Acetate and malonate

Incorporation of [l- C]acetate into a polyketide metabolite gives labelling throughout the chain, of alternate carbon atoms, which are of course the normally oxygenated ones. Labelled malonate, on the other hand, tends to label the second and successive acetate-derived units more heavily than the first [ unit in (5.9)] which derives directly from acetate and not malonate. This first acetate unit is called starter acetate. 

Assimilation of [l- C]acetate into an organism may be followed by direct conversion into a polyketide metabolite, or the labelled acid may be turned through the citric acid cycle before use in polyketide biosynthesis. The result is, however, that [l- C]acetate is again generated. On the other hand, [2- C]acetate which enters the 

Some polyketides include extra carbon atoms as [CHRCO] , R = H and Me. The extra Cj units may arise from 5-adenosylmethionine with methylation of an intermediate enol [as 3,22) 3,23) Alternatively introduction is by substitution in 

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The overall biosynthetic pathway to the tetracychnes has been reviewed (74). Studies (75—78) utilising labeled acetate and malonate and nmr analysis of the isolated oxytetracycline (2), have demonstrated the exclusive malonate origin of the tetracycline carbon skeleton, the carboxamide substituent, and the folding mode of the polyketide chain. Feeding experiments using [1- 02] acetate and analysis of the nmr isotope shift effects, led to the location of... 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

C-N.m.r. spectroscopy is useful in determining the position(s) of substitution, as esters of acetic and malonic acid, in polysaccharides. This topic is covered in Section VI,6, 8, and 10. 

QUANTITATIVE CAROTENOID COMPOSITION (PG/CELL), OBTAINED BY HPLC ANALYSIS, OF H. PLUVIALIS CELLS IN THE STATIONARY PHASE IN CULTURES WITH DIFFERENT CONCENTRATIONS OF NITRATE (G/L), ACETATE AND MALONATE (% W/V)... 

Data are available for malonate complexes of the divalent ions of the first transition series from Mn2+ on, and also for H+, Table 4. For Cu2+, Zn2+ and H+, a comparison with the corresponding acetate systems is thus possible. Unfortunately, the comparison is made somewhat uncertain by the fact that the acetate and malonate data, except for H+, refer to media of very different ionic strength, which presumably has a consi-... 

In flavonoids acylated with aliphatic acids, the most common acids are acetic and malonic. In the MS fragmentation of the dicarboxylic acids (as malonic acid), a first loss of 44 mass units is observed (loss of the carboxylic radical, CO2), and this is due to decarboxylation. 

The so-called acetate-malonate pathway leads to three different kinds of natiu al products depending on the detailed pathway followed. Fatty acids result from a reductive pathway to be described here, but acetate and malonate are also precursors for the isoprenoids (terpenes and sterols) produced via mevalonic acid (Ce) and... 

In the de novo pathway, acetate and malonate react through a series of steps converting acetate first to butanoate (C4), then to hexanoate (Cg), and then sequentially, two carbon atoms at a time, to palmitate (Cig). At this stage, a thioesterase liberates the acyl chain from ACP. The thioesterase is not completely specific, and acids of other chain lengths may be produced. This is obviously true in the lauric oils where the specificity of their thioesterases causes lauric acid (12 0) to be the major saturated acid, accompanied by lower levels of caprylic (8 0), capric (10 0), myristic (14 0), and palmitic acid (16 0). 

This four-step cycle includes condensation of acetate and malonate to give ketobu-tanoate with subsequent reduction to butanoate in three further steps. These are reduction to the 3R hydroxy acid, dehydration to the 2t acid, and reduction again. Reduction is affected by NADPH and a proton. The process is then repeated to add further two-carbon units until a thioesterase liberates the free acid. This sequence requires a fatty acid synthase, which contains the enzymes needed for each of the four steps viz. p-ketoacyl-ACP synthase, p-ketoacyl-ACP reductase, p-ketoacyl-ACP dehydrase, and enoyl-ACP reductase, respectively. 

