If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Fatty Acid and Lipid Biosynthesis → Sterol Biosynthesis|
Some taxa known to possess this pathway include : Homo sapiens
Expected Taxonomic Range: Metazoa
Cholesterol is an important structural component of eukaryotic cell membranes. It is also a precursor of steroid hormones and bile acids. The impact of cholesterol and its regulation on human health is well known. Its role in arteriosclerosis, and drug therapy for this process, has been extensively studied (reviewed in [Knopp99]). Nobel prizes in Physiology or Medicine were awarded to Konrad Bloch in 1963 and to M. S. Brown and J. L. Goldstein in 1985 for their work on the metabolism and regulation of cholesterol. Several interesting historical reviews detail the discovery of cholesterol and work leading to the elucidation of its structure and function [Bloch65, Bloch92, Vance00].
The first nine reactions of cholesterol biosynthesis are shown in the linked mevalonate pathway in HumanCyc. Mevalonate is the precursor of isoprenoids, which are involved in the biosynthesis of many classes of compounds in addition to cholesterol. As shown in the mevalonate pathway, the cholesterol molecule is formed from acetate units (reviewed in [Rilling85, Bloch65]). The acetate units are joined in a series of reactions to form farnesyl pyrophosphate, a branch point for the biosynthesis of other isoprenoid compounds such as ubiquinone, dolichol, and farnesylated proteins. The mevalonate pathway is highly regulated, as reviewed in [Goldstein90]. HMG-CoA reductase from the mevalonate pathway is the rate-determining enzyme for the entire pathway from acetate to cholesterol. This enzyme is the target for the well-known cholesterol-lowering statin drugs (reviewed in [Knopp99]).
Farnesyl-diphosphate farnesyltransferase (squalene synthase) is the first specific enzyme in cholesterol biosynthesis. It catalyzes a 2-step reaction involving a "head-to-head" condensation of two molecules of farnesyl pyrophosphate to form presqualene pyrophosphate [Rilling69, Altman71, Corey76]. Presqualene pyrophosphate is then reduced to squalene. Details of the enzymatic reaction mechanism, which involves carbocation intermediates, have been described [Blagg02]. The importance of squalene synthase has been demonstrated in squalene synthase knockout mice, which results in embryonic lethality and retardation of development [Tozawa99]. Squalene epoxidase (monooxygenase) catalyzes oxygen-dependent epoxidation of squalene [Yamamoto70]. Oxidosqualene cyclase (lanosterol synthase) catalyzes its remarkable cyclization, in a single reaction, to form lanosterol [Dean67]. This cyclization mechanism was proposed by several researchers, but the correct structure was determined by R. B. Woodward and K. Bloch [Woodward53], and reviewed in [Bloch65].
The enzymology of the multistep conversion of lanosterol to cholesterol was largely determined in rat liver by J. L. Gaylor and coworkers (reviewed in [Gaylor02]). The human pathway is therefore inferred from this work. The order of some of the reactions in this pathway may vary. The pathway shown here represents the reactions in a frequently published order, updated as follows. The last three reactions, as illustrated in two reviews ([Gaylor02, Rilling85]), have more recently been reordered as shown here ([Bae97], in [Waterham01], and reviewed in [Gaylor02]). Previously, the isomerase reaction was followed by the Δ5-desaturase, the Δ7-reductase (producing desmosterol) and the Δ24-reductase, producing cholesterol (as shown by clicking on the pathway link above that leads to pathway cholesterol biosynthesis III (via desmosterol)) (reviewed in [Gaylor02, Rilling85], in [Salway04]). In the pathway shown here, the isomerase reaction is followed by the Δ24-reductase, the Δ5-desaturase and the Δ7-reductase, producing cholesterol. Desmosterol is not shown. This reordering is based on substrate specificity studies of the Δ24 reductase [Bae97]. However, the Δ24 double bond reduction can also occur at other times, using other intermediates as substrates [Bae97]. This is evidenced by the genetic disorder desmosterolosis. In this disorder, patients have elevated levels of desmosterol in their plasma and tissues due to mutations in the Δ24-reductase gene [Waterham01]. Another route involves reduction of the Δ24 double bond at the point of lanosterol and leads to the production of 24,25-dihydrolanosterol. Metabolism of this compound is shown by clicking on the pathway link above that leads to pathway cholesterol biosynthesis II (via 24,25-dihydrolanosterol).
The lanosterol-to-cholesterol conversion involves the oxidative removal of three methyl groups, reduction of double bonds, and migration of the lanosterol double bond to a new position in cholesterol (in [Kawata85] and reviewed in [Gaylor02]). Unlike the mostly cytosolic reactions of the mevalonate pathway, the reactions shown here are catalyzed by membrane-bound enzymes (reviewed in [Gaylor02, Rilling85]). In rat liver, they have been localized in the endoplasmic reticulum [Reinhart87]. Solubilization and characterization of these enzymes had to be achieved before this segment of the pathway could be determined, as reviewed in [Gaylor02]. Human genes have been identified for all the enzymes in this pathway and human disorders of cholesterol metabolism have been associated with genetic defects in most of these enzymes. Mouse models also exist for several of these human disorders ([Marijanovic03] and reviewed in [Herman03]).
In the pathway shown here, the first of the three methyl groups, the 14α methyl group, is removed from lanosterol by a cytochrome P450 isozyme [Trzaskos84]. The reaction proceeds through alcohol and aldehyde intermediates [Shafiee86]. This is followed by decarbonylation with formate release, and Δ14 double bond formation. Details of this reaction mechanism have been published and involve formation of a formyl ester intermediate (not shown), suggesting a Bayer-Villiger reaction mechanism ([Fischer91], and reviewed in [Gaylor02]). The resulting Δ14 double bond of the conjugated diene is then reduced by an NADPH-dependent reductase [Paik84].
Another multienzyme complex, C4 methyl sterol oxidase, removes the second (4α) and third (4β) methyl groups in a sequential oxidative series of reactions that include carboxylic acid intermediates [Miller70, Miller71, Miller67, Miller70a]. This enzyme is not a cytochrome P450. It utilizes cytochrome b5 as an electron carrier [Kawata86, Fukushima81]. The first of the two decarboxylation reactions is accompanied by epimerization of the 4β methyl group to the 4α position [Rahimtula72]. Thus, after subsequent reduction, the remaining methyl group is in the 4α orientation for further stereospecific oxidation by this enzyme ([Gaylor75, Sharpless68] and in reviews [Gaylor02, Rilling85, Schroepfer82]).
Following decarboxylation, a 3-ketosteroid intermediate is formed and is reduced by 3-ketosteroid reductase [Swindell68, Billheimer81]. The final demethylated intermediate then undergoes isomerization of the Δ8(9) double bond, which moves to the Δ7(8) position of the sterol B-ring [Bechtold72, Paik86]. The Δ24-reductase then catalyzes reduction of the side chain double bond [Bae97]. The Δ5-desaturase is a multienzyme system utilizing cytochrome b5 as an electron carrier. It catalyzes formation of the Δ5 double bond to form 7-dehydro-cholesterol ([Kawata86, Kawata85], and in [Taton00]). The terminal enzyme of the pathway reduces the Δ7 double bond to form cholesterol [Lee97].
Acknowledgment: We thank Drs. Anne Venturelli and Ripudaman Malhotra, SRI International, for their help in providing chemical names for many of the intermediates shown in this pathway.
Superpathways: superpathway of cholesterol biosynthesis
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