Ribulose bisphosphate enzyme

Figure 4.8 The active site in all a/p barrels is in a pocket formed by the loop regions that connect the carboxy ends of the p strands with the adjacent a helices, as shown schematically in (a), where only two such loops are shown, (b) A view from the top of the barrel of the active site of the enzyme RuBisCo (ribulose bisphosphate carboxylase), which is involved in CO2 fixation in plants. A substrate analog (red) binds across the barrel with the two phosphate groups, PI and P2, on opposite sides of the pocket. A number of charged side chains (blue) from different loops as welt as a Mg ion (yellow) form the substrate-binding site and provide catalytic groups. The structure of this 500 kD enzyme was determined to 2.4 A resolution in the laboratory of Carl Branden, in Uppsala, Sweden. (Adapted from an original drawing provided by Bo Furugren.)...

Bromo-l,4-dihydroxybutan-2-one 1,4-bisphosphate (31) has been prepared from the protected bromohydrin of cw-but-2-ene-l,4-diol.95 Nucleophiles rapidly displace bromide ion from (31), and the latter has been used as an affinity label for ribulose-bisphosphate carboxylase. In this case two sites are labelled in each enzyme molecule, the two molecules of (31) being linked to the enzyme by two different lysine residues.96... 

In this reaction, one molecule of ribulose-1,5-bisphosphate (metabolite 1) and one molecule of CO2 (metabolite 2) give rise to two molecules of 3-phosphoglycerate (metabolite 3). The enzyme responsible has the EC number 4.1.1.39. The annotated enzyme list shows that this refers to ribulose bisphosphate carboxylase ( rubisco for short). Rubisco belongs to enzyme class 4 (the lyases) and, within that group, to subclass 4.1 (the car-boxy-lyases). It contains copper as a cofactor ([Cu]). 

The enzyme responsible for the fixation of CO2, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), also catalyses the oxygenation of ribulose bisphosphate to form one molecule of phosphoglycerate and one molecule of phosphoglycolate. Under normal C02 and 02 concentrations six ribulose bisphosphate molecules react with oxygen for every 15 that react with carbon dioxide. 

Phosphate and bicarbonate ions are important substrates for many enzymatic processes and as such have regulatory functions. Bicarbonate controls the key enzyme of photosynthesis, ribulose bisphosphate carboxylase, by carbamate formation (Fig. 13-12). Chloride ions activate amylases and may affect the action of "G proteins" that mediate hormone actions. Other observed effects of ions are too numerous to mention. 

An analogous use of ATP is found in photosynthetic reduction of carbon dioxide in which ATP phos-phorylates ribulose 5-P to ribulose bisphosphate and the phosphate groups are removed later by phosphatase action on fructose bisphosphate and sedoheptulose bisphosphate (Section J,2). Phosphatases involved in synthetic pathways usually have a high substrate specificity and are to be distinguished from nonspecific phosphatases which are essentially digestive enzymes (Chapter 12). 

The following substances are either inhibitors or activators of rubisco, the enzyme that catalyzes the condensation of C02 with ribulose bisphosphate to yield 3-phosphoglycerate. State whether the substance should be an activator or an inhibitor... 

The reaction catalyzed by ribulose bisphosphate carboxylase involves 2-carboxy-3-ketoarabinitol-1,5-bisphosphate as an enzyme-bound intermediate. The intermediate probably forms by the addition of C02 to the enolate of ribulose-1,5-bisphosphate. The substrate is known to be C02 rather than bicarbonate. 

The same active site of ribulose bisphosphate carboxylase also catalyzes a competing reaction in which 02 replaces C02 as a substrate. The products of this oxygenase reaction are 3-phosphoglycerate and a two-carbon acid, 2-phosphoglycolate (fig. 15.27). Phosphoglycolate is oxidized to C02 by 02 in additional reactions involving enzymes in the cytosol, mitochondria, and another organelle, the peroxisome. This photorespiration is not coupled to oxidative phosphorylation, and it appears to constitute a severe... 

