Intermediates in the ribulose-1,5-bisphosphate carboxylase reaction

Multiple catalytic roles of His 287 of Rhodospirillum rubrum ribulose 1,5-bisphosphate carboxylase/oxygenase

Active-site His 287 of Rhodospirillum rubrum ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase interacts with the C3-hydroxyl of bound substrate or reaction-intermediate analogue (CABP), water molecules, and ligands for the activator metal-ion (Andersson I, 1996, J Mol Biol 259:160-174; Taylor TC, Andersson I, 1997, J Mol Biol 265:432-444). To test structure-based postulates of catalytic functionality, His 287 was replaced with Asn or Gln. The mutants are not affected adversely in subunit assembly, activation (binding of Mg2+ and carbamylation of Lys 191), or recognition of phosphorylated ligands; they bind CABP with even greater tenacity than does wild-type enzyme. H287N and H287Q are severely impaired in catalyzing overall carboxylation (approximately 10(3)-fold and > 10(5)-fold, respectively) and enolization (each mutant below threshold for detection) of RuBP. H287N preferentially catalyzes decarboxylation of carboxylated reaction intermediate instead of forward processing to phosphoglycerate. Analysis of RuBP turnover that occurs at high concentrations of mutants over extended time periods reveal > 10-fold reduced CO2/O2 specificities, elevated misprotonation of the enediol intermediate, and misprocessing of the oxygenated intermediate of the oxygenase pathway. These results are consistent with multifaceted roles for His 287 in promoting enediol formation, enediol tautomerization, and forward-processing of carboxylated intermediate.

Facilitation of the terminal proton transfer reaction of ribulose 1,5-bisphosphate carboxylase/oxygenase by active-site Lys166

The terminal step in the carboxylation pathway catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is stereospecific protonation of the C-2 aci-acid of 3-phosphoglycerate (PGA). X-ray crystallographic results favor the epsilon-amino group of Lys166 as the proton donor in this step [Knight et al. (1990) J. Mol. Biol. 215, 113]. Nonetheless, position-166 mutants are able to catalyze forward processing of isolated 2-carboxy-3-ketoarabinitol 1,5-bisphosphate (CKABP), the carboxylated reaction intermediate [Lorimer G.H., & Hartman, F.C. (1988) J. Biol. Chem. 263, 6468]. Prior assays for intermediate processing relied solely on formation of acid-stable radioactivity from acid-labile [2'-14C]CKABP. Therefore, PGA, the normal reaction product, may not have been distinguished from pyruvate, the product from beta-elimination of phosphate from the terminal aci-acid intermediate [Andrews, T.J., & Kane, H.J. (1991) J. Biol. Chem. 266, 9447]. If Lys166 indeed serves as the terminal proton donor, mutants lacking an ionizable side chain at position 166 might process the carboxylated intermediate predominantly to pyruvate. We have thus used anion exchange chromatography and enzyme coupling to separate and identify the products from turnover of [2'-14C]CKABP by wild-type, K166G, and K166S enzymes. Although PGA is the only labeled product of significance formed by wild-type enzyme, pyruvate is a major labeled product formed by the mutants. These results provide the first direct functionally-based evidence that Lys166 is crucial to the last step in Rubisco-catalyzed conversion of RuBP to PGA.

Chemical rescue by exogenous amines of a site-directed mutant of ribulose 1,5-bisphosphate carboxylase/oxygenase that lacks a key lysyl residue

Ligand binding to ribulose 1,5-bisphosphate carboxylase/oxygenase immobilizes the flexible loop 6 of the beta/alpha barrel domain in its closed conformation. Lys329, located at the apex of this loop, interacts electrostatically with Glu48 of the adjacent subunit and with the CO2-derived carboxylate of the carboxylated reaction intermediate [Knight et al. (1990) J. Mol. Biol. 215, 113-160]. Previous studies have implicated Lys329 in the addition of CO2 to the initial enediol(ate) intermediate: mutants at position 329 catalyze enolization of ribulose 1,5-bisphosphate and processing of isolated carboxyketone intermediate, but are severely impaired in overall carboxylation and the tight-binding of the carboxylated intermediate analogue 2-carboxyarabinitol 1,5-bisphosphate. Using the chemical rescue method of Toney and Kirsch [(1989) Science 243, 1485-1488], we show that these defects are partially overcome by exogenous amines. For example, ethylamine enhances the carboxylation rate of K329A by about 80-fold and strengthens complexation of 2-carboxyarabinitol 1,5-bisphosphate. The CO2/O2 specificity of K329A is increased by amines, but remains lower than the wild-type value. Despite the pronounced enhancement of carboxylase activity, amines do not influence the rate at which ribulose 1,5-bisphosphate is enolized by K329A. Rescue of K329A follows an apparent Brønsted relationship with a beta of 1, implying complete protonation of amine in the rescued transition state. Rate saturation with respect to amine concentration and the different steric preferences for amines between K329A and K329C suggest that the amines bind to the enzyme in the position voided by the mutation.(ABSTRACT TRUNCATED AT 250 WORDS)

