The Presence of Tightly Bound Copper in Ribulose Diphosphate Carboxylase from Spinach

Proteomics analysis reveals a dynamic diurnal pattern of photosynthesis-related pathways in maize leaves

Plant leaves exhibit differentiated patterns of photosynthesis rates under diurnal light regulation. Maize leaves show a single-peak pattern without photoinhibition at midday when the light intensity is maximized. This mechanism contributes to highly efficient photosynthesis in maize leaves. To understand the molecular basis of this process, an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomics analysis was performed to reveal the dynamic pattern of proteins related to photosynthetic reactions. Steady, single-peak and double-peak protein expression patterns were discovered in maize leaves, and antenna proteins in these leaves displayed a steady pattern. In contrast, the photosystem, carbon fixation and citrate pathways were highly controlled by diurnal light intensity. Most enzymes in the limiting steps of these pathways were major sites of regulation. Thus, maize leaves optimize photosynthesis and carbon fixation outside of light harvesting to adapt to the changes in diurnal light intensity at the protein level.

A novel copper-containing protein from spinach: a blue oxidase with unique properties

A copper-containing protein resembling in its optical and EPR spectra stellacyanin from latex was isolated from spinach leaves. The protein oxidizes ferrocyanide and catechol. The activity was highest at acidic pH. It was shown that similar proteins isolated from cucumber and squash also possess the oxidase activity to ferrocyanide.

Crystalline Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase from Spinach

Spinach fraction I protein (ribulose 1,5-bisphosphate carboxylase-oxy genase, E.C. 4.1.1.39) was crystallized on both an analytical and a preparative scale by vapor diffusion with polyethylene glycol (molecular weight, 6000) used as the precipitant. The identity of the crystalline material with fraction I protein was shown by gel electrophoresis in the presence of sodium dodecyl sulfate and immunological properties. The carboxylase and oxygenase activities copurify during crystallization, and the crystalline enzyme lacks copper and iron.

The effect of divalent metal ions on the activity of Mg(++) depleted ribulose-1,5-bisphosphate oxygenase

Ribulose-1,5-bisphosphate carboxylase-oxygenase is deactivated by removal of Mg(++). The enzyme activities can be restored to a different extent by the addition of various divalent ions in the presence of CO2. Incubation with Mg(++) and CO2 restores both enzyme activities, whereas, the treatment of the enzyme with the transition metal ions (Mn(++), Co(++), and Ni(++)) and CO2 fully reactivates the oxygenase: however, the carboxylase activity remains low. In experiments where CO2-free conditions were conscientiously maintained, no reactivation of RuBP oxygenase was observed, although Mn(++) ions were present. Other divalent cations such as Ca(++) and Zn(++), restore neither the carboxylase nor the oxygenase reaction. Furthermore, the addition of Mn(++) to the Mg(++) and CO2 preactivated enzyme significantly inhibited carboxylase reactions, but increased the oxygenase reaction.

Mechanism of action of ribulose bisphosphate carboxylase/oxygenase

RuBP carboxylase-oxygenase appears to catalyze carboxylation and oxygenation by homologous mechanisms. A common binding site exists on the enzyme for the acceptor substrate, RuBP. A mechanism is proposed whereby RuBP is isomerized, and a carbanion is generated at C2. Then, either CO2 or O2 is added as an electrophile at C2 to form the corresponding 3-keto-2-carboxy-RBP or 3-keto-2-hydroperoxy-RBP adduct. Hydrolytic cleavage at the C2-C3 bonds of these intermediates by the enzyme is envisioned to produce 2 molecules of 3-phosphoglycerate in the carboxylation sequence and 1 molecule of phosphoglycolate and 1 molecule of 3-phosphoglycerate in the oxygenation sequence. Further work will be necessary to establish the validity of the proposed mechanism.

