Mevalonate, decarboxylation

Methyl-l-propanol, l3C NMR spectrum of. 453 2-Methylpropene, heat of hydrogenation of. 187 Mevalonate, decarboxylation of, 1075 isopentenyl diphosphate from, 1072-1075... 

Mevalonate molecules are condensed to a 30-carbon compound, squalene. The alcohol groups of mevalonate are first phosphorylated. Then they multiply phosphorylated mevalonate decarboxylates to make the two compounds isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). 

D-Mevalonic acid is the fundamental intermediate in the biosynthesis of the terpenoids and steroids, together classed as poly-isoprenoids. The biogenetic isoprene unit is isopentenyl pyrophosphate which arises by enzymic decarboxylation-dehydration of mevalonic acid pyrophosphate. D-Mevalonic acid is almost quantitatively incorporated into cholesterol synthesized by rat liver homogenates. 

In keeping with its biogenetic origin m three molecules of acetic acid mevalonic acid has six carbon atoms The conversion of mevalonate to isopentenyl pyrophosphate involves loss of the extra carbon as carbon dioxide First the alcohol hydroxyl groups of mevalonate are converted to phosphate ester functions—they are enzymatically phosphorylated with introduction of a simple phosphate at the tertiary site and a pyrophosphate at the primary site Decarboxylation m concert with loss of the terti ary phosphate introduces a carbon-carbon double bond and gives isopentenyl pyrophos phate the fundamental building block for formation of isoprenoid natural products... 

The principal steps in the mechanism of polyisoprene formation in plants are known and should help to improve the natural production of hydrocarbons. Mevalonic acid, a key intermediate derived from plant carbohydrate via acetylcoen2yme A, is transformed into isopentenyl pyrophosphate (IPP) via phosphorylation, dehydration, and decarboxylation (see Alkaloids). IPP then rearranges to dimethylaHyl pyrophosphate (DMAPP). DMAPP and... 

Step 4 of Figure 27.7 Phosphorylation and Decarboxylation Three addition reactions are needed to convert mevalonate to isopentenyl diphosphate. Th first two are straightforward phosphorylations that occur by nucleophilic sul stitution reactions on the terminal phosphorus of ATP. Mevalonate is first cor verted to mevalonate 5-phosphate (phosphomevalonate) by reaction wit ATP in a process catalyzed by mevalonate kinase. Mevalonate 5-phosphat then reacts with a second ATP to give mevalonate 5-diphosphate (diphosphc mevalonate). The third reaction results in phosphorylation of the tertiar hydroxyi group, followed by decarboxylation and loss of phosphate ion. 

The final decarboxylation of mevalonate 5-diphosphate appears unusual because decarboxylations of acids do not typically occur except in /3-keto acids and malonic acids, in which the carboxylate group is two atoms away from an additional carbonyl group (Section 22.7). The function of this second carbonyl group is to act as an electron acceptor and stabilize the charge resulting from loss of CC>2- In fact, though, the decarboxylation of a /3-kelo acid and the decarboxylation of mevalonate 5-diphosphate are closely related. 

In Acanthomyops, the strikingly different incorporations following the two mevalonate feedings indicate that mevalonate is not degraded before being built into terpenes but rather is decarboxylated, as in the classical mevalonic acid pathway. 

Step 2—Formation of Isoprenoid Units Mevalonate is phosphorylated sequentially by ATP by three kinases, and after decarboxylation (Figure 26-2) the active isoprenoid unit, isopentenyl diphosphate, is formed. 

Mevalonic acid is then modified by phosphorylation and decarboxylation, and several molecules of it are condensed to form cholesterol in a complex series of eight reactions. 

Elimination usually involves loss of a proton together with a nucleophilic group such as -OH, -NH3+, phosphate, or pyrophosphate. However, as in Eq. 13-18, step c, electrophilic groups such as -COO-can replace the proton. Another example is the conversion of mevalonic acid-5-pyrophosphate to isopentenyl pyrophosphate (Eq. 13-19) This is a key reaction in the biosynthesis of isoprenoid compounds such as cholesterol and vitamin A (Chapter 22). The phosphate ester formed in step a is a probable intermediate and the reaction probably involves a carbo-cationic intermediate generated by the loss of phosphate prior to the decarboxylation. 

