II CHLOROPLAST II OUTSIDEOF CHLOROPLAST the chloroplasts (Fig. 1). There, phospho- II glycerate is produced, linking glycolate II ATP II metabolism to the Calvin cycle. Formation II and conversion of 2 mol of glycolate to 1 II t P-glyiolote 11 mol of phosphoglycerate lead to the uptake glycolate II of 3 mol O2 and the evolution of 1 mol CO2. IIII , glyc~ If photosynthetic oxygen evolution for II glyoxylote production of requii'ed reducing equiv- II alents is taken into account, an O~/CO2 triose-P II II [ ratio of 1 is obtained for the complete pro- II cess of photorespiration. The key reaction | Calvin cycle ) II -iHz gl~~'-IH]P 171 II of the process - the biosynthesis of glyco- II late in the chloroplasts - has been a con- II 5n/o._ O II troversial issue for several years. It now U pyruvote [~1 appears established [3-5] that .oxygen II 13-ph0 +yc.ro,. glycr~'~'otee 1 ATP NADPH U. Heber and G. tl. Krattse are at the Institute of II Botany and Pharrnaceutical Biology of the University II of Wgtrzburg and Institute of Botany of the University Fig. I. Scl, eme of photosynthetic and photorespiratory pathways showing energy-requiring steps. [It] denotes of Diisseldorf, F.R.G. reduced pyridine nucleotide. * Phosphorylation of glycerate is possible both inside and outside the chloroplasts. El~vier/North-HollandBiomedical Prr 1980 TIBS - February 1980 33 Bjfrkman [12] have suggested that photo- respiratory energy consumption may pro- tect plants from photo-oxidative damage. l | Several mechanisms are available to pre-~ vent light damage to the photosynthetic apparatus [13]. Carotenoids are known to play a role in the protection of chloroplast - - ~-- l rnin, ~(~ L.-..-....~._.~ pigments against photo-bleaching. Super- I' light on oxide dismutase, catalase, ascorbate and glutathione act together to degrade oir. oil. superoxide radicals and hydrogen peroxide Fig. 2. Effects o f gas phase composition on scattering o f a weak beam of 535 nm light by a spinach leaf.. Exciting red formed during reduction of oxygen. It light (intensity 125 W.m=) was switched on aj~er a 2 rain dark period. The gas phase surrounding the leaf was appeared possible that light energy not changed from air to nitrogen and vice versa, both containing 475 ppm C02. Numbers denote steady-state rates of used for photosynthesis could give rise to net photosynthetic CO~ uptake, as recorded with an infrared analyser (hoT,oiling chlorophyll h). destructive reactions which cannot be Dissipation of energy by photores- photorespiration would only be practical handled by the available protective piration can be demonstrated in vivo by when oxygenation of RuBP is selectively deVices. light scattering chlorophyll fluorescence suppressed without the impairment of Attempts have been made to test this measurements [6]. Upon illumination of carboxylase activity, a task that seems dif- concept. Isolated chloroplasts that are cap- leaves, a proton gradient is established ficult to accomplish. able of photosynthetic rates comparable to across the energy-conserving thylakoid There is the question of why evolu- those shown by the parent leaves were membranes in the chloroplasts. Together tionary pressures have not eliminated a exposed to light in the absence of CO2 [14]. with a light-dependent membrane poten- process as wasteful as photorespiration Oxidizable substrates were also largely tial this is thought to drive ATP synthesis. appears to be. Such pressures seem to have absent from the chloroplasts and little for- The magnitude of the proton gradient can produced insensitivity to oxygen inhibition mation of glycolate was possible during be used as a measure of the energization of of photosynthesis in the group of'Ca plants' illumination. Within only a few minutes of the chloroplast system. Formation of the in which primary photosynthetic CO2 exposure to light, the chloroplasts lost a proton gradient gives rise to an increase jn incorporation occurs into the Ca acid, significant proportion of their capacity to light scattering and a decrease in chloro- oxaloacetate. Even Ca plants are capable of photoreduce CO= (Table I). This loss of phyll fluorescence. Both phenomena'can photorespiration, but photorespiratory activity was irreversible. It seems that in be easily monitored in intact leaves. Fig. 2 rates are low. The rates are probably these chloroplasts, photophosphorylation shows that after a lag phase during which a decreased because Ca plants have a and, to a lesser degree, photosynthetic steady rate of photosynthesis and mechanism whereby CO2 is accumulated at electron transport were partly inactivated. photorespiration in air are established, the site of secondary CO= fixation by RuBP Oxygen appeared to play a role in the inac- light scattering by a leaf is kept at a low carboxylase. In isolated chloroplasts from tivation, as damage increased with increas- value. When photorespiration is suppres- spinach, a (23 plant, glycolate synthesis is ing oxygen concentration. Chloroplasts sed by exchange of air for Na + CO2, light almost completely suppressed by CO2 permitted to assimilate CO2 during illumi- scattering increases, indicating increased [3,4]. In leaves of (23 plants, the ratio of nation at a constant oxygen concentration energization of the membranes. This sup- CO= fixation to photorespiratory CO2 did not lose activity while exposed to light. ports a higher rate of CO2 f'Lxation; energy evolution increases with ambient CO2 con- Rather similar results were obtained that in air was required for photorespirat- centrations, although in apparent contra- with isolated leaf cells suspended in CO2- ory COa evolution is now available for diction to the results found hz vitro, absol- and bicarbonate-free