Phosphoribulokinase

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phosphoribulokinase
MhPRK crystal structure.jpg
3D cartoon depiction of a phosphoribulokinase protomer from Methanospirillum hungatei
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EC no. 2.7.1.19
CAS no. 9030-60-8
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Phosphoribulokinase (PRK) (EC 2.7.1.19) is an essential photosynthetic enzyme that catalyzes the ATP-dependent phosphorylation of ribulose 5-phosphate (RuP) into ribulose 1,5-bisphosphate (RuBP), both intermediates in the Calvin Cycle. Its main function is to regenerate RuBP, which is the initial substrate and CO2-acceptor molecule of the Calvin Cycle. [1] PRK belongs to the family of transferase enzymes, specifically those transferring phosphorus-containing groups (phosphotransferases) to an alcohol group acceptor. Along with ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo), phosphoribulokinase is unique to the Calvin Cycle. [2] Therefore, PRK activity often determines the metabolic rate in organisms for which carbon fixation is key to survival. [3] Much initial work on PRK was done with spinach leaf extracts in the 1950s; subsequent studies of PRK in other photosynthetic prokaryotic and eukaryotic organisms have followed. The possibility that PRK might exist was first recognized by Weissbach et al. in 1954; for example, the group noted that carbon dioxide fixation in crude spinach extracts was enhanced by the addition of ATP. [3] [4] The first purification of PRK was conducted by Hurwitz and colleagues in 1956. [5] [6] [7]

Contents

ATP + Mg2+ - D-ribulose 5-phosphate  ADP + D-ribulose 1,5-bisphosphate
Reaction scheme for the regeneration of ribulose 1,5-bisphosphate from ribulose 5-phosphate by phosphoribulokinase Prk reaction scheme mjp.tif
Reaction scheme for the regeneration of ribulose 1,5-bisphosphate from ribulose 5-phosphate by phosphoribulokinase

The two substrates of PRK are ATP and D-ribulose 5-phosphate, whereas its two products are ADP and D-ribulose 1,5-bisphosphate. PRK activity requires the presence of a divalent metal cation like Mg2+, as indicated in the reaction above. [3]

Structure

The structure of PRK is different in prokaryotes and eukaryotes. Prokaryotic PRK's typically exist as octamers of 32 kDa subunits, while eukaryotic PRK's are often dimers of 40 kDa subunits. [8] [9] Structural determinations for eukaryotic PRK have yet to be conducted, but prokaryotic PRK structures are still useful for rationalizing the regulation and mechanism of PRK. As of 2018, only two crystal structures have been resolved for this class of enzymes in Rhodobacter sphaeroides and Methanospirillum hungatei , with the respective PDB accession codes 1A7J and 5B3F.

Key residues that interact with RuP (labeled in blue) or with the hydroxyl group in RuP (red) within the active site of R. sphaeroides PRK. Generated from 1A7J. Click to view enlarged. RsPRK labeled.png
Key residues that interact with RuP (labeled in blue) or with the hydroxyl group in RuP (red) within the active site of R. sphaeroides PRK. Generated from 1A7J. Click to view enlarged.

Rhodobacter sphaeroides

In Rhodobacter sphaeroides, PRK (or RsPRK) exists as a homooctomer with protomers composed of seven-stranded mixed β-sheets, seven α-helices, and an auxiliary pair of anti-parallel β-strands. [10] The RsPRK subunit exhibits a protein folding analogous to the folding of nucleotide monophosphate (NMP) kinases. [3] Mutagenesis studies suggest that either Asp 42 or Asp 169 acts as the catalytic base that deprotonates the O1 hydroxyl oxygen on RuP for nucleophilic attack of ATP, while the other acts a ligand for a metal cation like Mg2+ (read mechanism below for more details). [10] Other residues present at the active site for RsPRK include His 45, Arg 49, Arg 168, and Arg 173, which are purportedly involved in RuP binding. [10] (See image at right).

