1964 Nobel Prize in Physiology or Medicine — Bloch and Lynen: Unraveling the 36 Steps of Cholesterol Biosynthesis

What You Will Learn in This Article

You will understand how cholesterol, a 27-carbon molecule synthesized only in animal cells, is assembled in 36 steps from the simple 2-carbon molecule, acetate. You will also learn how Bloch, a biochemist who emigrated from Germany to the United States to escape Nazi persecution, painstakingly reconstructed this pathway over 20 years at Columbia, Chicago, and Harvard. Finally, you will discover how this research laid the theoretical foundation for the development of statin drugs, a class of cardiovascular medications, that are used today.

A Story Different from Common Knowledge — Cholesterol is a 27-Carbon Macromolecule

The advice to “be careful with cholesterol” is common today. However, what cholesterol actually is and how it is made in our bodies is often overlooked.

Let’s start with a precise definition. Cholesterol is a sterol molecule synthesized only in animal cells, made up of 27 carbon atoms. The cholesterol biosynthetic pathway runs through a full 36 steps, but if we compress it into landmarks, it condenses to four checkpoints: Acetate (C2) → Mevalonate (C6) → Isoprene (C5) → Squalene (C30) → Cholesterol (C27).

This summary is surprising. Starting with acetate, a 2-carbon molecule, cholesterol, a 27-carbon molecule, is created. Acetate molecules repeatedly combine to form larger molecules, and three carbon atoms are removed from squalene (30 carbons), resulting in the final 27-carbon cholesterol. Bloch and Lynen spent 20 years meticulously tracing each of these 36 steps.

Why was it important to elucidate this pathway? There are two reasons. First, cholesterol is an essential component of cell membranes and a precursor for various steroid hormones (cortisol, sex hormones) and bile acids. In other words, it is indispensable. Second, excess cholesterol is a major cause of cardiovascular disease. In other words, too much of it is also not good. To precisely regulate the required amount, we need to know where to intervene — the answer to this question lies somewhere within the 36-step pathway.

In the language of CS, this problem is about reverse engineering a long build pipeline. Starting from the final output (cholesterol), we measure each intermediate, identify the transformation steps and catalysts (enzymes) between them, and work our way backward. This is why it took 20 years.

The Historical Context — The Asian Cold War Turns into Actual War

1964 was the year the Asian Cold War escalated into actual war.

In world history, the August Tonkin Gulf incident and resolution occurred. Based on reports that U.S. destroyers were attacked by North Vietnamese torpedo boats (later partially proven to be fabricated), the U.S. Congress passed a resolution authorizing the president to take military action. This marked the beginning of the U.S. military involvement in the Vietnam War. In July, President Johnson signed the Civil Rights Act, dismantling the legal basis for racial discrimination. In October, the Tokyo Olympics — a symbolic event marking Japan’s return to the international stage after the war.

In Korean history, the June 3 incident occurred in Seoul. In response to the unfair terms of the Korea-Japan Claims Agreement (beginning with the Kim-Ohira Memorandum in 1962), students staged protests. The Park Chung-hee regime declared martial law, and the leaders of student organizations at Seoul National University and Korea University were arrested. This remains one of the most important events in the history of the Korean student movement, alongside the April 19 Revolution.

In this year, the Nobel Committee recognized the cholesterol biosynthetic pathway. In a year when politics was reshaping the world through intense conflict, a metabolic pathway within the body was quietly elucidated.

The Two Laureates — An Immigrant Who Fled the Nazis and a Munich Native

Konrad E. Bloch (1912-2000) was an American biochemist. He was not, however, American by birth. The years in which he was finishing his degree coincided with the year Hitler came to power. From the Nuremberg Rally onward, the Nazi persecution of Jews began in earnest, and Bloch — who was Jewish — closed the German chapter of his life. He spent a short time as a research fellow at the Swiss National Institute before immigrating to the United States in 1936.

