Cholesterol is most important for its role in formation and maintenance of cell membranes and structures as well as being a precursor for the synthesis of crucial steroid hormones, bile acids and vitamin D. Cholesterol can be synthesised naturally and obtained from the diet. It is transported through the circulation in lipoproteins. Normal healthy adults synthesise 1g/day and consume approximately 0.3g/day as keeping a rather constant level in the blood (150-200 mg/d). It is regulated by controlling the rate at which cholesterol is synthesised naturally. Excess levels of cholesterol can have negative side effects as they lead up to the build-up of low density lipoproteins (LDLs) within the artery walls. This causes damage to the endothelium lining of the blood vessels and can cause multiple health concerns such as blood clots which can lead to strokes or heart attacks. For this reason, it is important that the synthesis of cholesterol is regulated.
Cholesterol is made via the synthetic pathway it begins with the transport of Acetyl-CoA from within the mitochondria to the cytosol. Two acetyl-CoA molecules combine to form Acetoacetyl CoA, which then reacts with a third Acetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). HMG-CoA is converted to mevalonate by HMG-CoA reductase, HMGR. HMGR requires NADPH as a cofactor as 2 molecules of NADPH are consumed during the conversion of HMG-CoA to mevalonate. The reaction catalysed by HMG-CoA is the rate limiting step of cholesterol biosynthesis.
Mevalonate once formed undergoes a series of 3 phosphorylation’s with the use of ATP to produce 3-phospho-5-pyrophosphomevalonate, this molecule is very unstable and undergoes a decarboxylation reaction where isopentenyl pyrophosphate (IPP) is produced. IPP can isomerise to form dimethylallyl pyrophosphate, they then both combine to form geranyl pyrophosphate by use of dimethylallyl transferase. Geranyl pyrophosphate combines with IPP to give Farnesyl-PP (FPP), this is the precursor for producing cholesterol and other compounds including, CoQ10, dolichol and isoprenylated proteins. Two FPPs combine to form pre-sqaulene, which then undergoes phosphate elimination to form squalene. Squalene then undergoes a two-step cyclisation to yield lanosterol, lanosterol then undergoes numerous reactions and is finally converted to cholesterol.
The synthesis of cholesterol can be regulated by targeting the enzymes used in these reactions by various methods, however, it is most commonly exerted at the beginning of the pathway at the HMG-CoA reductase step. Evidence of this can be seen by the sterol mediated regulation of transcription. Sterol regulatory element binding protein (SREBP) binds to a short DNA sequence called the ‘sterol regulatory element’ (SRE) when there are reduced levels of cholesterol, this then enhances the gene for HMG-CoA reductase. It does this by being escorted by SREBP cleavage activating protein (SCAP) in small membrane vesicles to the golgi complex. It is then dissociated by proteolytic cleavages which allows SREBP to travel to the nucleus and bind to the SRE of HMG-CoA reductase to increase transcription. When levels of cholesterol become too high, the proteolytic release of SREBP is blocked and is held in the endoplasmic reticulum and any SREBP that are present in the nucleus are degraded by proteasomes. As the cells are not able to synthesis cholesterol, the only source is LDL cholesterol found in the plasma. The cells synthesis and upregulate LDL receptors on the plasma cell surface allowing the LDL to bind and be taken up by the cell.