Squalene synthesis from mevalonic acid

After FPP synthesis, the biochemical reactions and enzymes involved have not been fully understood and well characterized. Akhila et al. [21] proposed a complete biosynthetic pathway for artemisinin, starting from mevalonic acid and IPP. The pathway branches at FPP. FPP is converted in to squalene by the enzyme squalene synthase (SQS) and subsequently into sterol. SQS is the key enzyme catalyzing the first step of the sterol biosynthetic pathway, a pathway in competition with that of artemisinin biosynthesis [26]. 

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. 

C. Reduction of HMG CoA to mevalonic acid is an early step in cholesterol synthesis. Inhibition of this step would lead to an increase in cellular levels of HMG CoA and a decrease in squalene, an intermediate beyond this step, and cholesterol. The decreased cholesterol levels in cells cause ACAT activity to decrease and synthesis of LDL receptors to increase. Because the receptors function (but at a less than normal rate), more receptors cause more LDL to be taken up from the blood. Consequently, blood cholesterol levels decrease, but blood triacylglyc-erol levels do not change much, since LDL does not contain much triacylglycerol. 

Theories on the in vivo synthesis of phytanic acid concentrate mainly on two possibilities. One of them deals with the possible formation of phytanic acid from isopren units or four molecules of mevalonic acid (or mevalonate), (Kahlke 1964 a, Kahlke and Richterich 1965). Instead of an end-to-end condensation of two molecules of famesyl pyrophosphate which results in the formation of squalene and finally cholesterol, a fourth active isoprenoid unit might be attached to famesyl pyrophosphate. From this intermediate several steps of hydrogenation and oxidation would be required for the formation of phytanic acid. This hypothesis now appears unlikely since Steinberg (1965) was unable to detect any activity in the phytanic acid fraction after administration of labeled mevalonate to a patient with HAP (case T.E. of Refsum). 

A different kind of enzymatic individuality in cancer cells is shown by three liver tumours (from mouse, rat, and human) which had lost feedback control of cholesterol synthesis while the surrounding healthy liver tissue was exercising normal restraint on this synthesis. The site of this failure of control was traced to HMG-reductase, the enzyme that converts p-hydroxy-P-methylglutaric acid to mevalonic acid, on the way to squalene (Siperstein and Fagan, 1964). 

Cholesterol is formed in the liver (85%) and intestine (12%) - this constitutes 97% of the body s cholesterol synthesis of 3.2 mmol/day (= 1.25 g/day). Serum cholesterol is esterized to an extent of 70-80% with fatty acids (ca. 53% linolic acid, ca 23% oleic acid, ca 12% palmitic acid). The cholesterol pool (distributed in the liver, plasma and erythrocytes) is 5.16 mmol/day (= 2.0 g/day). Homocysteine stimulates the production of cholesterol in the liver cells as well as its subsequent secretion. Cholesterol may be removed from the pool by being channelled into the bile or, as VLDL and HDL particles, into the plasma. The key enzyme in the synthesis of cholesterol is hydroxy-methyl-glutaryl-CoA reductase (HGM-CoA reductase), which has a half-life of only 3 hours. Cholesterol is produced via the intermediate stages of mevalonate, squalene and lanosterol. Cholesterol esters are formed in the plasma by the linking of a lecithin fatty acid to free cholesterol (by means of LCAT) with the simultaneous release of lysolecithin. (s. figs. 3.8, 3.9) (s. tab. 3.8)... 

---