Carbonate units

Fatty acids derived from animal and vegetable sources generally contain an even number of carbon atoms siace they are biochemically derived by condensation of two carbon units through acetyl or malonyl coenzyme A. However, odd-numbered and branched fatty acid chains are observed ia small concentrations ia natural triglycerides, particularly mminant animal fats through propionyl and methylmalonyl coenzyme respectively. The glycerol backbone is derived by biospeciftc reduction of dihydroxyacetone. 

IPP react with each other, releasiag pyrophosphate to form another allyl pyrophosphate containing 10 carbon atoms. The chain can successively build up by five-carbon units to yield polyisoprenes by head-to-tad condensations alternatively, tad-to-tad condensations of two units can yield squalene, a precursor of sterols. Similar condensation of two C2Q units yields phytoene, a precursor of carotenoids. This information is expected to help ia the development of genetic methods to control the hydrocarbon stmctures and yields. 

DiisononylPhthalate andDiisodeeylPhthalate. These primary plasticizers are produced by esterification of 0x0 alcohols of carbon chain length nine and ten. The 0x0 alcohols are produced through the carbonylation of alkenes (olefins). The carbonylation process (eq. 3) adds a carbon unit to an alkene chain by reaction with carbon monoxide and hydrogen with heat, pressure, and catalyst. In this way a Cg alkene is carbonylated to yield a alcohol a alkene is carbonylated to produce a C q alcohol. Due to the distribution of the C=C double bond ia the alkene and the varyiag effectiveness of certain catalysts, the position of the added carbon atom can vary and an isomer distribution is generally created ia such a reaction the nature of this distribution depends on the reaction conditions. Consequendy these alcohols are termed iso-alcohols and the subsequent phthalates iso-phthalates, an unfortunate designation ia view of possible confusion with esters of isophthaUc acid. 

AMP, ADP, and ATP = adenosine mono-, di-, and triphosphate IMP = inosine 5 -monophosphate AICAR = 5 -phosphoribosyl-5-amino-4-imida2olecarboxamide DAP = diaminopimelic acid PRPP = phosphoribosyl pyrophosphate a — KGA = a-ketoglutaric acid Orn = ornithine Cit = citnilline represents the one carbon unit lost to tetrahydrofolate as serine is converted to glycine. 

Sulfur combines directly with hydrogen at 150—200°C to form hydrogen sulfide. Molten sulfur reacts with hydrogen to form hydrogen polysulfides. At red heat, sulfur and carbon unite to form carbon disulfide. This is a commercially important reaction in Europe, although natural gas is used to produce carbon disulfide in the United States. In aqueous solutions of alkaU carbonates and alkaU and alkaline-earth hydroxides, sulfur reacts to form sulfides, polysulfides, thiosulfates, and sulfites. 

When the adsorption capacity of a carbon unit is exceeded, there is breakthrough of the contaminant in the treated stream. Eixed beds may be operated in series to allow continuous treatment while spent or exhausted units are replaced with fresh carbon. In series operation, there are two or more units. The majority of the contaminant is removed by the first unit in the series with the downstream units acting as polishing units. When breakthrough occurs in the first or primary unit, it is replaced with fresh carbon and becomes a polishing unit while the next unit in the series takes over and becomes the primary treatment unit. An example of this round-robin operation of a three-carbon bed is shown in Eigure 2. 

Tiansketolases (TK) aie widely used in oiganic synthesis to extend the chain of an acceptor aldose by two carbon units. 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

The role of the 1,1-bielectrophile in ring closures of this type is to provide a one-carbon unit (or heteroatbm) to close the cycle. Thus, the synthesis of the four-atom precursor with two nucleophilic centers 1,4 to each other is an appreciable challenge, especially to obtain a heterocycle at the desired oxidation level. The examples below illustrate the way this approach to synthesis may be gainfully utilized. 

Basic to establishing whether power recovery is even feasible, let alone economical, are considerations of the flowing-fluid capacity available, the differential pressure available for the power recovery, and corrosive or erosive properties of the fluid stream. A further important consideration in feasibihty and economics is the probable physical location, with respect to each other, of fluid source, power-production point, and final fluid destination. In general, the tendency has been to locate the power-recoveiy driver and its driven unit where dictated by the driven-unit requirement and pipe the power-recoveiy fluid to and away from the driver. While early installations were in noncorrosive, nonerosive services such as rich-hydrocarbon absorption oil, the trend has been to put units into mildly severe seiwices such as amine plants, hot-carbonate units, and hydrocracker letdown. 