Methanospirillum hungatei

In archaeal PRK of Methanospirillum hungatei, PRK (or MhPRK) exists as a homodimer of two protomers, each consisting of eight-stranded mixed β-sheets surrounded by α-helices and β-strands—similar to the structure of bacterial PRK from R. sphaeroides (see info. box above). [11] Although their quaternary structures differ and they have low amino acid sequence identity, MhPRK and RsPRK have structurally similar N-terminal domains as well as sequentially conserved residues like His 55, Lys 151, and Arg 154. [11]

Mechanism and Activity

PRK catalyzes the phosphorylation of RuP into RuBP. A catalytic residue in the enzyme (i.e. aspartate in RsPRK) deprotonates the O1 hydroxyl oxygen on RuP and activates it for nucleophilic attack of the γ-phosphoryl group of ATP. [10] As the γ-phosphoryl group is transferred from ATP to RuP, its stereochemistry inverts. [12] To allow for such inversion, the catalytic mechanism of PRK must not involve a phosphoryl-enzyme intermediate. [12]

Some studies suggest that both substrates (ATP and RuP) bind simultaneously to PRK and form a ternary complex. Others suggest that the substrate addition is sequential; the particular order by which substrates are added is still disputed, and may in fact, vary for different organisms. [13] [14] In addition to binding its substrates, PRK also requires ligation to divalent metal cations like Mg2+ or Mn2+ for activity; Hg2+ has been demonstrated to inactivate the enzyme. [3] [15]

Enzyme specificity

PRK shows high specificity for ribulose 5-phosphate. It does not act on any of the following substrates: D-xylulose 5-phosphate, fructose 6-phosphate, and sedoheptulose 7-phosphate. [15] However, at high concentrations, PRK may sometimes phosphorylate ribose 5-phosphate, a compound upstream the RuBP regeneration step in the Calvin Cycle. [15] Furthermore, PRK isolated from Alcaligenes eutrophus has been shown to use uridine triphosphate (UTP) and guanosine triphosphate (GTP) as alternative substrates to ATP. [8] [3]

pH effects

The phosphorylation reaction proceeds with maximal velocity at pH 7.9, with no detectable activity at pH's below 5.5 or above 9.0. [15]

Regulation

The mechanisms by which prokaryotic and eukaryotic PRK's are regulated vary. Prokaryotic PRK's are typically subject to allosteric regulation while eukaryotic PRK's are often regulated by reversible thiol/disulfide exchange. [16] These differences are likely due to structural differences in their C-terminal domains [11]

Allosteric regulation of prokaryotic PRK

NADH is known to stimulate PRK activity, while AMP and phosphoenolpyruvate (PEP) are known to inhibit activity. [3] AMP has been shown to be involved in competitive inhibition in Thiobacillus ferrooxidans PRK. [17] On the other hand, PEP acts as a non-competitive inhibitor of PRK. [18]

Regulation of eukaryotic PRK

Eukaryotic PRK is typically regulated through the reversible oxidation/reduction of its cysteine sulfhydryl groups, but studies suggests that its activity can be regulated by other proteins or metabolites in the chloroplast. Of such metabolites, 6-phosphogluconate has been shown to be the most effective inhibitor of eukaryotic PRK by competing with RuP for the enzyme's active site. [19] This phenomenon may arise from the similarity in molecular structure between 6-phosphogluconate and RuP.

More recent work on the regulation of eukaryotic PRK has focused on its ability to form multi-enzyme complexes with other Calvin cycle enzymes such as glyceraldehyde 3-phosphate dehydrogenase (G3PDH) or RuBisCo. [20] In Chlamydomonas reinhardtii , chloroplast PRK and G3PDH exist as a bi-enzyme complex of 2 molecules of dimeric PRK and 2 molecules of tetrameric G3PDH thorough association by an Arg 64 residue, which may potentially transfer information between the two enzymes as well. [21]

Multi-enzyme complexes are likely to have more intricate regulatory mechanisms, and studies have already probed such processes. For example, it has been shown that PRK-glyceraldehyde 3-phosphate dehydrogenase complexes in Scenedesmus obliquus only dissociate to release activated forms of its constituent enzymes in the presence of NADPH, dithiothreitol (DTT), and thioredoxin. [22] Another topic of interest has been to compare the relative levels of PRK activity for when it is complexed to when it is not. For different photosynthetic eukaryotes, the enzyme activity of complexed PRK may be enhanced as opposed to free PRK, and vice versa. [23] [24]

Other names

The systematic name of this enzyme class is ATP:D-ribulose-5-phosphate 1-phosphotransferase. Other names in common use include phosphopentokinase, ribulose-5-phosphate kinase, phosphopentokinase, phosphoribulokinase (phosphorylating), 5-phosphoribulose kinase, ribulose phosphate kinase, PKK, PRuK, and PRK.

Related Research Articles

<span class="mw-page-title-main">Glycolysis</span> Catabolic pathway

Glycolysis is the metabolic pathway that converts glucose into pyruvate, and in most organisms, occurs in the liquid part of cells, the cytosol. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

<span class="mw-page-title-main">Phosphorylation</span> Chemical process of introducing a phosphate

In biochemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology. Protein phosphorylation often activates many enzymes.