In the United States, he obtained his Ph.D. in 1938 from the Department of Biochemistry at the Columbia University College of Physicians and Surgeons with funding from the Werner Fund, under the guidance of Clarke. Then came the crucial encounter. He joined Schoenheimer’s research team at Columbia and met Schoenheimer along with his colleague Rittenberg. After this meeting he accelerated his research sharply, and from 1942 he spent nearly 20 years wrestling with cholesterol biosynthesis alongside Rittenberg.

Schoenheimer and Rittenberg were pioneers in the isotope labeling method. They introduced radiolabeled or heavy isotope-labeled acetate into cells and tracked where the label appeared in the final molecules and which carbon atoms it was incorporated into. This technique, which allowed them to visualize the journey of individual molecules, became a crucial tool for Bloch’s 20-year endeavor.

Bloch’s career continued as follows:

• Assistant Professor of Biochemistry at the University of Chicago in 1946, promoted to Professor. While there, he also conducted research on the biosynthesis of glutathione, a tripeptide, with Snoke.
• In 1953, he received a Guggenheim Fellowship and spent a brief period at the Zurich Institute of Organic Chemistry. He returned to the United States with a large research grant for a biochemical study of terpenes and steroids, which had been developed in Switzerland.
• In 1954, he became a professor of biochemistry in the Department of Chemistry at Harvard University and served as head of the department from 1968.

Feodor F. K. Lynen (1911-1979) was a German biochemist who received his doctorate from the University of Munich in 1937, then became a professor at the University of Munich (1947-1953), and later the director of the Max Planck Institute for Cell Biochemistry (1954-1979). Because he was not Jewish, he was able to remain in Germany during the Nazi regime and became a leading figure in biochemistry based in Munich after the war.

The Crucial Discovery — From Acetate to Mevalonate

Their crucial discovery can be summarized this way. In 1950, Bloch showed experimentally that the basic raw material of cholesterol is acetate, a two-carbon compound, and identified mevalonic acid as the first substance produced from it. Then, in 1951, Lynen probed the activated form and reaction pathway of acetate, pinning down both the activation principle and the chemistry of the subsequent binding steps.

Behind this short summary lies a great deal of weight. The fact that acetate is the raw material had been theoretically predicted for some time, but Bloch was the one who confirmed it through experimentation. Furthermore, the discovery that mevalonic acid is the first stable intermediate produced from acetate was crucial. Mevalonic acid is the first commitment point in this pathway — once this point is reached, the pathway is irreversibly directed toward cholesterol synthesis.

Lynen discovered that acetate is actually activated in the form of acetyl-CoA to participate in the reaction. That is, it must bind to the coenzyme A to become reactive. This discovery became a representative example of the activation principle common to many metabolic pathways.

The summary of the pathway is as follows:

• Multiple Acetyl-CoA (C2) molecules combine → Mevalonic acid (C6) — This is the first commitment point, and HMG-CoA reductase catalyzes the reaction (the target of statin drugs today).
• Mevalonic acid (C6) → Isoprene units (C5) — Multiple isoprene units combine to form materials for various other compounds.
• Isoprene units combine → Squalene (C30) — A direct precursor of cholesterol.
• Squalene (C30) → Lanosterol → Multiple intermediates → Cholesterol (C27) — Multiple oxidation, reduction, and demethylation reactions.

The 20-year effort involved meticulously tracing each of these 36 steps.

The CS Framework of the Build Pipeline

Let’s now look at this metabolic pathway in the language of CS.

A build pipeline defines the various stages from source code to the final deployable output. Source files → compilation → linking → optimization → packaging → deployment. Each stage involves a specific tool (compiler, linker, etc.), and the output of each stage becomes the input for the next stage.

Cholesterol biosynthesis is exactly this kind of pipeline.

