Molecular structure and subcellular localization of spinach leaf glycolate oxidase

Glycolate oxidase (E.C. 1.1.3.1) was purified from spinach leaves (Spinacia oleracea). The molecular weight of the native protein was determined by sucrose density gradient centrifugation to be 290,000 daltons (13S), whereas that of the monomeric form was 37,000 daltons. The quaternary structure of the holoenzyme is likely to be octameric, analogous to pumpkin cotyledon glycolate oxidase [Nishimura et al, 1982]. The subcellular localization of the enzyme was studied using linear sucrose density gradient centrifugation, and it was found that glycolate oxidase activity is detectable in both leaf peroxisomal and supernatant fractions, but not in chloroplasts and mitochondria; the activity distribution pattern is essentially similar to that for catalase, a known leaf peroxisomal enzyme. Ouchterlony double diffusion and immunotitration analyses, demonstrated that the rabbit antiserum against purified spinach leaf glycolate oxidase cross-reacted, identically, with the enzyme molecules present in two different subcellular fractions, i.e, the leaf peroxisome and supernatant fractions. It is thus concluded that the enzyme present in the supernatant is due to the disruption of leaf peroxisomes during the isolation, and hence glycolate oxidase is exclusively localized in leaf peroxisomes in spinach leaves.     

Primary structure of glycolate oxidase from spinach

The primary structure of glycolate oxidase from spinach has been determined. Six different types of peptide digest were investigated, utilizing CNBr, proteolytic enzymes, and chemical modifications to change a specificity of cleavage. In total, 90 peptides were purified and analyzed. The studies were aimed at correlation with crystallographic analysis of the same protein carried through in parallel and with cDNA studies which utilized initially determined amino acid sequences for synthesis of oligonucleotide probes. Continuous comparisons with the results from the crystallographic studies helped at an early stage to secure peptide overlaps, at the same time as the peptide data secured residue assignments in the electron density maps. In the end, all data agree and regions from all parts of the molecule have been checked by independent methods of analysis. The primary structure establishes the type of N-terminal post-translational processing, and yields information on segments not fully defined in electron density maps. Combined, the chemical, crystallographic, and cDNA data give extensive reliability. The peptide analysis shows that the N-terminus is blocked by acylation of the initiator methionine, which is in a primary structure typical for non-removal of the methionine in the processing events of the nascent protein chain. The molecule is comparatively rich in menthionine and some other generally less common residues, but has only one cysteine residue and no extensive hydrophobic segment. An amino acid sequence homology with flavocytochrome b2 from yeast, as expected from known similarities in tertiary structure, is observed (33% residue identities).     

Refined structure of spinach glycolate oxidase at 2 A resolution

The amino acid sequence of glycolate oxidase from spinach has been fitted to an electron density map of 2.0 A nominal resolution and the structure has been refined using the restrained parameter least-squares refinement of Hendrickson and Konnert. A final crystallographic R-factor of 18.9% was obtained for 32,888 independent reflections from 5.5 to 2 A resolution. The geometry of the model, consisting of 350 amino acid residues, the cofactor flavin mononucleotide and 298 solvent molecules, is close to ideal with root-mean-square deviations of 0.015 A in bond lengths and 2.6 degrees in bond angles. The expected trimodal distribution with preference for staggered conformation is obtained for the side-chain chi 1-angles. The core of the subunit is built up from the eight beta-strands in the beta/alpha-barrel. This core consists of two hydrophobic layers. One in the center is made up of residues pointing in from the beta-strands towards the barrel axis and the second, consisting of two segments of residues, pointing out from the beta-strands towards the eight alpha-helices of the barrel and pointing from the helices towards the strands. The hydrogen bond pattern for the beta-strands in the beta/alpha-barrel is described. There are a number of residues with 3(10)-helix conformation, in particular there is one left-handed helix. The ordered solvent molecules are organized mainly in clusters. The average isotropic temperature factor is quite high, 27.1 A2, perhaps a reflection of the high solvent content in the crystal. The octameric glycolate oxidase molecule, which has 422 symmetry, makes strong interactions around the 4-fold axis forming a tight tetramer, but only weak interactions between the two tetramers forming the octamer.     

