214 Biochemical dissection of photorespiration Roland Douce* and Michel Neuburger Progress has been made in the understanding of each other. In the course of this pathway two molecules of photorespiration and related proteins (Rubisco, glycolate glycolate-2-P are metabolized to form one molecule each of oxidase and glycine decarboxylase) in the context of recent glycerate-3-P and CO2 and these carbon compounds are structural information. Numerous shuttles exist to support used immediately for the regeneration of RuBP via the transamination, ammonia refixation and the supply or export of Benson–Calvin cycle (C3 cycle) without the net synthesis reductants generated or consumed (via malate-oxaloacetate of triose phosphate. Once glycolate-2-P is formed, the pho- shuttles) in the photorespiratory pathway. A porin-like channel torespiratory cycle works forward to convert all the carbon that is anion selective represents the major permeability diverted out of the C3 cycle back to photosynthesis as pathway of the peroxisomal membrane. rapidly as possible [3]. Indeed, several reactions occuring in chloroplasts and peroxisomes strongly favor product forma- Addresses tion. Obviously, although very little is known about the DBMS, Laboratoire de Physiologie Cellulaire Végétale, feed-back mechanisms that might operate in photorespira- CEA Grenoble et Université Joseph Fourier, 17 rue des martyrs, tion [4], the most important control step is at the level of F 38054 Grenoble, Cedex 9, France competition between O and CO for binding to Rubisco. *e-mail: [email protected] 2 2 Current Opinion in Plant Biology 1999, 2:214–222 In C3 plants the C2 cycle is operating in the photosynthet- http://biomednet.com/elecref/1369526600200214 ically active mesophyll cells. In C4 plants the C2 cycle operates in the bundle sheath cells [5]. Using two geneti- © Elsevier Science Ltd ISSN 1369-5266 cally modified C4 plants, a mutant of Amaranthus edulis that Abbreviation is deficient in PEP carboxylase and a transgenic plant RuBP ribulose-1,5-bisphosphate Flaveria bidentis which has reduced levels of Rubisco, • Marocco et al [6 ] have demonstrated that when the C4 Introduction plant is ineffective in concentrating CO2 in the bundle The prime function of the C2 oxidative photosynthetic car- sheath cells there is a marked increase in photorespiration bon cycle — inappropriately named ‘photorespiration’ and when the C4 plant exhibits low levels of Rubisco there • [1 ] — is to salvage glycolate-2-P produced continuously in is a marked increase in bundle-sheath CO2 leakage. This the light by the oxygenase activity of ribulose-1,5-bisphos- observation provides definitive evidence that photorespi- phate carboxylase/oxygenase (Rubisco). In leaves under ration is insignifiant in C4 plants because they are capable ambient conditions the rate of oxygenation to carboxylation of concentrating CO2 in the bundle-sheath cells leading to has been estimated as high as 0.4. Low intercellular con- the suppression of the oxygenase reaction of Rubisco. centrations of CO2, as may occur, for example, under water stress (e.g. whenever the stomata are closed), can result in Functioning of key enzymes involved in even higher ratios. Given the voluminous literature on pho- photorespiration torespiration [2,3], in this short review we merely highlight Few enzymes involved in this cycle have been studied recent advances in this topic, laying emphasis on a few pho- carefully. Only Rubisco, glycolate oxidase and a sophisti- torespiratory enzymes (Rubisco, glycolate oxidase and cated set of proteins involved in glycine cleavage (glycine glycine decarboxylase) and molecular traffic between per- decarboxylase system) have been studied in an exhaustive oxisomes, chloroplasts, and mitochondria. manner. For this reason we have chosen to focus on a restricted set of enzyme systems. The photorespiratory pathway The value of numerous mutant plants (Hordeum, pea, and Mechanism of Rubisco: the triggering of photorespiration Arabidopsis thaliana) in the exquisite elucidation of the mech- Rubisco is present at a tremendous concentration in the –1 anism of photorespiration and its relationships with CO2 stroma of the chloroplasts (~0.2 g ml ) stromal extract and fixation and amino acid metabolism has been highlighted by catalyses both the carboxylation (the enzyme exhibits a low several groups (see [3] for a full list). These mutants were catalytic rate constant, 3.5 sec–1) and the oxygenation of unable to survive in air, but could thrive in atmospheres con- ribulose-1,5-bisphosphate [7–9,10••]. The two reactions taining a high concentration of CO2 (or low [O2]). involve the competition of molecular CO2 with O2 for the 2,3-enediol(ate) form of RuBP which is first generated at the The recycling