• Source file = Acetyl-CoA (the activated form of acetate)
• Tools for each stage = Specific enzymes (e.g., HMG-CoA reductase, squalene monooxygenase, etc.)
• Intermediate output for each stage = Mevalonic acid, isoprene, squalene, lanosterol, ...
• Final output = Cholesterol
• Dependency graph = Sequential dependencies of the 36 steps

The pathway illustrates the difficulty of reverse engineering. If only the source file and final output are known, and the intermediate steps are unknown, how long would it take to identify each intermediate and precisely determine each catalyst? The answer is 20 years. Cells maintain intermediates for a very short time and move on to the next step — transient cache. To capture and measure this, sophisticated techniques such as drugs that inhibit specific steps or isotope labeling are required.

HMG-CoA reductase = the rate-limiting step in the pipeline. Just as the slowest step in a software pipeline determines the overall throughput, the activity of this enzyme determines the overall cholesterol production. Therefore, statin drugs target and inhibit this enzyme today. If you find the bottleneck in the pipeline, you can control the entire system — this is exactly the principle of performance optimization in CS.

Limitations of the analogy: Of course, the metabolic pathway is not a static pipeline but a dynamic system with multiple feedback loops. For example, cholesterol itself inhibits the expression of HMG-CoA reductase, reducing its own production. However, the fundamental structure of “a series of sequential transformations and the existence of a rate-limiting step” is completely consistent.

The Legacy That Continues Today

The way in which the research of Bloch and Lynen lives on today is clear.

Statin drugs: Drugs that inhibit HMG-CoA reductase. Akira Endo, a Japanese scientist, discovered this class of drugs in the late 1970s, and they have since become one of the most prescribed cardiovascular drugs in the world. They are the standard treatment for the prevention of myocardial infarction and stroke. The fact that we know what the target of these drugs is (HMG-CoA reductase) is due to the fact that Bloch and Lynen identified that this enzyme is the rate-limiting enzyme in the pathway.

Establishment of the clinical significance of cholesterol levels: The fact that today we know that LDL (low-density lipoprotein) cholesterol is a marker of cardiovascular risk is a result of this research. Understanding the metabolic pathway has led to a more detailed understanding of which forms of cholesterol are harmful.

Understanding steroid hormone metabolism: The fact that cholesterol is a common precursor for various steroids, including cortisol, sex hormones, and vitamin D, has made it possible to develop approaches to regulate the metabolism of these hormones.

Applications of isoprene chemistry: Isoprene, an intermediate in the cholesterol pathway, is also used as a common raw material for various natural compounds (terpenoids). This is the industrial basis for fragrances, flavors, and anticancer drugs.

Why It Matters

What Bloch and Lynen left behind is the demonstration that even complex metabolic pathways can be completely deciphered in principle.

Many scientists at the time believed that the complex metabolism within cells was too intertwined to accurately determine the sequential steps. The two men disproved this belief — with 20 years of perseverance and sophisticated tools such as isotope labeling, they were able to identify each of the 36 steps.

This principle is the root of today’s metabolomics. The analytical techniques of today, which simultaneously quantify thousands of metabolites in cells, are an extension of this approach. It is the methodological root of systems biology and precision medicine.

Another implication is that “if you find the bottleneck in the pipeline, you can control the entire system.” The success story of statin drugs is a powerful demonstration of this principle. The 20-year effort identified the importance of a specific enzyme (HMG-CoA reductase), and this identification led to a drug that reduces the cardiovascular risk of hundreds of millions of people half a century later.

The story of a young Jewish scientist who immigrated to the United States to escape the Nazis and left a theoretical foundation for cardiovascular drugs today — politics can drive individuals to different continents, but what they will leave behind is unpredictable.

Summary of Bloch and Lynen in 1964: Elucidation of the 36-step biosynthetic pathway from acetate (C2) to cholesterol (C27), involving mevalonic acid, isoprene, and squalene. Establishment of the activation principle of acetyl-CoA and the rate-limiting role of HMG-CoA reductase. This laid the theoretical foundation for statin drugs, a class of cardiovascular medications, that are used today.

→ Previous: 1963 — Eccles, Hodgkin, and Huxley → Next: 1965 — Jacob, Lwoff, and Monod and the Operon Theory