Subperoxisomal localization of glycolate oxidase

The subperoxisomal distribution of glycolate oxidase (GO) in leaves and cotyledons of several plants was investigated using post-embedding immunogold labelling. In peroxisomes with amorphous nucleoids, all of the immunolabelling is associated with the matrix of the peroxisome, even in tissue embedded in Lowicryl, a resin that preserves antigenicity best. This same staining pattern was found after cytochemical staining for GO activity with cerium. In peroxisomes with crystalline inclusions, the inclusions are only lightly labelled, compared with the densely-labelled matrix. Cytochemical reactions are noted between the units of the crystal in these peroxisome types. Because cytochemical reactions for catalase are concentrated in the amorphous nucleoid and crystalline peroxisomal inclusions, the general lack of immunogold staining of GO and other peroxisomal proteins indicate that catalase may be the major (or in some cases the exclusive) constituent of these peroxisomal inclusions.     

Subperoxisomal localization of glycolate oxidase

Summary The subperoxisomal distribution of glycolate oxidase (GO) in leaves and cotyledons of several plants was investigated using post-embedding immunogold labelling. In peroxisomes with amorphous nucleoids, all of the immunolabelling is associated with the matrix of the peroxisome, even in tissue embedded in Lowicryl, a resin that preserves antigenicity best. This same staining pattern was found after cytochemical staining for GO activity with cerium. In peroxisomes with crystalline inclusions, the inclusions are only lightly labelled, compared with the denselylabelled matrix. Cytochemical reactions are noted between the units of the crystal in these peroxisome types. Because cytochemical reactions for catalase are concentrated in the amorphous nucleoid and crystalline peroxisomal inclusions, the general lack of immunogold staining of GO and other peroxisomal proteins indicate that catalase may be the major (or in some cases the exclusive) constituent of these peroxisomal inclusions.     

A simple method for estimating intactness of spinach leaf protoplasts by glycolate oxidase assay

A method was developed for the quantitative analysis of intactness of spinach leaf protoplasts using glycolate oxidase activity as an index. Since glycolate does not penetrate into protoplasts at neutral pH, the increase of O₂ consumption by the addition of glycolate to protoplast suspension was due to the glycolate oxidase activity released from damaged protoplasts. The proportion of damaged protoplasts in the whole preparation was calculated from the ratio of released and total glycolate oxidase activity. Freshly prepared spinach leaf protoplasts were found to be 80 to 90% intact as estimated by the method. The effect of osmolarity on the respiratory activities of spinach leaf protoplasts was also examined by applying the same principle.     

Mechanism of glycolate transport in spinach leaf chloroplasts

The incorporation of ¹⁴CO₂ into glycolate by intact spinach leaf (Spinacia oleracea L. var. Kyoho) chloroplasts exposed to ¹⁴CO₂ (NaH¹⁴CO₃, 1 millimolar) in the light was determined as a function of O₂ concentrations in the reaction media. A hyperbolic saturation curve was obtained, apparent K_m (O₂) of 0.28 millimolar, indicating that glycolate is produced predominantly by ribulose-1,5-bisphosphate carboxylase/oxygenase. A concentration gradient of glycolate was invariably observed between chloroplast stroma and the outside media surrounding chloroplasts during photosynthetic ¹⁴CO₂ fixation under an O₂ atmosphere.     

Expression of active spinach glycolate oxidase in Aspergillus nidulans

The biocatalytic production of glyoxylic acid from glycolic acid requires two enzymes: glycolate oxidase, which catalyzes the oxidation of glycolic acid by oxygen to produce glyoxylic acid and hydrogen peroxide, and catalase, which decomposes the byproduct hydrogen peroxide. As an alternative to isolation from the leaf peroxisomes of spinach, glycolate oxidase has now been cloned and expressed in transformants of Aspergillus nidulans T580 at levels ranging from 1.7 to 36 IU/g dry wt. cells. The glycolate oxidase of transformant strain T17 comprises ca. 1.9% of total cell protein and is expressed at near 100% activity. (c) 1996 John Wiley & Sons, Inc.     