<span class="mw-page-title-main">RuBisCO</span> Key enzyme of the photosynthesis involved in carbon fixation

Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in light-independent part of photosynthesis, including the carbon fixation by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose. It emerged approximately four billion years ago in primordial metabolism prior to the presence of oxygen on earth. It is probably the most abundant enzyme on Earth. In chemical terms, it catalyzes the carboxylation of ribulose-1,5-bisphosphate.

<span class="mw-page-title-main">Ribulose 1,5-bisphosphate</span> Chemical compound

Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal CO2 acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose called ribulose. Salts of RuBP can be isolated, but its crucial biological function happens in solution. RuBP occurs not only in plants but in all domains of life, including Archaea, Bacteria, and Eukarya.

A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 (aldotetroses) or a ketone functional group in position 2 (ketotetroses).

<span class="mw-page-title-main">Calvin cycle</span> Light-independent reactions in photosynthesis

The Calvin cycle,light-independent reactions, bio synthetic phase,dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes. These reactions take the products of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

<span class="mw-page-title-main">Glyceraldehyde 3-phosphate</span> Chemical compound

Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms. With the chemical formula H(O)CCH(OH)CH2OPO32-, this anion is a monophosphate ester of glyceraldehyde.

Dihydroxyacetone phosphate (DHAP, also glycerone phosphate in older texts) is the anion with the formula HOCH2C(O)CH2OPO32-. This anion is involved in many metabolic pathways, including the Calvin cycle in plants and glycolysis. It is the phosphate ester of dihydroxyacetone.

Methylotrophs are a diverse group of microorganisms that can use reduced one-carbon compounds, such as methanol or methane, as the carbon source for their growth; and multi-carbon compounds that contain no carbon-carbon bonds, such as dimethyl ether and dimethylamine. This group of microorganisms also includes those capable of assimilating reduced one-carbon compounds by way of carbon dioxide using the ribulose bisphosphate pathway. These organisms should not be confused with methanogens which on the contrary produce methane as a by-product from various one-carbon compounds such as carbon dioxide. Some methylotrophs can degrade the greenhouse gas methane, and in this case they are called methanotrophs. The abundance, purity, and low price of methanol compared to commonly used sugars make methylotrophs competent organisms for production of amino acids, vitamins, recombinant proteins, single-cell proteins, co-enzymes and cytochromes.

<span class="mw-page-title-main">3-Phosphoglyceric acid</span> Chemical compound

3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P). This glycerate is a biochemically significant metabolic intermediate in both glycolysis and the Calvin-Benson cycle. The anion is often termed as PGA when referring to the Calvin-Benson cycle. In the Calvin-Benson cycle, 3-phosphoglycerate is typically the product of the spontaneous scission of an unstable 6-carbon intermediate formed upon CO2 fixation. Thus, two equivalents of 3-phosphoglycerate are produced for each molecule of CO2 that is fixed. In glycolysis, 3-phosphoglycerate is an intermediate following the dephosphorylation (reduction) of 1,3-bisphosphoglycerate.

<span class="mw-page-title-main">Phosphoenolpyruvate carboxylase</span> Class of enzymes

Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; EC 4.1.1.31, PDB ID: 3ZGE) is an enzyme in the family of carboxy-lyases found in plants and some bacteria that catalyzes the addition of bicarbonate (HCO3−) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate:

<span class="mw-page-title-main">Phosphoglycerate kinase</span> Enzyme

Phosphoglycerate kinase is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate (1,3-BPG) to ADP producing 3-phosphoglycerate (3-PG) and ATP :

<span class="mw-page-title-main">6-Phosphogluconate dehydrogenase</span> Class of enzymes

6-Phosphogluconate dehydrogenase (6PGD) is an enzyme in the pentose phosphate pathway. It forms ribulose 5-phosphate from 6-phosphogluconate:

<span class="mw-page-title-main">Phosphopentose epimerase</span>

Phosphopentose epimerase encoded by the RPE gene is a metalloprotein that catalyzes the interconversion between D-ribulose 5-phosphate and D-xylulose 5-phosphate.

<span class="mw-page-title-main">Fructose-bisphosphate aldolase</span>

Fructose-bisphosphate aldolase, often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Aldolase can also produce DHAP from other (3S,4R)-ketose 1-phosphates such as fructose 1-phosphate and sedoheptulose 1,7-bisphosphate. Gluconeogenesis and the Calvin cycle, which are anabolic pathways, use the reverse reaction. Glycolysis, a catabolic pathway, uses the forward reaction. Aldolase is divided into two classes by mechanism.