Photorespiration results from the oxygenation reaction catalyzed by ribulose bisphosphate carboxylase/ oxygenase. 2-Phosphoglycolate generated by the reaction moves from the chloroplast to the cytosol, where other enzymes break it down to C02, H20, and Pj. The oxygenation reaction, like carboxylation, does not require light directly. It occurs mainly during illumination, however, because the formation of the substrate, ribulose-1,5-bisphosphate, requires ATP and NADPH (see fig. 15.25). 

The dark reactions (carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to fix carbon dioxide as carbohydrate sucrose and starch. The reactions form a cycle (the Calvin cycle) in which the enzyme ribulose bisphosphate carboxylase (rubisco), located in the stroma, condenses a C02 molecule with ribulose 1,5-bisphosphate to produce two molecules of 3-phosphoglycerate. Other reactions then regenerate the ribulose... 

Another unique property of at least some of the halobacteria is the ability to grow phototrophically by employing the light-driven proton pump bacteriorhodopsin. The proton gradient that is produced is used directly to generate ATP (Hartmann et al., 1980 Oesterhelt and Kripphal, 1983). Photoassimilation of CO2 by halobacteria was shown by Danon and Caplan (1977) and Oren (1983). In vivo C02 fixation was demonstrated by Javor (1988) and the existence of the enzyme ribulose-bisphosphate carboxylase activity in several halobacteria was shown by Altekar and Rajagopalan (1990). 

This pathway is sometimes called the Calvin-Benson cycle, after the biochemists who elucidated it. The 5-carbon, doubly phosphorylated carbohydrate, ribulose bisphosphate is the acceptor for CO2 the enzyme is called ribulose-bisphosphate carboxylase/oxygenase (called Rubisco). 

Ribulose phosphate kinase is active only when a cystine disulfide on the enzyme is reduced to two cysteines. An electron carrier, thioredoxin, carries out this reduction, and is then itself reduced by electrons from NADPH. Because the action of Photosystems I and II forms NADPH, this reduction ensures that ribulose bisphosphate is made only when enough light exists to support Photosynthesis. In other words, the light and dark reactions are coupled. 

While this efficiency is impressive, it also is rarely achieved. The difficulty is in the protein that carries out the first step of photosynthesis. Molecular oxygen, 02, competes with C02 for the active site of ribulose bisphosphate carboxylase, leading to an oxidation and loss of the ribulose bisphosphate acceptor. This competition is apparently intrinsic to the enzyme, because attempts to increase the discrimination for C02 by genetic engineering have resulted in a less-active enzyme, which fixes C02 very poorly. 

Monomeric intermediates during the assembly of oligomeric proteins are usually inactive and the formation of native quaternary structures are often a prerequisite for catalytic activity. One clear reason for this is that the active sites of some enzymes are located at the interface between subunits and are formed by amino acid residues from different subunits. Such examples are aspartate transcarbamoylase from K coli5) and ribulose bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum.6)... 

Most plants reduce CO2 to carbohydrate according to the well-known Calvin-Benson or C3 pathway, where the initial product of photosynthesis is the 3C compound phosphoglycerate. Fixation of CO 2 to phosphoglycerate occurs with the assistance of the enzyme ribulose bisphosphate (RuBP) carboxylase, which discriminates heavily against C02 (11). Consequently, plants with C3 photosynthesis have 6 values that average -27.0 (12). Plants with the Hatch-Slack or Ci,... 

The first stable product of carbon fixation by the enzyme, ribulose bisphosphate carboxylase (Rubisco), is glyceraldehyde 3-phosphate, a 3-C sugar. This 3-C sugar is fed into biosynthetic pathways and forms the basis for all organic compounds produced by photosynthetic organisms. Fixed carbon and major and trace elements... 

The reaction is catalyzed by ribulose bisphosphate carboxylase, "Rubisco", which is probably the most abundant enzyme on earth (it is 10-25% of leaf protein). 

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