Metabolism of glycolate and glyoxylate in intact spinach leaf peroxisomes

Intact and broken (osmotically disrupted) spinach (Spinacia oleracea) leaf peroxisomes were compared for their enzymic activities on various metabolites in 0.25 molar sucrose solution. Both intact and broken peroxisomes had similar glycolate-dependent o₂ uptake activity. In the conversion of glycolate to glycine in the presence of serine, intact peroxisomes had twice the activity of broken peroxisomes at low glycolate concentrations, and this difference was largely eliminated at saturating glycolate concentrations. However, when glutamate was used instead of serine as the amino group donor, broken peroxisomes had slightly higher activity than intact peroxisomes. In the conversion of glyoxylate to glycine in the presence of serine, intact peroxisomes had only about 50% of the activity of broken peroxisomes at low glyoxylate concentrations, and this difference was largely overcome at saturating glyoxylate concentrations. In the transamination between alanine and hydroxypyruvate, intact peroxisomes had an activity only slightly lower than that of broken peroxisomes. In the oxidation of NADH in the presence of hydroxypyruvate, intact peroxisomes were largely devoid of activity. These results suggest that the peroxisomal membrane does not impose an entry barrier to glycolate, serine, and O₂ for matrix enzyme activity; such a barrier does exist to glutamate, alanine, hydroxypyruvate, glyoxylate, and NADH. Furthermore, in intact peroxisomes, glyoxylate generated by glycolate oxidase is channeled directly to glyoxylate aminotransferase for a more efficient glycolate-glycine conversion. In related studies, application of in vitro osmotic stress to intact or broken peroxisomes had little effect on their ability to metabolize glycolate to glycine.     

Preliminary crystallographic data for glycolate oxidase from spinach.

Glycolate oxidase, an enzyme that plays an important role in photorespiration in plants, has been purificant from spinach and crystallized in two different crystal forms. Form A which was obtained with tertiary butanol as precipitating agent belongs to space group I 422 with unit cell dimensions a = b = 148.1 A and c = 134.9 A. This form diffracts to high resolution and will be used for further crystallographic studies. Form B is also tetragonal, space group P42212, with cell dimensions a = b = 145.4 A and c = 104.2 A. This form was obtained from ammonium sulfate precipitations. Sodium dodecyl sulfate polyacrylamide gel electrophoresis shows that the enzyme is built up from subunits of molecular weight 37,000. The asymmetric units of both crystal forms contain at least two such subunits.     

Direct expression of active spinach glycolate oxidase in Escherichia coli.

Spinach glycolate oxidase (GAO) was expressed in Escherichia coli using the T7 RNA polymerase promotor. The enzyme accounts for approx. 1% of the soluble protein fraction and is expressed as a soluble and active enzyme. Comparison with GAO expressed in Saccharomyces cerevisiae (Macheroux, P., Massey, V., Thiele, D.J. and Volokita, M. (1991) Biochemistry 30, 4612-4619) showed that the GAO expressed in E. coli has identical physico-chemical features to the wild-type enzyme, but is expressed at a level approx. 15-fold higher than in the yeast system.     

Cytochemical localization of glycolate oxidase in microbodies of Klebsormidium

The filamentous green alga Klebsormidium flaccidum A.Br. was fixed with glutaraldehyde, incubated in a cytochemical medium designed to detect glycolate-oxidase activity, and prepared for electron microscopy. Heavy deposits of stain were observed in microbodies following incubation with either glycolate or L-lactate as substrate, but not after incubation with D-lactate or H₂O. When Chlamydomanas reinhardi Dangeared cells were treated in the same way, their microbodies did not appear stained. The results establish that in Klebsormidium glycolate-oxidase occurs in microbodies (peroxisomes), as it does in angiosperms; also, they emphasize the dichotomy between those green algae which contain glycolate-oxidase and those, such as Chlamydomonas, which possess the mitochondrial enzyme glycolate dehydrogenase.     

