We now turn our attention to the synthesis of the fundamental lipid cholesterol. This steroid modulates the fluidity of animal cell membranes (Section 12.5) and is the precursor of steroid hormones such as progesterone, testosterone, estradiol, and cortisol. All 27 carbon atoms of cholesterol are derived from acetyl CoA in a three-stage synthetic process (Figure 26.7).

1. Stage one is the synthesis of isopentenyl pyrophosphate, an activated isoprene unit that is the key building block of cholesterol.

2. Stage two is the condensation of six molecules of isopentenyl pyrophosphate to form squalene.

3. In stage three, squalene cyclizes and the tetracyclic product is subsequently converted into cholesterol.

The first stage takes place in the cytoplasm, and the next two in the endoplasmic reticulum.

The first stage in the synthesis of cholesterol is the formation of isopentenyl pyrophosphate from acetyl CoA. This set of reactions 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 26.8). Recall that, alternatively, 3-hydroxy-3-methylglutaryl CoA may be generated in the mitochondria and processed to form ketone bodies, which are subsequently secreted to provide fuel for other tissues, notably the brain under starvation conditions (Section 22.3).

The synthesis of mevalonate is the committed step in cholesterol formation. The enzyme catalyzing this irreversible step, 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), is an important control site in cholesterol biosynthesis, as will be discussed shortly.

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HMG-CoA reductase is an integral membrane protein in the endoplasmic reticulum.

Mevalonate is converted into 3-isopentenyl pyrophosphate in three consecutive reactions requiring ATP (Figure 26.9). In the last step, the release of CO₂ yields isopentenyl pyrophosphate, an activated isoprene unit that is a key building block for many important biomolecules throughout the kingdoms of life.

Squalene is synthesized from isopentenyl pyrophosphate by the reaction sequence

C₅ → C₁₀ → C₁₅ → C₃₀

This stage in the synthesis of cholesterol starts with the isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate.

These two isomeric C₅ units (one of each type) condense to form a C₁₀ compound: isopentenyl pyrophosphate attacks an allylic carbocation formed from dimethylallyl pyrophosphate to yield geranyl pyrophosphate (Figure 26.10). The same kind of reaction takes place again: geranyl pyrophosphate is converted into an allylic carbonium ion, which is attacked by isopentenyl pyrophosphate. The resulting C₁₅ compound is called farnesyl pyrophosphate. The same enzyme, geranyl transferase, catalyzes each of these condensations.

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The last step in the synthesis of squalene is a reductive tail-to-tail condensation of two molecules of farnesyl pyrophosphate catalyzed by the endoplasmic reticulum enzyme squalene synthase.

The reactions leading from C₅ units to squalene, a C₃₀ isoprenoid, are summarized in Figure 26.11.

The final stage of cholesterol biosynthesis starts with the cyclization of squalene (Figure 26.12). Squalene is first activated by conversion into squalene epoxide (2,3-oxidosqualene) in a reaction that uses O₂ and NADPH. Squalene epoxide is then cyclized to lanosterol by oxidosqualene cyclase. The enzyme holds squalene epoxide in an appropriate conformation and initiates the reaction by protonating the epoxide oxygen. The carbocation formed spontaneously rearranges to produce lanosterol. Lanosterol is converted into cholesterol in a multistep process by the removal of three methyl groups, the reduction of one double bond by NADPH, and the migration of the other double bond (Figure 26.13).

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