Role of tryptophan in the spectral and catalytic properties of the copper enzyme, galactose oxidase

Previous results indicate that a tryptophan residue(s) may interact with the sugar substrate and Cu(II) atom of galactose oxidase (Ettinger, M. J., and Kosman, D. J. (1974), Biochemistry 13, 1248). We now show that N-bromosuccinimide (NBS) reduces enzymatic activity to 2% as two tryptophans are oxidized; only four residues are easily oxidized in the holoenzyme. An enzymatic activity vs. number of residues oxidized profile suggests that this inactivation is probably associated with only one of the first 2 residues oxidized. There is no evidence for chain cleavage or modification of amino acids other than tryptophan. While substrate protection is not afforded by the sugar substrate, the activity-related tryptophan is placed within the active-site locus by spectral evidence. NBS oxidation of two tryptophans results in a marked diminution of the large copper optical-activity transition at 314 nm. Under some reaction conditions, a doubling of ellipticity in the 600-nm region of copper CD is also observed. The effects of the NBS oxidation on the CD spectra of galactose oxidase permit the assignment of the 314-nm CD band to a charge-transfer transition and the 229-nm extremum to a specific tryptophan contribution. The AZZ parameter from electron spin resonance spectra is also markedly reduced by the NBS oxidation. Moreover, while cyanide binds to the native enzyme without reducing the Cu(II) atom, cyanide rapidly reduces the Cu(II) atom to Cu(I) in the NBS-oxidized enzyme. These CD and ESR results are taken to suggest that one aspect of the inactivation by NBS oxidation may be a conversion of the pseudosquare planar copper complex in the native enzyme to a more distorted, towards tetrahedral, complex in the inactivated enzyme. Since the inactivation can be accomplished without affecting binding of the sugar substrate, tryptophan oxidation must affect catalysis per se.

Ribulose biophosphate carboxylase from Thiobacillus A2. Its purification and properties

Ribulose bisphosphate carboxylase (EC 4.1.1.39) from Thiobacillus A2 has been purified to homogeneity on the basis of polyacrylamide gel electrophoresis and U.V. analysis during sedimentation velocity studies. The enzyme had an optimum pH of about 8.2 with Tris-HCl buffers. The molecular weight was about 521000 with an Srel. of 16.9. Km for RuBP was 122 muM, for total "CO2" it was 4.17 mM, and for Mg2+ 20.0 muM. The absolute requirement for a divalent cation was satisfied by Mg2+ which was replaceable to a certain extent by Mn2+. Activity was not significantly affected by SO(2-4), SO(2-3), or S(2)O(2-3) at 1.0 mM. At this concentration S(2-) caused a 27% stimulation. All mercurials tested were inhibitory. pHMB was the most potent causing about 60% inhibition at 0.04 mM. This inhibition was reversible by low concentrations of cysteine. Cyanide was also inhibitory. Its mode of inhibition with respect to RuBP was un-competitive and with a Ki of 20 muM. Lost activity could be restored partially by GSH or Cu2+. Although azide at the concentration tested had no significant effect on enzyme activity, 2, 4-dinitrophenol at 1.0 mM caused 91% inhibition. Finally, activity was also affected by energy charge.

Ribulose diphosphate carboxylase from autotrophic microorganisms

Thiobacillus denitrificans was grown anaerobically with nitrate as an acceptor in both sterile and nonsterile media. Ribulose diphosphate carboxylase was stable throughout the exponential growth phase and declined slowly only after cells reached the stationary phase. Reversible inactivation of the carboxylase occurred in extracts as a result of bicarbonate omission. The enzyme was purified 32-fold with excellent recovery of a preparation which was 50 to 60% pure by the criterion of polyacrylamide gel electrophoresis. This purified preparation catalyzed the fixation of 1.25 mumoles of CO(2) per min per mg of protein at pH 8.1 and 30 C, and the molecular weight of ribulose diphosphate carboxylase was approximately 350,000 daltons. A striking biphasic time course of CO(2) fixation that was independent of protein and ribulose diphosphate concentration was observed. The optimal pH of the enzyme assay was fairly broad, ranging from 7 to 8.2. Kinetic dependence upon bicarbonate, ribulose diphosphate, and Mg(2+) was characterized and indicated that bicarbonate and Mg(2+) must combine with enzyme prior to addition of ribulose diphosphate. Antiserum to ribulose diphosphate carboxylase from Hydrogenomonas eutropha was only slightly inhibitory when added to the enzyme from T. denitrificans, and the mixture did not precipitate. Cyanide (4 x 10(-5)m) gave 61% inhibition of the enzyme from T. denitrificans. Ribulose diphosphate carboxylase in extracts of H. eutropha, H. facilis, Chromatium D, Rhodospirillum rubrum, and Chlorella pyrenoidosa were also inhibited to varying extents by cyanide and antiserum to the H. eutropha enzyme.