Isopentenyl pyrophosphate, formation from acetyl

Terpenoids do not necessarily contain exact multiples of five carbons and allowance has to be made for the loss or addition of one or more fragments and possible molecular rearrangements during biosynthesis. In reality the terpenoids are biosynthesized from acetate units derived from the primary metabolism of fatty acids, carbohydrates and some amino acids (see Fig. 2.10). Acetate has been shown to be the sole primary precursor of the terpenoid cholesterol. The major route for terpenoid biosynthesis, the mevalonate pathway, is summarized in Fig. 2.16. Acetyl-CoA is involved in the generation of the C6 mevalonate unit, a process that involves reduction by NADPH. Subsequent decarboxylation during phosphorylation (i.e. addition of phosphate) in the presence of ATP yields the fundamental isoprenoid unit, isopentenyl pyrophosphate (IPP), from which the terpenoids are synthesized by enzymatic condensation reactions. Recently, an alternative pathway has been discovered for the formation of IPP in various eubacteria and plants, which involves the condensation of glyceraldehyde 3-phosphate and pyruvate to form the intermediate 1-deoxy-D-xylulose 5-phosphate (Fig. 2.16 e.g. Eisenreich et al. 1998). We consider some of the more common examples of the main classes of terpenoids below. 

The first stage in the synthesis of cholesterol is the formation of isopentenyl pyrophosphate from acetyl CoA. This set of reactions, which takes place in the cytosol, starts with the formation of 3-hydroxy-3-methylglutaryl CoA (HMG CoA) from acetyl CoA and acetoacetyl CoA. This intermediate is reduced to mevalonate for the synthesis of cholesterol (Figure 26,7). Recall that mitochondrial 3-hydroxy-3-methylglutaryl CoA is processed to form ketone bodies (Section 22.3.5). 

Cholesterol is a steroid component of eukaryotic membranes and a precursor of steroid hormones. The committed step in its synthesis is the formation of mevalonate from 3-hydroxy-3-methylglutaryl CoA (derived from acetyl CoA and acetoacetyl CoA). Mevalonate is converted into isopentenyl pyrophosphate (C5), which condenses with its isomer, dimethylallyl pyrophosphate (C5), to form geranyl pyrophosphate (Cjo)- The addition of a second molecule of isopentenyl pyrophosphate yields famesyl pyrophosphate (C15), which condenses with itself to form squalene (C30). 

The answer is d. (Murray, pp 505-626. Scrivei, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) In the first stage ol cholesterol formation, acetyl coenzyme A condenses to form mevalonate, which is then phosphorylated and decarboxylated to form isopentenyl pyrophosphate. Half of the isopentenyl pyrophosphate isomerizes to form dimethylallyl pyrophosphate. These two isomeric C5 pyrophosphate units (isopentenyl pyrophosphate and dimethylallyl pyrophosphate) condense to form a CIO compound called geranyl pyrophosphate. Isopentenyl pyrophosphate then condenses with geranyl pyrophosphate to form the C15 compound farne-syl pyrophosphate. Finally, two farnesyl pyrophosphates condense in the presence of NADPH to form the C30 compound squalene. Squalene is ultimately cyclized through a series of steps to form cholesterol. Thus, the correct sequence of events leading from C5 units to C30 squalene is sequential condensation of 5-carbon units until a 15-carbon unit is formed, then condensation of two 15-carbon units to form squalene. 

Cholesterol is synthesized mainly in the liver by a three-stage process. All 27 carbon atoms in the cholesterol molecule are derived from acetyl-CoA. The first stage is the synthesis of the activated five-carbon isoprene unit, isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate then condense to form squalene in a sequence of reactions that also synthesize isoprenoid intermediates that are important in protein isoprenylation modifications. The characteristic four-ring structure of cholesterol is then formed by cycUzing of the linear squalene molecule. Several demethylations, the reduction of a double bond, and the migration of another double bond result in the formation of cholesterol. Figure 34-1 provides an overview of cholesterol biosynthesis. 

The isoprenoids are a vast array of biomolecules that contain repeating five-carbon structural units known as isoprene units (Figure 11.10). Isoprenoids are not synthesized from isoprene (methylbutadiene). Instead, their biosynthetic pathways all begin with the formation of isopentenyl pyrophosphate from acetyl-CoA (Chapter 12). 

The biosynthesis of natural rubber has been studied from the viewpoint of an elucidation of initiation and propagation mechanisms mainly by tracer techniques. The steps in the formation of isopentenyl pyrophosphate from acetyl-coA via mevalonate are now well established in the in vitro synthesis of rubber. It has also been confirmed that chain extension occurs on the surface of existing rubber particles by successive additions of isopentenyl pyrophosphate to build up chains of 5000-7000 isoprene units (4,5). The initiation step of rubber formation, however, remains unknown due to the lack of detailed information concerning the direct precursor of the chain extension. 

A, The Formation of Isopentenyl Pyrophosphate (Fig. 93). All isoprenoids originate from isopentenyl pyrophosphate, which is usually formed from acetyl CoA (C 1.2), but may also be derived in higher plants and animals from leucine (D 14). An important intermediate is mevalonic acid. Isopentenyl pyrophosphate is an activated molecule, which easily undergoes nucleophilic substitution at C-1 and electrophilic substitution at the double bond ... 

Carotenoids are synthesized from the basic C5 terpenoid precursor isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP). These precursors can be obtained from two distinct pathways the mevalonate pathway (MVA) and the non-MVA pathway also known as 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway [84, 85]. All eukaryotes use the MVA pathway, whereas plant plastids and most bacteria use the MEP pathway [86,87]. Some bacteria also use the MVA pathway [84]. In the MEP pathway, the first step in IPP biosynthesis is the formation of l-deoxy-D-xylulose-5-phosphate (DXP) from pyruvate and glyceraldehyde-3-phosphate catalyzed by DXP synthase (Figure 10.7). DXP is then reduced to MEP by DXP reductase. Additional MEP pathway enzymes are then used in subsequent reactions for converting MEP to IPP, which is isomer-ized to DMAPP by the enzyme IPP isomerase. The MVA pathway begins with the conversion of three molecules of acetyl-CoA to MVA through acetoacetyl-CoA... 

There are conflicting reports about whether the chloroplast is completely autonomous with respect to carotenoid biosynthesis or whether early biosynthetic intermediates are imported into the chloroplast. For example. Buggy et al reported that isolated chloroplasts can form carotenoids from mevalonic acid (MVA), with MVA itself being formed intrachloroplastidi-cally from acetyl-CoA. More recently, the formation of /3-carotene from " C-labeled bicarbonate by isolated spinach chloroplasts, under photosynthetic conditions, has been reported. In contrast, however, Kreuz and Kleinig have concluded that isopentenyl pyrophosphate must be imported... 

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