In the remaining steps, acetic and malonic acids react, not as CoA esters, but as thiol esters of acyl carrier protein (ACP), a small protein with a prosthetic group quite similar to CoA. These esters are formed by (2) and (3). which we recognize as examples of transesterification. 

In order to check Geissmann s proposal by experimentally Whiting et al. (36, 83) injected [1- C] and [2- C] sodium acetate and malonate, [1- CJ- and [3-Nc]-phenylalanine, and sodium H-ferulate into the roots of Curcuma tonga. After a few days of incubation... 

The polyketide synthesis chemically and biochemically resembles that of fatty acids. The reaction of fatty acid synthesis is inhibited by the fungal product ceru-lenin [9]. It inhibits all known types of fatty acid synthases, both multifunctional enzyme complex and unassociated enzyme from different sources like that of some bacteria, yeast, plants, and mammalians [10]. Cerulenin also blocks synthesis of polyketides in a wide variety of organisms, including actinomycetes, fungi, and plants [11, 12]. The inhibition of fatty acid synthesis by cerulenin is based on binding to the cysteine residue in the condensation reaction domain [13]. Synthesis of both polyketide and fatty acids is initiated by a Claisen condensation reaction between a starter carboxylic acid and a dicarboxylic acid such as malonic or methylmalonic acid. An example of this type of synthesis is shown in Fig. 1. An acetate and malonate as enzyme-linked thioesters are used as starter and extender, respectively. The starter unit is linked through a thioester linkage to the cysteine residue in the active site of the enzymatic unit, p-ketoacyl ACP synthase (KS), which catalyzes the condensation reaction. On the other hand, the extender... 

The biosynthesis of 2-alkyl- and 2-arylquinoline alkaloids was investigated by Luckner and co-workers. For example, anthranilic acid was incorporated intact into 2-heptyl-4-hydroxyquinoline 339 in the microorganism Pseudomonas aeruginosa. Acetate and malonate are also precursors, and degradation experiments showed that the side chain was... 

The facts that radioactively labeled acetate and malonate label acetylenes in the same manner as fatty acids and that few branched-chain acetylenic compounds are known strongly suggest that naturally occurring acetylenes are derived from fatty acids. Additionally, many acetylenic compounds with a Z double bond have the double bond in the same position as the double bond of oleic acid. In a similar manner to fatty acids, introduction of double bonds into acetylenic compounds requires molecular oxygen. Although the triple bond is usually introduced into a position where a double bond would be expected, in some instances no prior double-bond formation is observed. The enzymes responsible for the introduction of triple bonds have been little studied. 

Polyketide or polyacetate compounds are derived from ace-tate-malonate pathways and, in terms of biosynthesis, are related to fatty acids. Polyketides are assembled by condensation of acetate and malonate units however, in contrast to fatty acid biosynthesis, the carbonyl groups may not be reduced and intermediate compounds typically condense to produce aromatic ring systems, usually with phenolic substitutions (Packter, 1980). 

Few areas of natural products chemistry have seen as many major advances in the study of biosynthetic pathways as have occurred in polyketide compounds. Birch and Donovan (1953) demonstrated that a wide range of structural types are derived from acetate (later shown to be acetate and malonate). In experiments witfi deuterated precursors, acetate serves preferentially as a starter unit for the formation of 6-methylsalicylic acid in Penicillium griseofulvum (Simpson, 1983). Thus, polyketides are derived from the same precursors as fatty acids and the initial step seems to be similar (Fig. 5.1). Extensive purifrcation of 6-methylsalicylate synthetase from Penicillium patulum has been performed. This enzyme system is distinct and separable from the co-occurring fatty acid synthetase and has a molecular weight approximately half that of the former enzyme. NADPH is required as a coenzyme for methylsalicylate synthetase from this source (O Hagan, 1990 Packter, 1980). 

E are biosynthesized from a phenylpropane unit such as cinnamic acid, which contributes the aromatic ring B, and C-atoms 2, 3 and 4. The remaining C-atoms are added by head-to-tail condensation of acetate and malonate (see Stilbenes). 

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