Three-carbon unit elongation

BIOCATALYTIC THREE-CARBON UNIT ELONGATION REACTIONS... 

Aldol reactions catalyzed by aldolases are useful for the elongation of aldehydes by a two- or three-carbon unit. 

A review of the four reagent systems developed for direct C-H oxidation and establishing three carbon-chain elongations via straightforward installation of carbon unit to C-H bonds is published. Highly reactive chemical species acting like oxygen radical are utilized as a key C-H activator. ... 

BIOCATALYTIC ELONGATION REACTIONS BY GREATER THAN A THREE-CARBON UNIT... 

CoA to form malonyl CoA using C02 in the form of bicarbonate HC03 (Fig. 2). This reaction is catalyzed by the enzyme acetyl CoA carboxylase which has biotin as a prosthetic group, a common feature in C02-binding enzymes. One molecule of ATP is hydrolyzed in the reaction, which is irreversible. The elongation steps of fatty acid synthesis all involve intermediates linked to the terminal sulfhydryl group of the phosphopantetheine reactive unit in ACP phosphopantetheine is also the reactive unit in CoA. Therefore, the next steps are the formation of acetyl-ACP and malonyl-ACP by the enzymes acetyl transacylase and malonyl transacylase, respectively (Fig. 2). (For the synthesis of fatty acids with an odd number of carbon atoms the three-carbon propionyl-ACP is the starting point instead of malonyl-ACP.)... 

Acetyl CoA is converted to malonyl CoA, which provides the 2-carbon units for elongation of the growing fatty acyl chain on the fatty acid synthase complex. Acetyl CoA carboxylase, the enzyme that catalyzes the conversion of acetyl CoA to malonyl CoA, is controlled by three of the major mechanisms that regulate enzyme activity (Fig. 36.5). It is activated by citrate, which causes the enzyme to... 

Before doing this, however, it is useful to briefly consider the biosynthesis of anacardic acids. It is believed that anacardic acids are synthesized from long chain fatty acids via a modification of the methyl salicylate synthetase scheme (11. 12). In effect a fatty acid, such as palmitate (Cie) is elongated by six carbons by the addition of three acetate units in a series of condensation and dehydration steps, followed by ring closure and aromatization to form a C22 saturated anacardic acid (5. 10. U ). Similarly, margaric acid (C17) is synthesized to a C23 saturated anacardic acid. A comparative study on the biosynthesis of anacardic acids in resistant and susceptible plants, using fatty acid precursors will be the subject of a separate publication. 

The structural variety of the compounds that form fibers is as diverse as their chemistries. From glasses (fiberglass), and partially crystalline materials (carbon), to special three-dimensional arrays, including polymers, the small, elongate solids may have aspect ratios up to 5000. From our research and compilation (Appendices 1, 2) we noted many mineral and synthetic compounds that have structures characterized by basic linear units. Amphi-boles, the major mineral group mined as asbestos, are characterized as doublechain structures. Many of the minerals in Appendix 1 are polymorphic (di-or trimorphs), and where one member of a mineral series has been described as fibrous the others in the same series are likely to be able to grow as fibers as well. Probably all compounds with similar structures and compositions, mineral or synthetic, can form fibers, even though they are not presently listed. It is also clear that fibrous formation is not confined to compounds with linear structural units indeed the variety of crystalline structure patterns is remarkably diverse. 

The molecular geometry of 3 comprises a central magnesium atom pairwise linked via four two-electron three-center bonded methyl groups to the two dimethylaluminium units. The four carbon-to-magnesium bonds (2.20, 2.22, 2.19 and 2.22 A) are slightly elongated compared to the C-Mg distances observed in linear bis(neopentyl)magnesium... 

In this section we consider peptide analogues containing the amide surrogates 1 to 11 (Scheme 1). These can be isosteric with the amide group in the sense that consecutive a-carbons are separated by three bonds, as in link 1, the (nitrono) peptides, and link 2, the [methyleneamino(hydroxy)] or (TV-hydroxy reduced amide) peptides. They also can be an N-modified amide, as in link 3, the (TV-hydroxy amide) peptides, and link 4, the (V-aminoamide) peptides. Elongation of the peptide unit by one covalent bond has been realized by the introduction of a heteroatom or a methylene into the backbone, as in link 5, the (hydrazide) peptides, link 6, the (amidoxy) peptides, link 7, the (oxomethyleneamino) peptides, link 8, the [(hydroxy)ethyleneamino] peptides, link 9, the (ethyleneamino) peptides, and link 10 the (oxime) peptides. Finally, insertion of an ethylenic bond (two covalent bonds) between the a-carbon and the carbonyl gives rise to link 11, the (but-2-enamide) or (vinylogous amide) peptides. 

The structure is quite different from that of KCg. Instead of every carbon atom bonded to three other carbon atoms, as in graphite and KC, calcium carbide contains discrete C-i ions with carixm-carbon triple bonds. The structure of CaC2 is similar to the Natl structure, but because Cz is a linear anion (not spherical like Cl") the unit cell of CaCl2 is elongated in one direction making the unit cell tetragonal (see Figure 14.14). This carbide is, hence, ionic. 

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