The conversion of lanosterol to cholesterol

The conversion of lanosterol to cholesterol involves 19 steps and is described in the article Cholesterol Biosynthesis Lanosterol to Cholesterol on pp 377-384 in the March 2002 issue of the Journal of Chemical Education... 

The conversion of lanosterol to cholesterol involves removal of the three methyl groups at the 4,4- and 14-positions, shift of the double bond at the B/C junction to between C5 and C6, and reduction of the C24-C25 double bond. The methyl groups are indicated by tracer experiments to be eliminated by oxidation to carbon dioxide. 

Two pathways for the conversion of lanosterol to cholesterol. The major route in mammals proceeds through 7-dehy drocho l es terol. 

Figure 2 In the first steps of the conversion of lanosterol to cholesterol, an enzyme, cytochrome P-4S0, oxidizes the three methyl groups that will be removed while leaving the rest of the molecule untouched, because of geometric control that is typical in enzymes.

The importance of cholesterol biosynthesis in embryonic development and formation of the central nervous system is reflected in patients with disorders in the pathway for the conversion of lanosterol to cholesterol. Three enzyme deficiencies have been identified (Figure 19-16) ... 

A partial pathway for the conversion of lanosterol to cholesterol. The complete process consists of 19 steps. The C24 = C25 double bond can be reduced by 3 j8-hydroxylsteroid-A -reductase at several steps along the pathway (indicated by I), and deficiency of this enzyme leads to accumulation of desmosterol. Deficiency of enzyme 2 and enzyme 3 results in CDPX2 and SLO syndromes, respectively (see text). 

Cytochrome P450 enzyme system The cytochromes P450 are mixed-function oxidases that require both NADPH and O. They are involved in a number of reactions in the conversion of lanosterol to cholesterol, as well as important steps in the synthesis of steroid hormones. Cytochromes P450 are very important in the detoxification of xenobiotics and in the metabolism of drags. 

For the conversion of lanosterol to cholesterol, there are a number of excellent reviews that consider the sequence of events and the participants therein [1,2,5,105]. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

Fig. 8 is a schematic diagram of a cell which shows the known sites in which sterol carrier proteins are involved in cholesterol biosynthesis, utilization and intracellular transfer. SCP, participates in the conversion of squalene to lanosterol and SCP2 participates in the conversion of lanosterol to cholesterol, the conversion of cholesterol to cholesterol ester by ACAT, and probably also in the conversion of cholesterol to 7a-hydroxycholesterol. SCPj transfers cholesterol from cytoplasmic lipid inclusion droplets to mitochondria in the adrenal and SCPj also translocates cholesterol from the outer to the inner mitochondrial membrane. 

Figure 21.29 shows the conversion of squalene to cholesterol. The details of this conversion are far from simple. Squalene is converted to squalene epoxide in a reaction that requires both NADPH and molecular oxygen (O2). This reaction is catalyzed by squalene monooxygenase. Squalene epoxide then undergoes a complex cyclization reaction to form lanosterol. This remarkable reaction is catalyzed by squalene epoxide cyclase. The mechanism of the reaction is a concerted reaction—that is, one in which each part is essential for any other part to take place. No portion of a concerted reaction can be left out or changed because it all takes place simultaneously rather than in a sequence of steps. The conversion of lanosterol to cholesterol is a complex process. It is known that 20 steps are required to remove three methyl groups and to move a double bond, but we shall not discuss the details of the process. 

Thorough investigation has delineated 19 steps in the conversion of lanosterol to cholesterol. An abbreviated version of the path is provided in Scheme 11.76 where it is intended to show that the conversion requires a series of methyl group oxidations in the sequence expected (viz. methyl [-CH3] hydroxymethyl... 

The conversion of lanosterol to cholesterol involves the loss of three... 

A biosynthetic relation is represented by ergosterol, where the methyl of methionine is the source of its C-24 (Alexander et of., 1958 Alexander and Schwenk, 1968). A cell-free ergosterol-synthesizing system has been prepared from yeast (Parks, 1958). Oxidative removal of angular methyls in the conversion of lanosterol to cholesterol by animal tissues does not appear to be folic-mediated (Bloch, 1957). 

Cholesterol consists of four fused rings and an eight-membered hydrocarbon chain. It is synthesized from acetyl CoA. The first two reactions of the pathway are similar to that of ketogenesis, with the formation of HMG CoA. The rate limiting step is the synthesis of mevalonic acid, catalysed by HMG CoA reductase. It requires the reducing properties of 2NADPH and releases acetyl CoA. A five-carbon isoprene unit is then formed from mevalonic acid using ATP. A series of condensation reactions between isoprene units follows, which ends in the formation of squalene, a 30-carbon compound. Squalene is converted to lanosterol by hydroxylation then cyclization. The conversion of lanosterol to cholesterol is a multi-step process that involves many enzymes located in the endoplasmic reticulum. Thus, cholesterol synthesis occurs in the endoplasmic reticulum and the cytoplasm of all cells in the body. 

The conversion of lanosterol to cholesterol involves a 19-step reaction sequence catalysed by microsomal enzymes. The exact order of the reactions has not been delineated and, indeed, there may be more than one pathway. The main features of the transformation are the removal of three methyl groups, reduction of the 24(25)-double bond and isomerization of the 8(9)-double bond to position 5 in cholesterol. 

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