Petroleum Locate and eliminate seepage. Activated carbon will adsorb oil and gasoline (most hydrocarbons) on a short term basis. Airstrip with (40 1 air/water ratio) followed by 2 ft carbon units in series... 

Folic acid derivatives (folates) are acceptors and donors of one-carbon units for all oxidation levels of carbon except that of CO2 (where biotin is the relevant carrier). The active coenzyme form of folic acid is tetrahydrofolate (THF). THF is formed via two successive reductions of folate by dihydrofolate reductase (Figure 18.35). One-carbon units in three different oxidation states may be bound to tetrahydrofolate at the and/or nitrogens (Table 18.6). These one-carbon units... 

Oxidation States of Carbon in 1-Carbon Units Carried by Tetrahydrofolate ... 

FIGURE 18.35 Formation of THF from folic acid by the dihydrofolate reductase reaction. The R group on these folate molecules symbolizes the one to seven (or more) glutamate units that folates characteristically contain. All of these glutamates are bound in y-carboxyl amide linkages (as in the folic acid structure shown in the box A Deeper Look Folic Acid, Pterins, and Insect VFingis). The one-carbon units carried by THF are bound at N, or at or as a single carbon attached to both... 

CoA to form L-malate. The net effect is to conserve carbon units, using two acetyl-CoA molecules per cycle to generate oxaloacetate. Some of this is converted to PEP and then to glucose by pathways discussed in Chapter 23. 

To this point, the pathway has generated a pool of pentose phosphates. The AG° for each of the last two reactions is small, and the three pentose-5-phosphates coexist at equilibrium. The pathway has also produced two molecules of N/ DPH for each glucose-6-P converted to pentose-5-phosphate. The next three steps rearrange the five-carbon skeletons of the pentoses to produce three-, four-, six-, and seven-carbon units, which can be used for various metabolic purposes. Why should the cell do this Very often, the cellular need for... 

N/ JDPH is considerably greater than the need for ribose-5-phosphate. The next three steps thus return some of the five-carbon units to glyceraldehyde-3-phos-phate and fructose-6-phosphate, which can enter the glycolytic pathway. The advantage of this is that the cell has met its needs for N/VDPH and ribose-5-phosphate in a single pathway, yet at the same time it can return the excess carbon metabolites to glycolysis. 

Fatty acid chains are constructed by the addition of two-carbon units derived from acctyl-CoA. 

The addition of two-carbon units to the growing chain is driven by decarboxylation of malonyl-CoA. 

Other enzymes then add double bonds and additional carbon units to the chain. 

FIGURE 25.7 The pathway of palmhate synthesis from acetyl-CoA and malonyl-CoA. Acetyl and malonyl building blocks are introduced as acyl carrier protein conjugates. Decarboxylation drives the /3-ketoacyl-ACP synthase and results in the addition of two-carbon units to the growing chain. Concentrations of free fatty acids are extremely low in most cells, and newly synthesized fatty acids exist primarily as acyl-CoA esters. 

The net result of this biosynthetic cycle is the synthesis of a four-carbon unit, a butyryl group, from two smaller building blocks. In the next cycle of the process, this butyryl-ACP condenses with another malonyl-ACP to make a... 

As seen already, palmitate is the primary product of the fatty acid synthase. Cells synthesize many other fatty acids. Shorter chains are easily made if the chain is released before reaching 16 carbons in length. Longer chains are made through special elongation reactions, which occur both in the mitochondria and at the surface of the endoplasmic reticulum. The ER reactions are actually quite similar to those we have just discussed addition of two-carbon units... 

Instead of thiophenes the readily available di-2-thienylmethane can be used as a chain extender, introducing a nine carbon unit upon desulfurization. Similarly the two reactive positions of 2,2-bithienyl and 2,2, 5",2"-terthienyl have been utilized for the introduction of eight and twelve carbons, respectively. ... 

Terpenes (and terpenoids) are further classified according to the number of 5-carbon units they contain. Thus, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, sesquiterpenes are 15-carbon molecules from three isoprene units, diterpenes are 20-carbon substances from four isoprene units, and so on. Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenoids occur in both plants and animals, and many have important biological roles. The triterpenoid lanosterol, for example, is the precursor from which all steroid hormones are made. 

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