<span class="mw-page-title-main">Ribose-5-phosphate isomerase</span>

Ribose-5-phosphate isomerase (Rpi) encoded by the RPIA gene is an enzyme that catalyzes the conversion between ribose-5-phosphate (R5P) and ribulose-5-phosphate (Ru5P). It is a member of a larger class of isomerases which catalyze the interconversion of chemical isomers. It plays a vital role in biochemical metabolism in both the pentose phosphate pathway and the Calvin cycle. The systematic name of this enzyme class is D-ribose-5-phosphate aldose-ketose-isomerase.

In enzymology, a [isocitrate dehydrogenase (NADP+)] kinase (EC 2.7.11.5) is an enzyme that catalyzes the chemical reaction:

In enzymology, a triokinase is an enzyme that catalyzes the chemical reaction

Bisphosphate may refer to:

<span class="mw-page-title-main">2-Phosphoglycolate</span> Chemical compound

2-Phosphoglycolate (chemical formula C2H2O6P3-; also known as phosphoglycolate, 2-PG, or PG) is a natural metabolic product of the oxygenase reaction mediated by the enzyme ribulose 1,5-bisphosphate carboxylase (RuBisCo).

References

  1. 1 2 Berg, Jeremy M.; Tymoczko, John L.; Gatto, Jr., Gregory J.; Stryer, Lubert (2015-04-08). Biochemistry (Eighth ed.). New York. ISBN   978-1464126109. OCLC   913469736.{{cite book}}: CS1 maint: location missing publisher (link)
  2. ↑ Marsden WJ (September 16, 1983). "Purification and Molecular and Catalytic Properties of Phosphoribulokinase from the Cyanobacterium Chlorogloeopsis fritschii". Journal of General Microbiology. 130 (4): 999–1006. doi: 10.1099/00221287-130-4-999 .
  3. 1 2 3 4 5 6 7 Miziorko HM (2000). "Phosphoribulokinase: Current Perspectives on the Structure/Function Basis for Regulation and Catalysis". In Purich DL (ed.). Advances in Enzymology and Related Areas of Molecular Biology. Advances in Enzymology - and Related Areas of Molecular Biology. Vol. 74. John Wiley & Sons, Inc. pp. 95–127. doi:10.1002/9780470123201.ch3. ISBN   9780470123201. PMID   10800594.
  4. ↑ Weissbach A, Smyrniotis PZ, Horecker BL (July 1954). "Pentose phosphate and CO2 fixation with spinach extracts". Journal of the American Chemical Society. 76 (13): 3611–3612. doi:10.1021/ja01642a090.
  5. ↑ Hurwitz J, Weissbach A, Horecker BL, Smyrniotis PZ (February 1956). "Spinach phosphoribulokinase". The Journal of Biological Chemistry. 218 (2): 769–83. PMID   13295229.
  6. ↑ Racker E (July 1957). "The reductive pentose phosphate cycle. I. Phosphoribulokinase and ribulose diphosphate carboxylase". Archives of Biochemistry and Biophysics. 69: 300–10. doi:10.1016/0003-9861(57)90496-4. PMID   13445203.
  7. ↑ Jakoby WB, Brummond DO, Ochoa S (February 1956). "Formation of 3-phosphoglyceric acid by carbon dioxide fixation with spinach leaf enzymes". The Journal of Biological Chemistry. 218 (2): 811–22. PMID   13295232.
  8. 1 2 Siebert K, Schobert P, Bowien B (March 1981). "Purification, some catalytic and molecular properties of phosphoribulokinase from Alcaligenes eutrophus". Biochimica et Biophysica Acta (BBA) - Enzymology. 658 (1): 35–44. doi:10.1016/0005-2744(81)90247-3. PMID   6260209.
  9. ↑ Buchanan, Bob B. (2003-11-28). "Role of Light in the Regulation of Chloroplast Enzymes". Annu. Rev. Plant Physiol. 31: 341–374. doi:10.1146/annurev.pp.31.060180.002013.
  10. 1 2 3 4 Harrison DH, Runquist JA, Holub A, Miziorko HM (April 1998). "The crystal structure of phosphoribulokinase from Rhodobacter sphaeroides reveals a fold similar to that of adenylate kinase". Biochemistry. 37 (15): 5074–85. doi:10.1021/bi972805y. PMID   9548738.
  11. 1 2 3 Kono T, Mehrotra S, Endo C, Kizu N, Matusda M, Kimura H, Mizohata E, Inoue T, Hasunuma T, Yokota A, Matsumura H, Ashida H (January 2017). "A RuBisCO-mediated carbon metabolic pathway in methanogenic archaea". Nature Communications. 8: 14007. Bibcode:2017NatCo...814007K. doi:10.1038/ncomms14007. PMC   5241800 . PMID   28082747.