Intermediates formed by the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase from spinach studied by electron paramagnetic resonance spectroscopy

Two enzyme-metal-bound intermediates formed by the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) have been studied by electron paramagnetic resonance (EPR) spectroscopy. Their rates of approach to a stationary state are different and their relative amounts at steady state are dependent on the concentration of ribulose 1,5-bisphosphate. It is therefore proposed that enzyme-metal-coordinated ribulose 1,5-bisphosphate and an enzyme-metal-coordinated enediolate anion of it, where bound ribulose 1,5-bisphosphate appears first, constitute the two EPR-detectable intermediates, respectively.

Ribulose 1,5-bisphosphate carboxylase-oxygenase. I. Structural, immunochemical and catalytic properties

Some structural, immunochemical and catalytic properties are examined for ribulose 1,5-bisphosphate carboxylase-oxygenase from various cellular organisms including bacteria, cyanobacteria, algae and higher plants. The native enzyme molecular masses and the subunit polypeptide compositions vary according to enzyme sources. The molecular masses of the large and small subunits from different cellular organisms, on the other hand, show a relatively high homology due to their well-conserved primary amino acid sequence, especially that of the large subunit. In higher plants, the native enzyme and the large subunit are recognized by the antibodies raised against either the native or large subunit, whereas the small subunit apparently cross-reacts only with the antibodies directed against itself. A wide diversity exists, however, in the serological response of the native enzyme and its subunits with antibodies directed against the native enzyme or its subunits from different cellular organisms. According to numerous kinetic studies, the carboxylase and oxygenase reactions of the enzyme with ribulose 1,5-bisphosphate and carbon dioxide or oxygen require activation by carbon dioxide and magnesium prior to catalysis with ribulose 1,5-bisphosphate and carbon dioxide or oxygen. The activation and catalysis are also under the regulation of other metal ions and a number of chloroplastic metabolites. Recent double-labeling experiments using radioactive ribulose 1,5-bisphosphate and 14CO2 have elucidated the carboxylase/oxygenase ratios of the enzymes from different organisms. Another approach, i.e., genetic experiments, has also been used to examine the modification of the carboxylase/oxygenase ratio.

Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate

In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP+ by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, tau L = 0.4 s, which is nearly two orders of magnitude shorter than the corresponding value, tau D = 11 +/- 3 s, for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible light-dark cycles of the NADP+/NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitol-1-phosphate binding to the enzyme active site.

The synthesis of mannose 1-phosphate in brain

The interconversion of mannose-6-P and mannose-1-P in brain has been shown to be catalyzed by a distinct enzyme. The enzyme has been separated from most of the phosphoglucomutase activity of the brain. The residual phosphoglucomutase activity (less than 1%) may be associated with phosphomannomutase itself. Mannose-1,6-P2 or glucose-1,6-P2 is required for the reaction as well as a divalent cation (Mg2+ greater than Co2+ greater than Ni2+ greater than Mn2+). Glucose-1-P, glucose-6-P, and 2-deoxyglucose-6-P are also substrates or inhibitors. Other phosphorylated sugars tested, glucosamine-6-P, N-acetylglucosamine-6-P, galactose-6-P, fructose-6-P, ribose-5-P, and arabinose-5-P, do not affect the rate of the reaction when assayed in the presence of mannose-6-32P.

Carbon isotope effect on carboxylation of ribulose bisphosphate catalyzed by ribulosebisphosphate carboxylase from Rhodospirillum rubrum

The carbon isotope effect at CO2 has been measured in the carboxylation of ribulose 1,5-bisphosphate by the ribulosebisphosphate carboxylase from Rhodospirillum rubrum. The isotope effect is obtained by comparing the isotopic composition of carbon 1 of the 3-phosphoglyceric acid formed in the reaction with that of the carbon dioxide source. A correction is made for carbon 1 of 3-phosphoglyceric acid which arises from carbon 3 of the starting ribulose bisphosphate. The isotope effect is k12/k13 = 1.0178 +/- 0.0008 at 25 degrees C, pH 7.8. This value is smaller than the corresponding value for the spinach enzyme. It appears that substrate addition with the R. rubrum enzyme is principally ordered, with ribulose bisphosphate binding first, whereas substrate addition is random with the spinach enzyme. The carboxylation step is partially rate limiting with both enzymes.