Expression of spinach glycolate oxidase in Saccharomyces cerevisiae: purification and characterization.

Glycolate oxidase from spinach has been expressed in Saccharomyces cerevisiae. The active enzyme was purified to near-homogeneity (purification factor approximately 1400-fold) by means of hydroxyapatite and anion-exchange chromatography. The purified glycolate oxidase is nonfluorescent and has absorbance peaks at 448 (epsilon = 9200 M-1 cm-1) and 346 nm in 0.1 M phosphate buffer, pH 8.3. The large bathochromic shift of the near-UV band indicates that the N(3) position is deprotonated at pH 8.3. A pH titration revealed that the pK of the N(3) is shifted from 10.3 in free flavin to 6.4 in glycolate oxidase. Glycolate oxidase is competitively inhibited by oxalate with a Kd of 0.24 mM at 4 degrees C in 0.1 M phosphate buffer, pH 8.3. Three pieces of evidence demonstrate that glycolate oxidase stabilizes a negative charge at the N(1)-C(2 = O) locus: the enzyme forms a tight sulfite complex with a Kd of 2.7 x 10(-7) M and stabilizes the anionic flavosemiquinone and the benzoquinoid form of 8-mercapto-FMN. Steady-state analysis at pH 8.3, 4 degrees C, yielded a Km = 1 x 10(-3) M for glycolate and Km = 2.1 x 10(-4) M for oxygen. The turnover number has been determined to be 20 s-1. Stopped-flow studies of the reductive (k = 25 s-1) and oxidative (k = 8.5 x 10(4) M-1 s-1) half-reactions have identified the reduction of glycolate oxidase to be the rate-limiting step.     

Effect of {alpha}-hydroxy-2-pyridinemethanesulfonate on glycolate metabolism in spinach leaf protoplasts

The addition of -hydroxy-2-pyridinemethanesulfonate (-HPMS)to spinach leaf protoplasts caused a marked inhibition of photosyntheticCO₂ fixation in both air and O₂ atmospheres. In the O₂ atmosphere,¹⁴CO₂ was incorporated into glycine, but upon addition of -HPMS(10 mM), there was a suppression of ¹⁴CO₂ incorporation intothe glycine and serine plus isoleucine fractions, accompaniedby an accumulation of ¹⁴C-glycolate. A marked stimulation ofalanine labeling due to the -HPMS treatment was also observed.Feeding protoplasts with [l–¹⁴C)-glycolate resulted inthe formation of ¹⁴C-labeled glycine, serine, and sugar phosphatesin both light and dark conditions, and a sizable amount of ¹⁴CO₂evolved concomitantly. The results support the notion concerningthe operation of the glycolate pathway in leaf tissue duringphotorespiratory environments. The suitability of protoplastsfor photosynthetic research in conjunction with the use of inhibitorsubstances are discussed, although the results of the presentresearch indicate that the effect of a-HPMS is not confinedto the specific inhibition of the glycolate-oxidase reaction. ¹ This is paper 42 in the series "Structure and Function ofChloroplast Proteins", and the research was supported in partby the grant from the Ministry of Education of Japan (11912,147106), the Toray Science Foundation (Tokyo), the Nissan ScienceFoundation (Tokyo), and the Matsunaga Foundation (to M. N.). (Received July 21, 1977; )     

Glycolate metabolism and subcellular distribution of glycolate oxidase in Spatoglossum pacificum (Phaeophyceae, Chromophyta)