  12. 1 2 Miziorko HM, Eckstein F (November 1984). "The stereochemical course of the ribulose-5-phosphate kinase-catalyzed reaction". The Journal of Biological Chemistry. 259 (21): 13037–40. PMID   6490643.
  13. ↑ Lebreton S, Gontero B, Avilan L, Ricard J (December 1997). "Information transfer in multienzyme complexes--1. Thermodynamics of conformational constraints and memory effects in the bienzyme glyceraldehyde-3-phosphate-dehydrogenase-phosphoribulokinase complex of Chlamydomonas reinhardtii chloroplasts". European Journal of Biochemistry. 250 (2): 286–95. doi: 10.1111/j.1432-1033.1997.0286a.x . PMID   9428675.
  14. ↑ Wadano A, Nishikawa K, Hirahashi T, Satoh R, Iwaki T (1998-04-01). "Reaction mechanism of phosphoribulokinase from a cyanobacterium, Synechococcus PCC7942". Photosynthesis Research. 56 (1): 27–33. doi:10.1023/A:1005979801741. S2CID   21409736.
  15. 1 2 3 4 Hurwitz J (1962). [28c] Phosphoribulokinase. Methods in Enzymology. Vol. 5. pp. 258–261. doi:10.1016/s0076-6879(62)05214-3. ISBN   9780121818050.
  16. ↑ Tabita FR (September 1980). "Pyridine nucleotide control and subunit structure of phosphoribulokinase from photosynthetic bacteria". Journal of Bacteriology. 143 (3): 1275–80. doi:10.1128/JB.143.3.1275-1280.1980. PMC   294495 . PMID   6251028.
  17. ↑ Gale NL, Beck JV (September 1966). "Competitive inhibition of phosphoribulokinase by AMP". Biochemical and Biophysical Research Communications. 24 (5): 792–6. doi:10.1016/0006-291X(66)90396-2. PMID   5970515.
  18. ↑ Ballard RW, MacElroy RD (August 1971). "Phosphoenolpyruvate, a new inhibitor of phosphoribulokinase in pseudomonas facilis". Biochemical and Biophysical Research Communications. 44 (3): 614–8. doi:10.1016/s0006-291x(71)80127-4. PMID   4330777.
  19. ↑ Gardemann, A.; Stitt, M.; Heldt, H.W. (1983-01-13). "Control of CO2 fixation. Regulation of spinach ribulose-5-phosphate kinase by stromal metabolite levels". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 722 (1): 51–60. doi:10.1016/0005-2728(83)90156-1.
  20. ↑ Müller, Bruno (1972-08-01). "A Labile CO2-Fixing Enzyme Complex in Spinach Chloroplasts". Zeitschrift für Naturforschung B. 27 (8): 925–932. doi: 10.1515/znb-1972-0814 .
  21. ↑ Avilan L, Gontero B, Lebreton S, Ricard J (December 1997). "Information transfer in multienzyme complexes--2. The role of Arg64 of Chlamydomonas reinhardtii phosphoribulokinase in the information transfer between glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase". European Journal of Biochemistry. 250 (2): 296–302. doi: 10.1111/j.1432-1033.1997.0296a.x . PMID   9428676.
  22. ↑ Nicholson S, Easterby JS, Powls R (January 1987). "Properties of a multimeric protein complex from chloroplasts possessing potential activities of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase". European Journal of Biochemistry. 162 (2): 423–31. doi: 10.1111/j.1432-1033.1987.tb10619.x . PMID   3026812.
  23. ↑ Rault M, Gontero B, Ricard J (May 1991). "Thioredoxin activation of phosphoribulokinase in a chloroplast multi-enzyme complex". European Journal of Biochemistry. 197 (3): 791–7. doi: 10.1111/j.1432-1033.1991.tb15973.x . PMID   1851485.
  24. ↑ Gontero B, Mulliert G, Rault M, Giudici-Orticoni MT, Ricard J (November 1993). "Structural and functional properties of a multi-enzyme complex from spinach chloroplasts. 2. Modulation of the kinetic properties of enzymes in the aggregated state". European Journal of Biochemistry. 217 (3): 1075–82. doi: 10.1111/j.1432-1033.1993.tb18339.x . PMID   8223631.
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