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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
Since its discovery from gallstones in 1769 by Francois Poulletier de la Salle, cholesterol (see Figure 15-1) has attracted the attention of scientists.
As early as 1932, the first total synthesis was attempted, and by 1949 two very famous groups were competing to be the first to synthesize cholesterol. In 1951, the first synthetic route was published by Robinson and Cornforth.[1] The following year, another synthesis was published by Woodward and Sondheimer.[2] Since then, at least three other synthetic routes have been reported with the final product either being a racemic (50:50) mixture of the natural product and its enantiomer (non-superimposable mirror image) or just pure enantiomer, ent-cholesterol.
Each of the chemical routes to synthesis started with a naturally occurring compound that could either be extracted from natural sources or bought from a chemical company that synthesized the starting material from an even simpler natural product. Numerous intermediates, including various catalysts, reagents, and other substrates, also had to be purchased or prepared. The shortest reported route to ent-cholesterol (not the naturally occurring compound) consists of 16 linear steps with a 2% overall yield from the starting compound (S)-citronellol.[3] The work by Rychnovsky and Belani introduces the AB rings of cholesterol using annulation (ring forming reaction) that closely resembles Sir Robert Robinson’s annulation reaction. The synthesis of pure, naturally occurring cholesterol took many more steps, and the yield was also minuscule compared to the volume of starting material used.[4] In mammals, cholesterol is synthesized mainly in the liver, adrenal glands, intestines, and gonads.[5] Its entire carbon backbone is made from one molecule, an acetyl group (CH3CO). The first stage of biological synthesis utilizes three acetyl groups and the enzyme coenzyme A to produce mevalonate, a sixcarbon intermediate. The next step involves adding three phosphates (PO 3- 4 ) to the two hydroxyl (OH) groups using a kinase (the enzyme) and three adenosine triphosphates (ATPs). Initially, kinase just phosphorylates (adds a phosphate group) and later performs phosphorylation and decarboxylation (removal of carbon dioxide).
The product isopentyl pyrophosphate (IPP) is the result of this decarboxylation. Some of the IPP is stored in that form, while a larger portion is converted to dimethylallyl pyrophosphate (DMAPP) by a process called isomerization. In this isomerization, an external double bond is moved internally. With both isoprene units (a fivecarbon unit with four carbons in a straight line and a CH3 group on carbon number 2) at hand, the isomers are combined to produce geranyl pyrophosphate. One more DMAPP is added to make the 15-carbon farnesyl pyrophosphate. Another farnesyl pyrophosphate produced in an identical fashion is added to the first to make squalene, with 30 carbons.
Squalene has all the carbons needed to make cholesterol, but the famous ABCD four-ringed structure has not been formed. To achieve this, one of the two double bonds at the end of the squalene chain out of a possible six choices is oxidized with O2 and nicotinamide adenine dinucleotide phosphate (NADPH) to make squalene oxide. Cleavage of the oxide activates a beautiful cascade reaction in which all the internal double bounds of squalene react in sequence to form the ABCD steroid ring system.
Lanosterol is the next major intermediate. It is formed via a number of hydride and methyl group shifts and an elimination of hydrogen to produce the double bond. It takes another 19 steps to form cholesterol. These steps include the removal of three of the eight methyl groups present in lanosterol (demethylation), plus addition and removal of double bonds. Although this may seem like a lot of steps, the effort is worthwhile because many of these intermediates are used to make other useful molecules such as vitamin D, fatty acids, and other steroids.
Cholesterol has eight chiral centers, which means that based on chirality alone, a possible 256 (2n; n = number of chiral centers) compounds could be synthesized in the body. However, only one isomer is produced. The body metabolism is designed to determine how each isoprene unit should be stored and assembled, as well as how to differentiate between the six double bonds in squalene and the eight methyl groups in lanosterol. The enzyme’s selectivity and specificity is astounding from the perspective of a synthetic organic chemist.
Many of the scientific queries regarding the biosynthetic pathway of cholesterol can be specifically answered. Where is cholesterol made? In the liver, adrenal glands, intestines, and gonads. What is cholesterol made from? From manipulation of one molecule, an acetyl group. How is it made? Through a complex sequence of steps, detailed above.
Two questions remain unanswered at this point: When was cholesterol first made, and by whom? If the scientific community considered it appropriate to honor with Nobel Prizes the synthetic feats of Sir Robert Robinson, John Cornforth, and Robert B. Woodward, a truly scientific mind must ask the same question about nature. Who created these synthetic pathways? Considering this is just one of an uncounted number of compounds necessary for life, I propose that the real prize belongs to the Author of life and nature.
NOTES
[1] HME Cardwell, JW Cornforth, SR Duff, H Holtermann, R Robinson. Total synthesis of androgenic hormones. Chemistry & Industry 1951; 101:389–390.
[2] RB Woodward, F Sondheimer, D Taub, K Heusler, WM McLamore. The total synthesis of steroids. Journal of the American Chemical Society 1952; 74:4223–4251.
[3] JD Belani, SD Rychnovsky. A concise synthesis of ent-cholesterol. The Journal of Organic Chemistry 2008; 73(7):2768–2773.
[4] Cardwell et al., op. cit.; Woodward et al., op. cit.
[5] WD Nes. Biosynthesis of cholesterol and other sterols. Chemical Reviews 2011; 111(10):6423–6451.
Glenn Phillips is an instructional technologist in the North American Division Learning Interactives Department at Loma Linda University and is an academic coach for the chemistry department at Lamar University. He holds a PhD in Synthetic Organic Chemistry from Michigan State University. He has written two books of lecture notes in organic chemistry based on his former experience as professor of chemistry at Oakwood University and the University of South Alabama. He is currently working as the subject matter expert for a project titled “Reactions: Chemistry Lab Simulations".