In animals all isoprenoid compounds are apparently synthesized from mevalonate, which is converted by the consecutive action of two kinases21 23 into mevalonate 5-diphosphate (Fig. 22-1, step b). Mevalonate kinase is found predominantly in peroxisomes, which are also active in other aspects of steroid synthesis in humans.2124 A deficiency of this enzyme is associated with mevalonic aciduria, a serious hereditary disease in which both blood and urine contain very high concentrations of mevalonate.23 Mevalonate diphosphate kinase, which is also a decarboxylase, catalyzes phosphorylation of the 3-OH group of mevalonate (step c, Fig. 22-1) and decarboxylative elimination of phosphate (step d)25 to form isopentenyl diphosphate. 

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 Lire same biosynlheLic scheme (Fig. 3). The sell-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 (22). Phosphorylation of (22) followed by concomitant decarboxylation and dehydration processes... 

Formation of the biological isoprene unit from mevalonic acid has been shown to proceed by stepwise phosphorylation of both alcohol groups, then elimination and decarboxylation to yield 3-methyl-3-butenyl pyrophosphate, 9 (often called A3-isopentenyl pyrophosphate) ... 

The biosynthesis of steroids begins with the conversion of three molecules of acetyl-CoA into mevalon-ate, the decarboxylation of mevalonate, and its conversion to isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate are polymerized into squalene, which is cyclized to yield lanosterol. Lanos-terol is converted to cholesterol, which is the precursor of bile acids and steroid hormones. 

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

The IBP and its products are displayed in Figure 12.1. HMG-CoA, ultimately derived from acetyl-CoA is converted to mevalonate via the enzyme HMG-CoA reductase (HMGR) [8]. This reaction is the rate-limiting step in the pathway. Mevalonate is then phosphorylated via mevalonate kinase (MK) to yield 5-phosphomevalonate [9]. IPP is formed following additional phosphorylation and decarboxylation steps [10]. Isomerization of IPP via the enzyme IPP isomerase yields DMAPP [11]. In mammals, the enzyme farnesyl pyrophosphate synthase (FDPS) catalyzes the synthesis of both GPP and FPP [12]. In plants, a separate GPP synthase has been identified [13]. GPP is a key intermediate in plants as it serves as the precursor for all monoterpenes. In animals, however, GPP appears to serve only as an intermediate in the synthesis of FPP. Very low basal levels of GPP have been measured in cell culture, although cellular GPP levels can become markedly increased in the setting of FDPS inhibition [14]. 

Mevalonate is converted into 3-isopentenylpyrophosphate in three consecutive reactions requiring ATP (Figure 26.8). Decarboxylation yields isopentenyl pyrophosphate, an activated isoprene unit that is a key building block for many important biomolecules throughout the kingdoms of life. We will return to a discussion of this molecule later in the chapter. 

Figure 26.8. Synthesis of Isopentenyl Pyrophosphate. This activated intermediate is formed from mevalonate in three steps, the last of which includes a decarboxylation.

C,2iS- H,3R]mevalonate, Bloxham and Akhtar showed that a tritium atom was lost whereas when [3a- H,26,27- C2]lanosterol was used the tritium was retained. The latter result was also observed by Hornby and Boyd. Presumably NAD is necessary for the oxidation at C-3 to a ketone prior to decarboxylation. Similarly, Miller and Gaylor showed that 4a-methyl-5a-cholest-7-en-3) -ol was oxidized only as far as the 4a-carboxylic acid, with retention of tritium at C-3 but loss from a 4a-C H3 group. In the latter case, the recovered 4a-methyl sterol showed no sign of tritium enrichment due to isotope effects. In banana, alkylation at C-24 seems to precede loss of the 4a-methyl groups. When the rat liver system was inhibited by cholestane-3, 5a,6 -triol, sterols accumulated which retained a methyl group at C-4. Both 4,4-dimethyl- and 4 -methyl-cholest-8-en-3/I-ol Uere isolated, and were shown to be converted into cholesterol under normal conditions. 

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