Enzymes carboxydismutase

The carboxylation of ribulose diphosphate by cell-free extracts of Chlorella was demonstrated by Quayle et al. (1954), who named the enzyme carboxydismutase. The enzyme has been purified from spinach leaves (Weissbach et al, 1956 Mayaudon et al., 1957). The purified enzyme mediates the addition of water and CO2 to ribulose diphosphate, with an intramolecular oxidation-reduction reaction, or dismutation, leading to the formation of two molecules of PGA. In the carboxylation part of the reaction the carbon atom of CO2 becomes bonded to the number 2 carbon atom of ribulose diphosphate. 

CO2, present in the form of HCO3—, is fixed in the acceptor, ribulose-1,5-diphosphate, by means of the enzyme carboxydismutase. An intermediate with 6 C atoms is formed, the identity of which is still unknown. This substance is unstable. It decomposes into two molecules of 3-phosphoglyceric acid. The latter is then reduced to 3-phosphoglyceraldehyde by means of the ATP and NADPH + H+ formed in the primary processes. 3-Phosphoglyceraldehyde exists in equilibrium with its isomer, dihydroxy acetone phosphate. The equilibrium is controlled by the enzyme triose phosphate isomerase. 3-Phosphoglyceralde-hyde and dihydroxy acetone phosphate ar referred to collectively as triose phosphate. 

Once inside the cell, HCO3 is converted to CO2 by the enzyme, carbonic anhydrase. CO2 is then fixed by carboxydismutase and OH is excreted to maintain ionic balance. Carbonic anhydrase is also associated with the extracellular carbonate dissolution by boring organisms (Schneider, 1976) and with the C02-transfer system for intracellular calcification. It represents a key enzyme in the biological cycling of carbonate (Degens, 1976 Raven, 1974). 

Band 2 of Figure 7 was occasionally seen to contain a second component in addition to the -subunit of CFi. This component is probably the large subunit of carboxydismutase which sometimes tends to attach to thylakoids although it is a soluble enzyme. The small subunit of carboxydismutase, and plastocyanin, which was identified by a specific antiserum, are located in the area of band 9. The protein of band 3 is probably ferredoxin-NADP-reductase. The bands 5 to 7 remain unidentified, as are the 7 to 9 faint bands which are not visible in Figure 7. 

Racker 56) demonstrated the synthesis of carbohydrates from carbon dioxide and hydrogen in a cell-free system by bringing together many of the enzymes listed in Table IV. A spinach fraction furnished the phospho-pentokinase, carboxydismutase, phosphopentosisomerase, transketolase, transaldolase, and hexose diphosphatase. To this fraction were added the other enzymes, DPN+, ATP, and a hydrogenase preparation. The hydro-genase enzyme furnished DPNH in the presence of hydrogen. When this mixture was incubated at 25° for 60 minutes, the synthesis of fructose 6-phosphate could be demonstrated. 

However it still remains, in this enzymatic scheme, to explain the fixation of CO2 when entering this series of reactions. Calvin has isolated from spinach leaves and from ultrasonic macerates of the green alga Chlorella a soluble enzyme which, in the presence of bicarbonate, converts Ru-PP to two molecules of PGA. He has called it carboxydismutase, because the carboxylation depends on the oxidation of C-3 of ribulose to a carboxyl group. 

Fig. 36. The Calvin cycle (black lines) and pentose phosphate cycle (red lines). PGA = 3-Phosphoglyceric acid, PGAL = 3-phosphoglyceraldehyde, Rib. = ribose-5-phosphate, Xyl = xylulose-5-phosphate, Ru-diP = ribulose-1,5-diphosphate, C4 = erythrose-4-phosphate, FDP = fructose-1,6-diphosphate. A few of the enzymes participating are encoded, 1 = carboxydismutase, 2 = triose phosphate dehydrogenase, 3 = triose phosphate isomerase, 4 = aldolase, 5 = phosphatase, 6 = phosphoglucoisomerase. Details of the conversion of glucose-6-P into ribulose-5-P are given in Fig. 43. It should be pointed out that the pentose phosphate cycle presents only here and there a true reversal of the Calvin cycle. In many instances the mechanisms and enzymes are different.

According to Kreutz enzymes are located between the crystallites. Muhlethaler and other scientists have been able to show that other enzyme systems, the carboxydismutase and an ATPase (an enzyme that controls the equilibrium ATP + HOH ADP -f- P and thus might play a role in photophosphorylation) are located on the outside of the thylacoid membranes. 

What holds true for the structural elements of the plastids need not be valid for all of the enzyme systems of photosynthesis. A key enzyme in the secondary processes of photosynthesis is carboxydismutase which fixes CO2 into ribulose-1, 5-diphosphate (page 51). The enzyme is already present before the light-dependent differentiation of the plastid structure, at least in rye seedlings. Nonetheless, illumination induces an... 

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