AIR carboxylase

Ishiguro, J. (1989). An abnormal cell division cycle in an AIR carboxylase-deficient mutant of the fission yeast Schizosaccharomyces pomhe. Curr. Genet. 15,71-74. 

This process (106— 107) required the incubation of an aqueous mixture of AIR (106), potassium carbonate, and enzyme (AIR carboxylase). The equilibrium of the carboxylation reaction was shown to lie to the left (59JBC1799), but in the presence of 0.3 M aqueous potassium bicarbonate solution, yields of C-AIR (107) approaching 50% were obtained. 

A bifunctional enzyme, comprising the activities of AIR carboxylase and SAICAR synthetase, catalyzes reactions 6 and 7 of the purine pathway (AIR—> CAIR— SAICAR Fig. 15-16). A second bifunctional enzyme, IMP synthase, containing the activities of AICAR transformylase and IMP cyclohydrolase, catalyzes reactions 9 and 10 of the pathway (AICAR — FAICAR— IMP Fig. 15-16). Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A... 

Fig. 15-16 The de novo purine biosynthetic pathway. Rib-5-P, ribose 5-phosphate P-Rib-PP, 5-phosphoribosyl 1-pyrophosphate PRA, 5-phosphoribosylamine IO-CHO-FH4, /Vl0-formyl tetrahydrofolate GAR, glycineamide ribotide FGAR. /V-formylglycineamide ribotide FGAM, /V-formylglycineamidine ribotide AIR, 5-aminoimidazole ribotide CAIR, 4-carboxy-5-aminoimidazole ribotide SAICAR, iV-succino-5-aminoimidazole-4-carboxamide ribotide AICAR, 5-aminoimidazole-4-carboxamide ribotide FAICAR, 5-formamidoimidazole-4-carboxamide ribotide sAMP, /V-succino-AMP. Enzymes (1) amido phosphoribosyltransferase (2) GAR synthetase (3) GAR transformylase (4) FGAM synthetase (5) AIR synthetase (6) AIR carboxylase (7) SAICAR synthetase (8) adenylosuecinase (9) AICAR transformylase (10) IMP cyclohydrolase (11) sAMP synthetase (12) adenylosuecinasc (13) IMP dehydrogenase (14) GMP synthetase.

AIR carboxylase A -CAIR mutase SAICAR synthetase (9) SAICAR lyase ) AICAR transformylase (Q IMP synthase... 

In reaction (10), carbon dioxide was added to AIR to produce the car-boxylated product of AIR (carboxy-AIR) (Fig. 10). It is not known whether carbon dioxide or bicarbonate was the actual substrate for this condensation. An interesting feature of this reaction was the absence of a requirement for ATP. Other carbon dioxide fixation reactions were found to be coupled with TPNH oxidation 1S7,128), participation of phosphate 129, ISO), or degradation of ATP ISl). The enzyme which catalyzed reaction (10) was called AIR-carboxylase, and has been partially purified 1S2). In the next step [Eq. (11)] succino-AICAR was formed when carboxy-AIR reacted with aspartic acid and ATP other nucleoside triphosphates such as GTP, uridine triphosphate (UTP), CTP were ineffective in replacing ATP. This finding of ATP speiMcity may be contrasted with the specific... 

Fig. 3. The pathway of de novo purine ribonucleotide biosynthesis. The pathway includes the synthesis of PRPP, which is also used in the synthesis of pyrimidines, pyridine nucleotides, histidine, and tryptophan in plants. The enzymes catalyzing the numbered reactions are (1) PRPP synthetase, (2) PRPP amidotransferase, (3) GAR synthetase, (4) GAR transformylase, (5) FGAR amidotransferase, (6) AIR synthetase, (7) AIR carboxylase, (8) succino-AICAR synthetase, (9) adenylosuccinase, (10) AICAR transformylase, and (11) IMP cyclohydrolase.

The C-4 pathway requires the cooperation of two types of cells. Mesophyll cells (left) take up C02 from the air and export malate to the bundle sheath cells (right). The bundle sheath cells return pyruvate to the mesophyll cells and fix the C02 using ribulose bisphosphate carboxylase and the reductive pentose cycle. 

