Acetoacetyl unit

Two plus three to make four. Propose a reaction mechanism for the condensation of an acetyl unit with a malonyl unit to form an acetoacetyl unit in fatty acid synthesis. 

The subsequent reactions leading from 7V-methylpyrrolinium to tropinone remain doubtful since no enzymes have yet been demonstrated. For a long time, it was believed that the formation of the tropane ring from the jV-methyl-A -pyrrolinium cation occurred by condensation with an acetoacetyl unit, with the release of CO2, to form hygrine. This was... 

When both building block units are m place on the acyl carrier protein carbon-carbon bond formation occurs between the a carbon atom of the malonyl group and the carbonyl carbon of the acetyl group This is shown m step 1 of Figure 26 3 Carbon-carbon bond formation is accompanied by decarboxylation and produces a four carbon acetoacetyl (3 oxobutanoyl) group bound to acyl earner protein... 

It is also worth noting that the carbon of the carboxyl group that was added to drive this reaction is the one removed by the condensing enzyme. Thus, all the carbons of acetoacetyl-ACP (and of the fatty acids to be made) are derived from acetate units of acetyl-CoA. 

Ketone bodies are formed in the liver mitochondria by the condensation of three acetyl-CoA units. The mechanism of ketone body formation is one of those pathways that doesn t look like a very good way to do things. Two acetyl-CoAs are condensed to form acetoacetyl-CoA. We could have had an enzyme that just hydrolyzed the acetoacetyl-CoA directly to acetoacetate, but no, it s got to be done in a more complicated fashion. The acetoacetyl-CoA is condensed with another acetyl-CoA to give hydroxymethylglutaryl-CoA (HMG-CoA). This is then split by HMG-CoA lyase to acetyl-CoA and acetoacetate. The hydroxybutyrate arises from acetoacetate by reduction. The overall sum of ketone body formation is the generation of acetoacetate (or hydroxybutyrate) and the freeing-up of the 2 CoAs that were trapped as acetyl-CoA. 

The second five-carbon branched unit, in which the branch is one carbon further down the chain, is an intermediate in the biosynthesis of polyprenyl (isoprenoid) compounds and steroids. Three two-carbon units are used as the starting material with decarboxylation of one unit. Two acetyl units are first condensed to form acetoacetyl-CoA. Then a third acetyl unit, which has been transferred from acetyl-CoA onto an SH group of the enzyme, is combined with the acetoacetyl-CoA through an ester condensation. The thioester linkage to the enzyme is hydrolyzed to free the product 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This sequence is illustrated in Eq. 17-5. The thioester group of HMG-CoA is reduced to the... 

The Condensation Reaction. In the condensation reaction the acetyl group is initially transferred from ACP on to a SH group of 3-ketoacyl-ACP synthase. This acetyl moiety then reacts with malonyl-ACP (step 3 in fig. 18.12 ) so that the acetyl component becomes the methyl terminal two carbon unit of the acetoacetyl-ACP. The release of C02 in this condensation reaction provides the extra thermodynamic push to make the reaction highly favorable. 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

Concomitant to the enhancement of the signal for C-3 in the C NMR spectrum, the signals for H2 and H4 decreased in the H-NMR spectrum of all the tropine moieties compared with those in authentic samples. This indicates the incorporation of deuterium at C2 and C4 derived from C Hj CXX)". As the diminution of the signals for H2 and H4 was not identical at both positions, we suppose a sequential incorporation of labeled acetate (example given in Fig. 15b). From these data we assume that the biosynthesis from N-methyl-pyrrolidinium ion to tropine is a two-step process which does not involve a four carbon unit (acetoacetyl coenzyme A) but two units of acetyl coenzyme A, which were added sequentially as has been suggested for the biosynthesis of cocaine [28, 37]. On the other hand it has been reported that l,2- C2-acetate was incorporated with an equal efficiency at C2 and C4 by non-transformed root cultures of H. albus [36]. Further investigations are required to clarify this matter. 

There are two discrete locations of enzymes for the biosynthesis of acetoacetyl-CoA and HMG-CoA one is mitochondrial and serves the purpose of generating ketone bodies (acetoacetate and 3-hydroxybutyrate). The other is cytoplasmic and provides precursors for isoprene units for the biosynthesis of cholesterol and other terpenoids. 

