Malonyl coenzyme formation

Formation of malonyl coenzyme A is followed by a nucleophilic acyl substitution which transfers the malonyl group to the acyl carrier protein as a thioester... 

The four carbon atoms of the butanoyl group originate m two molecules of acetyl coenzyme A Carbon dioxide assists the reaction but is not incorporated into the prod uct The same carbon dioxide that is used to convert one molecule of acetyl coenzyme A to malonyl coenzyme A is regenerated m the decarboxylation step that accompanies carbon-carbon bond formation... 

JEZ, J.M., FERRER, J.-L., BOWMAN, M.E., DIXON, R.A., NOEL, J.P., Dissection of malonyl-coenzyme A decarboxylation from polyketide formation in the reaction mechanism of a plant polyketide synthase, Biochemistry, 2000, 39, 890-902. 

Kreuzaler F, Hahlbrock K (1972) Enzymatic synthesis of aromatic compounds in higher plants formation of naringenin (5,7,4 -trihydroxyflavanone) from p-coumaroyl coenzyme A and malonyl coenzyme A. FEBS Lett 28(l) 69-72... 

Stuhlemmer, U. and Kreis, W. (1996) Does malonyl coenzyme A hydroxypregnane 21-hydroxymalonyltransferase catalyse the first step in butenolide ring formation Tetrahedron Lett., 37, 2221-i. 

The Formation of Malonyl Coenzyme A Is the Committed Step in Fatty Acid Synthesis... 

Fig. 2.13 Biosynthesis of saturated fatty acids in plants and animals. Palmitate is formed by successive additions of malonyl coenzyme A to the enzyme-bound chain, with C02 being lost at each addition.This results in chain elongation by a (CH2)2 unit at each step. Details of the formation of butyryl (C4) from acetyl (C2) are shown, while the subsequent six further additions, terminating in palmitate, proceed similarly.

Biotin Coenzyme in decarhoxylatiDn liemoval of carhon ditmtiel and camoxyiatiDn ladditinn of carbon dioxidel reactions of caibohydiate, fat, and protein metabolism for esara-pie, pyiuvic acid to oxaloaceiic acid, and acetyl CoA to malonyl CoA formation of purines torraation of urea deamination of ammo adds. 

The anthraquinone biosynthesis by microorganisms is closely related to the in vivo formation of fatty acids, e.g., palmitic acid (14). One molecule of acetyl-coenzyme A (acetyl-CoA) as a starting unit condenses with seven molecules of malonyl-coenzyme A (malonyl-CoA), whereby the formed carbonyl group is reduced after each reaction step. If these reductions fail... 

To clarify the characteristics of AMDase, the effects of some additives were examined using phenylmalonic acid as the representative substrate. The addihon of ATP and coenzyme A did not enhance the rate of the reaction, different from the case of malonyl-CoA decarboxylase and others in those, ATP and substrate acid form a mixed anhydride, which in turn reacts with coenzyme A to form a thiol ester of the substrate. In the present case, as both ATP and CoA-SH had no effect, the mechanism of the reaction will be totally different from the ordinary one described above. It is well estabhshed that avidin is a potent inhibitor of the formation of the biotin-enzyme complex. In the case of AMDase, addition of avidin has no influence on the enzyme activity, indicating that AMDase is not a biotin enzyme. 

Biotin (5) is the coenzyme of the carboxylases. Like pyridoxal phosphate, it has an amide-type bond via the carboxyl group with a lysine residue of the carboxylase. This bond is catalyzed by a specific enzyme. Using ATP, biotin reacts with hydrogen carbonate (HCOa ) to form N-carboxybiotin. From this activated form, carbon dioxide (CO2) is then transferred to other molecules, into which a carboxyl group is introduced in this way. Examples of biotindependent reactions of this type include the formation of oxaloacetic acid from pyruvate (see p. 154) and the synthesis of malonyl-CoA from acetyl-CoA (see p. 162). 

Biotin enzymes are believed to function primarily in reversible carboxvlahon-decarboxylation reactions. For example, a biotin enzyme mediates the carboxylation of propionic acid to methylmalonic add, which is subsequently converted to succinic acid, a dtric acid cycle intermediate. A vitamin Bl2 coenzyme and coenzyme A are also essential to this overall reaction, again pointing out the interdependence of the B vitamin coenzymes. Another biotin enzyme-mediated reaction is the formation of malonyl-CoA by carboxylation of acetyl-CoA ( active acetate ). Malonyl-CoA is believed lo be a key intermediate in fatly add synthesis. 

Like the related fatty acid synthases (FASs), polyketide synthases (PKSs) are multifunctional enzymes that catalyze the decarboxylative (Claisen) condensation of simple carboxylic acids, activated as their coenzyme A (CoA) thioesters. While FASs typically use acetyl-CoA as the starter unit and malonyl-CoA as the extender unit, PKSs often employ acetyl- or propionyl-CoA to initiate biosynthesis, and malonyl-, methylmalonyl-, and occasionally ethylmalonyl-CoA or pro-pylmalonyl-CoA as a source of chain-extension units. After each condensation, FASs catalyze the full reduction of the P-ketothioester to a methylene by way of ketoreduction, dehydration, and enoyl reduction (Fig. 3). In contrast, PKSs shortcut the FAS pathway in one of two ways (Fig. 4). The aromatic PKSs (Fig. 4a) leave the P-keto groups substantially intact to produce aromatic products, while the modular PKSs (Fig. 4b) catalyze a variable extent of reduction to yield the so-called complex polyketides. In the latter case, reduction may not occur, or there may be formation of a P-hydroxy, double-bond, or fully saturated methylene additionally, the outcome may vary between different cycles of chain extension (Fig. 4b). This inherent variability in keto reduction, the greater variety of... 