of glycolate-2-P into glycerate-3-P via the active site of the enzyme. At any given [CO2][O2], the frac- photorespiratory pathway and then further to ribulose-1,5- tional partitioning of RuBP between the carboxylation and bisphosphate (RuBP) is not only a very costly reaction, it oxygenation pathways is governed by the relative reactivity also requires a large machinery consisting of 16 enzymes of the enzyme-bound 2,3-enediol(ate) toward CO2 and •• and more than six translocators, distributed over the chloro- O2 [10 ]. From biochemical analyses of Rubisco purified plast, peroxisome and mitochondrion in close proximity to from several species, including photosynthetic bacteria, Biochemical dissection of photorespiration Douce and Neuburger 215 cyanobacteria, green algae, and higher plants, there are large The oxygenation of RuBP yields one molecule each of differences in specificity towards the substrates CO2 and O2: glycerate-3-P (formed from C-3, C-4 and C-5 of RuBP), evolutionary pressures seem to have directed Rubisco and glycolate-2-P. The oxygenation pathway has not been •• •• towards more efficient utilization of CO2 [10 ,11]. Rubisco dissected as deeply as the carboxylation counterpart [10 ]. from cyanobacteria, green algae and higher plants is assem- Very likely the oxygenation pathway is similar to the car- bled from eight large (L) subunits and eight small (S) boxylation pathway although the putative key labile subunits (four dimers of L subunits surrounded by two intermediate (2-peroxy-3-ketoarabinitol 1,5-bisphosphate) •• tetramers of S subunits; (L2)4(S4)2) [12], whereas Rubisco [10 ] postulated through the exquisite characterization of → → from Rhodospirillum rubrum (a nonsulfur purple bacteria) two-different site directed mutants (E60 Q, K334 A) consists of only two large subunits (L2). The large subunit [18,19], has never been characterized so far. This reaction from spinach can be divided into two domains, an amino may be an inevitable consequence of Rubisco’s inability to α β terminal domain and a carboxy-terminal / -barrel domain. protect its ene-diolate reaction intermediate from O2. Two active sites are located at the interface of the L-sub- Indeed, this notion is supported by the failure of numerous units in the L2 dimer (‘head to tail’ arrangement). The efforts to eliminate selectively its oxygenase activity by catalytic center is mostly situated at the carboxy-terminal genetic manipulation. The partitioning of RuBP between end of the α/β-barrel. The enhancement of catalytic rate by the carboxylation and oxygenation pathways is sensitive to S subunits can only mediated through induced conforma- the active site microenvironment and does not involve tional changes in catalytic subunits because S subunits are large movements within the structure [10••]. Given the far removed from the active site [13]. structural similarity of the two alternative substrates CO2 and O2 and the large difference in their concentration A large number of crystal structures of Rubiscos from var- within the chloroplasts, it is clear that Rubisco influences ious sources including Rhodospirillum rubrum, the selectivity for CO2 in some way [12]. Synechococcus, and spinach have been reported along with a variety of ligands (see [10••,], for a full list) and this, in Glycolate oxidase synergy with biochemical investigations, led to a careful Glycolate oxidase (an octamer composed of identical sub- dissection of the carboxylation pathway. The carboxyla- units of approximately 40 kDa) is one of the very few tion of RuBP involves multiple discrete steps and peroxisomal proteins for which a high resolution crystal associated transition states: removal of ligands such as 2- structure is available [20•]. The enzyme from spinach crys- carboxyarabinitol 1-phosphate from the inactive enzyme tallizes in an octameric form and the subunit contains an form (this process occurs slowly by simple dissociation, or eight-fold β/α barrel motif corresponding to the flavin rapidly when catalysed by the enzyme Rubisco activase); mononucleotide (FMN) domain which is also found in carbamylation of the ε-amino group of Lys201 (spinach) other FMN-dependent enzymes. The irreversible reaction residue in the active site by an activator CO2 molecule; catalysed by the enzyme can be divided into two half-reac- stabilization of this protein-bound carbamate by mon- tions. First glycolate is oxidised by the flavin which is odentate coordination to Mg2+ (three water molecules, deeply burried in the barrel. In the second part FMN is Asp203 and Glu204 complete the octahedral coordination reoxidized by O2 to produce H2O2 which is, in turn, sphere around this metal ion); binding of RuBP: it is ori- decomposed by catalase (a heme-containing enzyme). The ented