Steroids are members of a large class of lipid compounds called terpenes. Using acetate as a starting material, a variety of organisms produce terpenes by essentially the same biosynthetic scheme (Fig. 8). The self-condensation of two molecules of acetyl coenzyme A (CoA) forms acetoacetyl CoA. Condensation of acetoacetyl CoA with a third molecule of acetyl CoA, then followed by an NADPH-mediated reduction of the thioester moiety produces mevalonic acid [150-97-0] (72). Phosphorylation of (72) followed by concomitant decarboxylation and dehydration processes produce isopentenyl pyrophosphate. Isopentenyl pyrophosphate isomerase establishes an equilibrium between isopentenyl pyrophosphate and 3,3-dimethylallyl pyrophosphate (73). The head-to-tail addition of these isoprene units forms geranyl pyrophosphate. The addition of another isopentenyl pyrophosphate unit results in the sesquiterpene (C15) famesyl pyrophosphate (74). Both of these head-to-tail additions are catalyzed by prenyl transferase. Squalene synthetase catalyzes the head-to-head addition of two achiral molecules of famesyl pyrophosphate, through a chiral cyclopropane intermediate, to form the achiral triterpene, squalene (75). 

The isoprenoids are derived from mevalonic acid (MVA), which is formed from three molecules of acetyl-CoA (Fig. 3). Two molecules of acetyl-CoA are condensed, yielding acetoacetyl-CoA. Subsequently, this product is coupled with another molecule of acetyl-CoA to yield 35-hydroxy-3-methylglutaryl-CoA (HMG-CoA). By reduction of HMG-CoA MVA is obtained. MVA is further converted in some steps to yield the Cs-unit isopentenyl diphosphate (IPP), which is then isomerized to dimethy-lallyl diphosphate (DMAPP), the starter molecule of the isoprenoid pathway. Coupling of DMAPP with one or more IPP molecules yields the basic structures which form the backbone of terpenoid biosynthesis. A number of reviews on the early steps in the terpenoid biosynthesis have been published (70-77). 

The first stage in fatty acid biosynthesis is a condensation between acetyl CoA (the starter unit) and malonyl CoA with the loss of CO2. This reaction could be drawn like this, with CO2 being lost as the new C—C bond is formed. When chemists use malonates, we like to make the stable enol using both carbonyl groups, condense, and only afterwards release CO2 (Chapter 25). As you saw on p. 1158, nature does this in making acetoacetyl CoA during alkaloid biosynthesis, but here things work differently. 

As indicated in Fig. 1, the initial step in the pathway outlined by Langdon (19,57) was believed to be the condensation of two acetyl CoA units to yield acetoacetyl CoA. Reduction of acetoacetyl CoA then followed, and in this mammalian liver system reduced diphosphopyridine nucleotide w as the hydrogen donor for this reaction. The resulting a-hydroxy-acyl-CoA com-... 

The metabolic pathways utihzed to produce poly[(/ )-3-hydroxybutyrate-co-(/ )-3-hydroxyhexanoate] (PHBHx) copolymer is shown in Fig. 4 (Noda et al. 2005a). Two units of acetyl-CoA forms the acetoacetyl-CoA with phaA thiolase, which is then converted to 3-hydroxybutyryl-CoA with phaB reductase. Parallel to these steps are the other metabolic pathways involving fatty acid biosynthesis (phaG) and... 

The first step of the biochemical pathway of PHB synthesis consists in the conversion of a selected carbon source to acetate. Then, an enzyme cofactor is attached via the formation of a thioester bond. The enzyme, called coenzyme A (CoA), is a universal carrier of acyl groups in biosynthesis and acetyl-CoA is a basic metabolic molecule found in all PHA-producing organisms. A dimer acetoacetyl-CoA is formed via reversible condensation and subsequently reduced to a monomer unit (R)-3-hydroxybutyryl-CoA. PHB is formed via the polymerization of the latter, maintaining the asymmetric centre [5]. The basic simplified process is shown in Scheme 22.1. 

Isoprene units all originate by the same biochemical route through isopentenyl pyrophosphate. This latter compound is formed from mevalonic acid in a series of enzyme-assisted steps using energy transfer from ATP ADP hydrolysis. Mevalonic acid (mevalonate) is obtained by condensation of acetyl-CoA with acetoacetyl-CoA. 

An example in which reactivity of both the attacking nucleophile and the electrophilic acceptor is dependent on the special character of acyl thioesters is in the condensation of two acetyl coenzyme A units to form acetoacetyl coenzyme A. This is the reverse of the thiolase reaction... 

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