The biosynthesis of polyketides is analogous to the formation of long-chain fatty acids catalyzed by the enzyme fatty acid synthase (FAS). These FASs are multi-enzyme complexes that contain numerous enzyme activities. The complexes condense coenzyme A (CoA) thioesters (usually acetyl, propionyl, or malonyl) followed by a ketoreduction, dehydration, and enoylreduction of the [3-keto moiety of the elongated carbon chain to form specific fatty acid products. These subsequent enzyme activities may or may not be present in the biosynthesis of polyketides. 

As a result of the reduced activity of the mutase in vitamin B12 deficiency, there is an accumulation of methyhnalonyl CoA, some of which is hydrolyzed to yield methylmalonic acid, which is excreted in the urine. As discussed in Section 10.10.3, this can be exploited as a means of assessing vitamin B12 nutritional status. There may also be some general metabolic acidosis, which has been attributed to depletion of CoA because of the accumulation of methyl-malonyl CoA. However, vitamin B12 deficiency seems to result in increased synthesis of CoA to maintain normal pools of metabolically useable coenzyme. Unlike coenzyme A and acetyl CoA, neither methylmalonyl CoA nor propionyl CoA (which also accumulates in vitamin B12 deficiency) inhibits pantothenate kinase (Section 12.2.1). Thus, as CoA is sequestered in these metabolic intermediates, there is relief of feedback inhibition of its de novo synthesis. At the same time, CoA may be spared by the formation of short-chain fatty acyl carnitine derivatives (Section 14.1.1), which are excreted in increased amounts in vitamin B12 deficiency. In vitamin Bi2-deficient rats, the urinary excretion of acyl carnitine increases from 10 to 11 nmol per day to 120nmolper day (Brass etal., 1990). 

Fatty acids have predominantly even numbers of carbon atoms because they are effectively formed from acetyl (C2) units, which are derived from glucose in the presence of various enzymes, coenzymes and carrier proteins. An overall scheme for saturated fatty acid biosynthesis is presented in Fig. 2.13, in which it can be seen that the first step is the formation of acetyl coenzyme A (often abbreviated to acetyl-CoA). One molecule ofacetyl-CoA undergoes addition of CO, to form malonyl-CoA, while the acetyl group on another molecule is transferred to an enzyme (fatty acid synthase). The malonyl unit (C3) is added to the enzyme-bound acetyl unit, which produces a butyryl group following loss of C02, dehydration and reduction. Six further steps of combined malonyl addition, decarboxylation, dehydration and reduction occur to yield palmitate (C16). Higher acids are built from palmitate in a similar... 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

One form of methionine synthase common in bacteria uses lV -methyltetrahydrofolate as a methyl donor. Another form of the enzyme present in some bacteria and mammals uses A/ -methyltetrahydro-folate, but the methyl group is first transferred to cobalamin, derived from coenzyme B12, to form methylcobalamin as the methyl donor in methionine formation. This reaction and the rearrangement of L-methyl-malonyl-CoA to succinyl-CoA (see Box 17-2, Fig. la) are the only known coenzyme Bi2-dependent reactions in mammals. In cases of vitamin B12 deficiency, some symptoms can be alleviated by administering not only vitamin B12 but folate. As noted above, the methyl group of methylcobalamin is derived from W -methyltetrahy-drofolate. Because the reaction converting the methylene form to the 7V -methyl form of tetrahydrofo-... 

The homolytic cleavage of the Co - C bond of the protein-boimd organo-metallic cofactor AdoCbl (2) is the initial step of the coenzyme Bi2-catalyzed enzymatic reactions. Halpern quoted that adenosyl cobamides can be considered as reversibly functioning sources for organic radicals [119]. A neutral aqueous solution of 2 is remarkably stable with a half-Ufe of 10 s (in the dark at room temperature), but decomposes, mainly with the homolysis of the Co-C bond, at higher temperatures [119,123]. The coenzyme B12-catalyzed enzyme reactions occur with maximal rates of approximately 100 s [173,239]. Rapid formation of Co(ll)corrins occurs only with addition of substrate to a solution of holoenzyme (or of apoenzymes and 2), as demonstrated in most of the known coenzyme Bi2-dependent enzymes, e.g., in methyl-malonyl-CoA mutase [121], glutamate mutase [202] and ribonucleotide reductase [239]. 

Scheme 12.23. The formation of naringenin chalcone, the progenitor for flavonoids. The chalcone forms from 4-coumaroyl-CoA (produced from coumatic acid using the enzyme 4-coumarate-CoA ligase, EC 6.2.1.12, coenzyme A, and ATP) and 3 equivalents of malonyl-CoA (the enzyme is naringenin chalcone synthase, EC 2.3.1.74) followed by a cyclization.

Formation of a poly-j -keto-acyl-CoA [as (5.9)] occurs as for fatty acid biosynthesis by condensation of acetyl-coenzyme A with malonyl-CoA. Malonyl-CoA is generally derived by carboxylation of 3.1) (Scheme 3.3). An alternative path to malonyl-CoA is via oxaloacetate, an intermediate in the citric acid cycle. 

On the basis of an analogy with the biogenesis of fatty acids via malonyl coenz3nne A, Lynen (1959) and Lynen and Tada (196I) predicted the participation of methylmalonyl coenz3nne A in the formation of the erythronoUde lactone or its Cgi acid. According to this hypothesis, the propionyl coenzyme A acts as... 

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