Recall that the oxygenase activity of ruhisco increases more rapidly with temperature than does its carboxylase activity. How then do plants, such as sugar cane, that grow in hot climates prevent very high rates of wastefiil photorespiration Their solution to this problem is to achieve a high local concentration of CO2 at the site of the Calvin cycle in their photosynthetic cells. The essence of this process, which was elucidated by M. D. Hatch and C. R. Slack, is that four-carbon (Cf compounds such as oxaloacetate and malate carry CO2 from mesophyll cells, which are in contact with air, to bundle-sheath cells, which are the major sites ofphotosynthesis (Figure 20.17). Decarboxylation of the four-carbon compound in a bundle-sheath cell maintains a high concentration of CO2 at the site of the Calvin cycle. The three-carbon compound pyruvate returns to the mesophyll cell for another round of carboxylation. 

Then in the biosynthesis reactions, the NADPH and ATP are used to capture carbon from the environment, for use in biology. Three ATP and two NADPH, with two H combine with a water and a CO2 molecule to form carbohydrate. In sum, a dozen quanta of light energy are needed to incorporate one molecule of CO2. This process is accomplished by the enzyme ribulose-l,5-bispho-sphate carboxylase oxygenase, or rubisco, which can in effect work both ways, either capturing carbon dioxide from the air, or oppositely to return it, depending on the O2 CO2 ratio it is exposed to (Lorimer and Andrews, 1973, Lorimer, 1981). 

C4 plants possess two types of photosynthesizing cells in their leaves mesophyll cells and bundle sheath cells. (In C3 plants, photosynthesis occurs in mesophyll cells.) Most mesophyll cells in both plant types are positioned so that they are in direct contact with air when the leaf s stomata are open. In C4 plants, C02 is captured in specialized mesophyll cells that incorporate it into oxaloacetate (Figure 13B). Phosphoenolpymvate carboxylase (PEP carboxylase) catalyzes this reaction. Oxaloacetate is then reduced to malate. Once formed, malate diffuses into bundle sheath cells. (As their name implies, bundle sheath cells form a layer around vascular bundles, which contain phloem and xylem vessels.) Within bundle sheath cells, malate is decarboxylated to pyruvate in a reaction that reduces NADP+ to NADPH. The pyruvate product of this latter reaction diffuses back to a mesophyll cell, where it can be reconverted to PEP. Although this reaction is driven by the hydrolysis of one molecule of ATP, there is a net cost of two ATP molecules. An additional ATP molecule is required to convert the AMP product to ADP so that it can be rephos-phorylated during photosynthesis. This circuitous process delivers CO, and NADPH to the chloroplasts of bundle sheath cells, where ribulose-1,5-bisphosphate carboxylase and the other enzymes of the Calvin cycle use them to synthesize triose phosphates. 

The first of them is supplied by the enzyme RUBP-C/0 which catalyses the BC cycle reactions, the other one catalyses the reactions with the yet unknown enzyme x-carboxylase which leads to acetyl-CoA formation. Thus it is supposed that in chloroplasts of isoprenereleasing plants there are at least two compartments of light conversion of CO2 carbon that comes to the leaves from the air, from two different carboxilation systems or from one system but along two channels. In fig.l they are located in immediate vicinity. But the centres of carboxylation are separated, which is to be understood as a sign of the presence either of two different enzymes or two different carboxylizing centres of the same enzyme. 

In ambient air, 2nd leaf (LPI-2) showed higher Net Carbon dioxide Exchange Rate (NCER) as well as RuBP carboxylase activity than the 5th leaf (LPI-5I) of the same age, while the 12 day old leaf had higher NCER and RuBP carboxylase activity than the 18 day old 5th leaf (LPI-5II). On enrichment with 600 and 900 ul 1 1 CO2 2nd leaf showed a much higher NCER than the 5th leaf of same age. Similarly, the younger 5th leaf showed more response than the older 5th leaf. It appears that leaf position as well as leaf age, both affect the response to elevated CO2 but the leaf position is more effective of the two (Table 1). 

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