Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Lipid Metabolism
Cholesterol Biosynthesis

In the 1940s and 1950s, K. Bloch et al., in experiments using acetate labeled with ¹⁴C at the methyl and carboxyl groups, demonstrated that both carbon atoms of acetic acid are incorporated into Liver Cholesterol in approximately equal amounts. Furthermore, it was proven that all carbon atoms of cholesterol originate from acetate.

Later on, through the pioneering work of F. Lynen, G. Popják, J. Cornforth, A.N. Klimov, and other researchers, the fundamental details of the enzymatic synthesis of cholesterol—which encompasses more than 35 enzymatic reactions—were elucidated. Cholesterol synthesis can be divided into three main stages: Stage I involves The conversion of active acetate into mevalonic acid; Stage II entails The formation of squalene from mevalonic acid; and Stage III is the cyclization of squalene into cholesterol.

Let us examine the conversion of active acetate into mevalonic acid. The initial step in the synthesis of mevalonic acid from acetyl-CoA is the formation of acetoacetyl-CoA via a reversible thiolase reaction:

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Subsequently, the Condensation of acetoacetyl-CoA with a third molecule of acetyl-CoA, catalyzed by hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase), yields ß-hydroxy-ß-methylglutaryl-CoA:

Next, under the action of the regulatory enzyme NADPH-dependent hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase), ß-hydroxy-ß-methylglutaryl-CoA is converted into mevalonic acid through the reduction of one of its carboxyl groups and the release of HS-CoA:

The HMG-CoA reductase reaction is the first practically irreversible step in the cholesterol Biosynthesis pathway. It proceeds with a significant loss of Free energy (approximately 33.6 kJ). This reaction has been established as the rate-limiting step in cholesterol biosynthesis.

Alongside the classical pathway of mevalonic acid biosynthesis, a second pathway exists in which the intermediate substrate appears to be ß-hydroxy-ß-methylglutaryl-S-ACP rather than ß-hydroxy-ß-methylglutaryl-CoA. The reactions of this pathway are identical to the Initial Stages of FATTY ACID BIOSYNTHESIS up to the formation of acetoacetyl-S-ACP. Acetyl-CoA carboxylase—the enzyme responsible for converting acetyl-CoA to malonyl-CoA—takes part in the formation of mevalonic acid via this route. The optimal ratio of malonyl-CoA to acetyl-CoA for mevalonic acid synthesis is 2 molecules of acetyl-CoA per 1 molecule of malonyl-CoA.

The participation of malonyl-CoA—the primary substrate in fatty acid biosynthesis—in the formation of mevalonic acid and various polyisoprenoids has been demonstrated in a range of biological systems: pigeon and rat liver, rabbit mammary gland, and Cell-free Yeast extracts. This pathway of mevalonic acid biosynthesis is observed predominantly in the Cytosol of liver Cells. A crucial role in mevalonate formation here is played by HMG-CoA reductase, which has been found in the soluble fraction of rat liver and differs from the microsomal enzyme in several kinetic and regulatory properties. The regulation of this second mevalonic acid biosynthesis pathway under various conditions (such as starvation, cholesterol feeding, or administration of the surfactant Triton WR-1339) differs from that of the first pathway involving the microsomal reductase. These findings indicate the existence of two autonomous systems for mevalonic acid biosynthesis. The Physiological Role of the second pathway is not yet fully understood. It is believed to be significant not only for the synthesis of non-steroid substances, such as the side chain of ubiquinone and the unique base N⁶-(∆²-isopentenyl)-adenosine found in certain tRNAs, but also for steroid biosynthesis (A.N. Klimov, E.D. Polyakova).

In Stage II of cholesterol synthesis, mevalonic acid is converted into squalene. Stage II reactions begin with the phosphorylation of mevalonic acid by ATP, resulting in the formation of mevalonate 5-phosphate and subsequently mevalonate 5-pyrophosphate:

Upon subsequent phosphorylation of its tertiary hydroxyl group, mevalonate 5-pyrophosphate forms an unstable intermediate, 3-phosphomevalonate 5-pyrophosphate, which undergoes decarboxylation and loss of a phosphate group to yield isopentenyl pyrophosphate. The latter then isomerizes into dimethylallyl pyrophosphate:

Next, both isomeric forms (dimethylallyl pyrophosphate and isopentenyl pyrophosphate) condense with the release of pyrophosphate to form geranyl pyrophosphate:

Another molecule of isopentenyl pyrophosphate is then added to geranyl pyrophosphate. This reaction yields farnesyl pyrophosphate:

In the final reaction of this stage, squalene is formed via an NADPH-dependent reductive condensation of 2 molecules of farnesyl pyrophosphate:

In Stage III of cholesterol biosynthesis, squalene is cyclized to lanosterol under the action of squalene epoxidase (squalene-oxidase). The subsequent conversion of lanosterol into cholesterol involves a series of reactions accompanied by the removal of three methyl groups, saturation of the side-chain double bond, and the shift of the double bond in ring B from the 8,9-position to the 5,6-position (the precise details of these final reactions are still under investigation):

Below is the General scheme of cholesterol synthesis:

Starting from squalene, all intermediate products of cholesterol biosynthesis (including cholesterol itself) are insoluble in an aqueous environment. Therefore, they participate in the final reactions of cholesterol biosynthesis while bound to sterol carrier Proteins (SCP). This ensures their solubility in The Cell cytosol and the progression of the respective reactions. This fact is also of great importance for the incorporation of cholesterol into cell membranes, its oxidation into Bile acids, and its conversion into Steroid Hormones. As noted earlier, the rate-limiting reaction of overall cholesterol biosynthesis is the reduction of ß-hydroxy-ß-methylglutaryl-CoA to mevalonic acid, catalyzed by HMG-CoA reductase. This enzyme is subject to regulatory influence by A number of factors. In particular, The rate of reductase synthesis in the liver exhibits distinct circadian rhythms, peaking at midnight and reaching its minimum in the morning hours.

The activity of HMG-reductase increases upon the administration of Insulin and THYROID HORMONES, which leads to enhanced cholesterol synthesis and elevated Blood cholesterol levels.

Conversely, during fasting, thyroidectomy, or the administration of Glucagon and glucocorticoids, cholesterol synthesis is suppressed, which is primarily associated with a decrease in HMG-CoA reductase activity.