C4 Acids

VI) was hydrogenated to the corresponding sorbitol derivative (VII) in which the only groups unsubstituted were those situated at Cl and C4. Acid-catalyzed ring closure gave a product identical with the tetramethyl derivative of arlitan. 

Physical Properties of Ct-C4 Acids Bolling Freezing Molecular Critical Properties... 

Figure 24-4. liquid heat capacity of C,-C4 acids from 0 C to 200 C... 

Figure 24-13. Surface tension of C,-C4 acids from OfC 10 <- 200LC...

The C4 acids give rise in the usual kind of way To oxaloacetic (by the citric cycle, say)... 

Figure 8-15. Carboxylase reactions and locations for the three photosynthetic pathways (a) C3, (b) C4, and (c) Crassulacean acid metabolism (CAM). The reactions for C3 and C4 plants occur during the daytime. The indicated decarboxylations of C4 acids occur in the cytosol of bundle sheath cells for C4 plants and the cytosol of mesophyll cells for CAM plants.

The C4 (dicarboxylic acid) pathway of photosynthetic carbon assimilation may be seen as a biochemical elaboration of the RPP cycle. In this pathway CO2 is transferred via the C-4 carboxyl of C4 acids to the reactions of the RPP cycle. Discovered in sugar cane, the pathway was first thought to be peculiar to tropical grasses but was later found in species of dicotyledons, Amaranthus (Amaranthaceae), and Atriplex (Chenopodiaceae). 

Two other types of C4 pathways are recognized. In type-2 plants, Atriplex spongiosa) and type-3 Panicum maximum) plants, malate is replaced by aspartate as the major C4 acid transported to the bundle sheath cells. After transport, aspartate is converted to OAA by transamination. In type-2 plants, OAA is reduced to malate, which in turn is decarboxylated by NAD-malic enzyme in the bundle sheath cell mitochondria to give NADH, CO2 and pyruvate. In type-3 plants, OAA is decarboxylated in the cytosol by PEP carboxykinase in the presence of ATP, yielding PEP, CO2 and ADP. The return of carbon to the mesophyll cells for regeneration of the CO2 acceptor occurs as pyruvate (or alanine to maintain nitrogen balance) in type-2 and as PEP (or again perhaps as alanine) in type-3. These variations in the C4 pathway are summarized in Table I (see also Ref. 14). 

As discussed above, the spatial separation of these processes in C4 plants necessitates a degreee of structural organization in the form of Kranz anatomy. CAM plants do not show such anatomy but have other specializations because the temporal separation of the synthesis and decarboxylation of C4 acids requires storage of large amounts of C4 acids in the vacuole. 

Certain features of CAM such as the diurnal rhythm in gas exchange and C4 acid... 

C4 photosynthesis is an adaptation to growth in hot climates. The first products of photosynthesis are C4 acids normal plants produce PGA, a 3 carbon acid, and hence are termed C3 plants. C4 photosynthesis has evolved independently in at least 16 families of flowering plemts. C4 metabolism is a way of getting round the problem of Rubisco s unfortunate oxygenase activity. C4 plants concentrate CO2 biochemically in a variety of ways, so Rubisco is exposed to a very high CO2/O2 ratio, which inhibits photorespiration. 

The C4 syndrome was discovery when radioactive i C02 was fed to sugarcane, and the first products seen were C4 acids, not C3 PGA. 

CO2 fixation into C4 acids occurs in the mesophyll. C4 acids are translocated into the bundle-sheath, through plasmodesmata, where they are decarboxylated to give CO2, and this CO2 enters the Calvin cycle as normal. The C3 fragment is returned to the mesophyll and is metabolised back to PEP (Fig. 13.16). 

This intermediate is subsequently reduced to butyryl-CoA, and the C4 acid is finally formed by reaction with acetate ... 

On alkaline oxidation of aldoses with (iV-chloro-/7-toluenesulfonamido) sodium (CAT), the monosaccharides 33, 40, D-mannose 54, D-arabinose 55, and D-ribose 56, belonging to the 4,5- or 3,4-ethythro-series, afford the C4-acids 59 and 60 in 35 to 49% yields while the yields of glyceric acid are low [64]. Thus, as illustrated in O Scheme 6, hexoses are cleaved at the C1/C2 (a) andC2/C3 (b) bonds, whereas pentoses break attheCl/Hl (a) andCl/C2 (b) bonds. 

The reaction products were analyzed by chromatography and chromatomass-spectrometry. The complex mixture of oxygen containing organic compounds and C - C6 hydrocarbons was formed. Oxygen containing products consist of C - C4 aldehydes (formaldehyde, propionic aldehyde, butiraldehyde), C2 - C4 acids (acetic, propionic, butyric), acetone, ethanol and the traces of C3 - C4 alcohols of normal structure. 

B. A deficiency in pyruvate carboxylase results in a diminution of oxaloacetate, the acid that acts as the acceptor for an acetyl group from acetyl-CoA. In order for the TCA cycle to continue efficiently, C4 acids must be replenished. Amino acids whose carbon skeletons feed into the TCA cycle and increase the C4 pool will accomplish this. Glutamine, which is converted to a-ketoglutarate, will lead to an increase in all of the C acids (succinate, fumarate, malate, and oxaloacetate). Alanine and serine are converted to pyruvate, which as a result of the deficiency in pyruvate carboxylase will not increase the C4 pool. Lysine and leucine are ketogenic amino acids and thus also do not increase the C pool. 

This series is shown in Table 2.4. The most important of these compounds is acetic acid, the essential component of volatile acidity. Its concentration, limited by legislation, indicates the extent of bacterial (lactic or acetic) activity and the resulting spoilage of the wine. As yeast forms a little acetic acid, there is some volatile acidity in all wines. Other C3 (propionic acid) and C4 acids (butyric acids) are also associated with bacterial spoilage. 

Condcnsalion of organic compowKls ----CondctKialuHi c4 acidic compounds... 

Total yield C3 aeid C4 acid C5 acids tert- sec- C6 ten acids - sec- C7 acids tert- sec- Cg acids... 

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