Biosynthesis of Cholesterol


Structure of Cholesterol
Structure of Cholesterol

 

 

 

Cholesterol is an isoprenoid lipid of the steroid group, esterol subclass, with an important rol in the structure of cell membrane, as a precursor of hormones, Vitamin D and bile acids, and in the pathology of vascular diseases.  

 

Cholesterol biosynthesis occurs in practically all the tissues, but it is more active in liver and in steroid producing organs, like suprarrenal cortex and gonads.

 

In the Cholesterol synthesis participates enzymes from the smooth endoplasmic reticulum  and the cytosol.

 

The main “materials” required for the synthesis of cholesterol are:

 

a)     Acetyl CoA, whose acetyl groups provide all the carbons of cholesterol.

 

b)      ATP, as an energy source.

 

c)       NADPH.H+ as provider of the reduction equivalents required for the synthesis.

 

Cholesterol biosynthesis is a very complex process that can be studied in different steps:

 

I.- Mevalonate synthesis

 

The first reactions of the synthesis of cholesterol are similar to the reactions involved in ketogenesis:

 

1.- Two Acetyl CoA molecules react to form acetoacetyl CoA, in a reaction catalyzed by Beta-keto-thiolase.

 

2.- Acetoacetyl CoA reacts then with another molecule of Acetyl CoA to form Beta-Hidroxymethyl Glutaryl Co A (HMGCoA), in a reaction catalyzed by the  HMGCoA synthase (this is a cytoplasmatic enzyme with an activity similar to the mitochondrial enzyme that participates in the ketogenesis).

 

3.- At this point, the reactions of cholesterol and ketone bodies synthesis diverge: while in ketogenesis HMGCoA is split, during the formation of cholesterol the HMGCoA is reduced by the enzyme HMGCoA reductasa, an enzime located in the Smooth Endoplasmic Reticulum, but with the active site oriented to the cytosol. 

HMGCoA----- Mevalonate

HMGCoA-----> Mevalonate

                   

 

 

This enzyme use NADPH.H+ as reducing agent, and it is the key enzyme in the control of cholesterol biosynthesis,  since it is inhibited physiologically by cholesterol and pharmacologically by statins, as will be discussed later.

 

 

II.- Conversion of Mevalonate to active isopren units.

 

Three consecutive phosphorylation (catalyzed by Kinases),  using ATP as (P) donor,  followed by a decarboxylation and a dephosphorylation, produce the active forms of isoprene, Isopentenyl pyrophosphate and its isomer, dimethylallyl pyrophosphate:

Isopentenyl Pyrophosphate ---- Dimethylallyl Pyrophosphate

Isopentenyl Pyrophosphate <----> Dimethylallyl Pyrophosphate

 

III. – Condensation of active isoprene units and formation of Squalene.

 

Active isoprene units can follow different pathways in the metabolism. In humans they follow mainly the synthesis of Cholesterol and synthesis of CoQ.

For the synthesis of Cholesterol:

 

Dimethyl allyl (P)~(P) + isopentenyl (P)~(P)-à Geranyl (P)~(P)

 

Geranyl (P)~(P) +Isopentenyl (P)~(P)à Farnesyl (P)~(P)

 

Farnesyl Pyrophosphate can bind to proteins to anchor them to the plasma membrane (prenylation), or continue in the synthesis of Cholesterol by binding to another molecule of Farnesyl (P)~(P), in a reaction catalyzed by Squalene Synthase, that also requires NADPH.H+ as donor of reduction equivalents:

 

Farnesyl (P)~(P) + Farnesyl (P)~(P)+ NADPH.H+ à Squalene +NADP+ + 2 (P)~(P)

 

Squalene (30 C)  already has all the carbon atoms required for the synthesis of Cholesterol (27 C). Next reactions will close the rings to form the Sterane system of rings that is characteristic of Cholesterol.

 

IV.-Squalene Cyclization and transformation in Cholesterol.

 

Squalene is oxidized in a reaction catalyzed by an oxido squalene cyclase enzyme and converted to the first metabolite that shows the system of rings of steroid compounds: Lanosterol. This compound, besides showing this characteristic system of rings, also present already the hydroxyl in C3 that is frequent in steroid compounds (the introduction of this hydroxyl ring requires NADPH.H+).

Squalene Cyclation

Squalene Cyclation

  A sequence of around 20 reactions will convert Lanosterol to Cholesterol.

 

 

Lanosterol to Cholesterol

Lanosterol to Cholesterol

 

 
Cholesterol is of great biological importance in the formation of membranes, not just because of its structural rol, but also because of its influence on membrane fluidity. Besides, Cholesterol is precursor of steroid hormones, as hormones of suprarenal cortex, (glucocorticoids and mineralocorticoids), and sex hormones and progesterone. Vitamin D and bile salts are also important derivatives of Cholesterol.  From the pathological point of view, it is well known also the relationship between cholesterol in blood and cardiovascular diseases.
 

 

 

Because of all the above reasons, the regulation of cholesterol synthesis is an important issue from the biological and medical point of view, and it is under different mechanisms of biochemical and physiological regulation. Statins are widely used as pharmacological agents in the regulation of Cholesterol synthesis. 

 

Regulation of the synthesis of Cholesterol will be considered in a specific post devoted to this topic.

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