Seven enzymes participating in glycolate metabolism were demonstrated to be present in crude extract of the brown alga Spatoglossum pacificum Yendo. These were phosphoglycolate phosphatase, glycolate oxidase, glutamate-glyoxylate aminotransferase, serine hydroxymethyltransferase, amino acid-hydroxy-pyruvate aminotransferase, hydroxypyruvate reductase and catalase. Malate synthase, which is involved in glycolate metabolism in the xanthophycean alga, could not be detected. On demonstration of subcellular distribution of glycolate oxidizing enzymes by linear sucrose density gradient centrifugation, glycolate oxidase was detected in the same fraction at a density of 1.23 g cm^?3 with catalase: that is, the marker enzyme of peroxisome and serine hydroxymethyltransferase was found in the same fraction at a density of 1.21 g cm^?3 with isocitrate dehydrogenase, the marker of mitochondria. From the present data, it is proposed that the brown alga Spatoglossum possesses the ability to metabolize glycolate to glycerate via the pathway which may be similar to that of higher plants.     

Oxidation-reduction properties of glycolate oxidase

This is the first report of the redox potentials of glycolate oxidase. The pH dependence of the redox behavior as well as the effects of activators and inhibitors was studied. At pH 7.1 in 10 mM imidazole-chloride, Eo1' (EF1ox/EF1-.) is -0.033 +/- 0.010 V and Eo2' (EF1-./EF1redH-) is -0.017 +/- 0.017 V vs. the standard hydrogen electrode at 10 degrees C. A maximum of 29% flavin mononucleotide (FMN) anion radical is stabilized at half-reduction at pH 7.1 and 10 degrees C. Both redox couples of glycolate oxidase are pH-dependent from pH 7 to pH 9, and the FMN anion radical is stabilized in this range. The redox potentials of glycolate oxidase are shifted markedly positive of those of unbound FMN, consistent with the enzyme's function. The midpoint potential of glycolate oxidase is more positive than that of the glyoxalate/glycolate couple, and two-electron reduction of glycolate oxidase is thermodynamically favorable. The redox behavior of glycolate oxidase markedly contrasts that of other flavoprotein oxidases. For most flavoprotein oxidases, Eo1' is independent of pH from pH 7 to pH 9 and is much more positive than Eo2', which is pH-dependent. We present a mechanism that suggests a structural basis for the positive shifts and pH dependence of both Eo1' and Eo2' of glycolate oxidase.     

Cytokinin control in subcellular localization of indoleacetic acid oxidase and peroxidase

IAA oxidase and peroxidase were found in all subcellular fractions of tobacco callus cells. The bound and cytoplasmic fractions differed greatly in IAA oxidase/peroxidase ratio and in isoperoxidase composition. The IAA oxidase/peroxidase ratio was particularly high in the plasma membrane-rich fraction. Kinetin had profound effects on IAA oxidase and peroxidase. The appearance of fast migrating isoperoxidases in response to 0·2 μM kinetin was found only in cytoplasmic, plasma membrane and ribosome-rich fractions; a high concentration of kinetin inhibited their formation. High kinetin concentrations also lowered the specific activity of IAA oxidase and peroxidase in all subcellular fractions, but the effect was much greater on peroxidase than on IAA oxidase, thus resulting in a drastic increase in IAA oxidase/peroxidase ratio. Evidently the activities of IAA oxidase and peroxidase were not equivalent and should be considered separately.     

Glycerate kinase from spinach leaves: Partial purification, characterization and subcellular localization

Spinach ( Spinacia oleracea L. cv. Melody hybrid) leaf glycerate kinase (EC 2.7.1.31) was partially purified and characterized. The enzyme did not exhibit any absolute stereospecificity towards the two enantiomers of glycerate, but the affinity for the D-isomer was 15-fold greater. The enzyme exhibited a broad pH optimum of 7.5–9.0 and a requirement for divalent cation, satisfied by Mg₂₊. The reaction product was identified as 3-phosphoglyceric acid. The observed high glycerate kinase activity together with its strategic localization exclusively in the chloroplast stroma are considered adequate for an efficient coupling of photosynthetic and photorespiratory carbon pathways.