Re: discoverer of proteins

> Who discovered proteins?

Hi Roman. Great question! So simple, yet opening a whole world of answers!

I guess I'll begin my answer in the late 1700's. Physiological chemists were trying to understand the materials that made up living cells. One group of substances were discovered that were called albuminoids, perhaps because they were first isolated from the albumin of eggs. These albuminoids were probably identified because they were easilly observable by simple experiments such as coagulation of egg white by heat when you cook an egg, curdling of milk with acid, or the spontaneous clotting of blood. All of these treatments cause a solid material to appear from an initially fluid substance.

Then sometime in the 1800s a widely known experiment demonstrated the physiological significance of albuminoids. Dogs fed a diet of only sugar and fat would not survive, but dogs whose diets included albuminoids would. So, chemists of the time set out to identify the chemical makeup of proteins. For example, they had already determined that water was made up of two parts hydrogen and one part oxygen (H₂O), and methane from one part carbon and four parts hydrogen (CH₄).

By 1838, a Dutch chemist called Gerardus Mulder presented his finding that proteins had high quantities of nitrogen and trace amounts of sulfur as well as the usual carbon, hydrogen and oxygen found in sugars. This roused considerable interest. Among those intrigued by Mulder's work was a Swedish biochemist Jöns Jakob Berzelius, who suggested that Mulder give the albuminoids another, more special name:

Because albuminoids appear to be a principal substance of animal nutrition, I propose to you the word protein, which I derive from the Greek proteios, meaning "of primary importance."

As it turns out, Mulder's work became the center of a great controversy because people could not reproduce the ratios of the elements that he found in proteins. (His work in 1938 gave the formula C₄₀H₆₂N₁₀O₁₂, which he changed later to C₃₆H₅₄N₈O₁₂.) Opposition to his work believed that proteins were the debris of microscopic organs in the cell - each containing many different sorts of things. That being the case, the opposition argued that they were much better suited for study by biologists rather then chemists. Fortunately, not all chemists agreed with this, and in the next hundred years it was discovered that proteins are long chains of 20 different kinds of molecules (called amino acids) strung together. Because every protein has a different chain of amino acids making it up, every protein has a slightly different ratio of atomic elements, which explains the irreproducability of Mulder's work. This is just one of many examples in science where strange results really are trying to tell you something, but in a way that you weren't thinking of in the first place.

Now that chemists had identified proteins, a really big question which remained was what did proteins do?

The answer to this question came from a totally different direction, started by the study of a process which has been known for about ten thousand years: fermentation. If you crush some grapes to release their juice, in time the juice will start bubbling, and when the reaction is finished, you have wine. This is due to a chemical reaction taking place in which glucose (a sugar) from the grapes is being converted to ethanol (an alcohol) and carbon dioxide gas, as shown in the following chemical reaction:

C₆H₁₂O₆ -> 2C₂H₅OH + 2CO₂

(Don't worry about this reaction if you have not had basic chemistry yet.)

In time, it was discovered that yeast on the grapes was necessary for this reaction, and a whole debate ensued. Did the fermentation process an example come about because of a "Vital Force" in yeast (in other words a unmeasurable part of living creatures, much like a soul), or from something in the yeast which in itself it not alive? Chemists were especially interested understanding the cause of this reaction because they were amazed that such a reaction could take place at all. The chemical reaction in itself was no big deal. It was already known that sugar could be broken down this way, but there was two important differences here.

1. The reaction always needed strong acid or base to take place. Yeast somehow did this reaction at neutral, physiological, pH.

2. The reaction was stereospecific. Glucose exists in two mirror forms, simillar to the way most people have left and right hands. No matter how you rotate a right hand, it can not superimpose with a left hand. The same is true for the two forms of glucose. Now, both forms of glucose break apart at the same rate in acid or base. But the reaction caused by yeast worked on only one form, and not the other. How could the yeast reaction cause one to react but not the other?

Eventually, from the work of many chemists over several decades time, science learned how to isolate smaller and smaller parts of yeast, and eventually they found that it was a protein which did the novel chemistry. I have always found the proof of this fun. People who believed in the "Vital Force" of yeast caused the fermentation reaction argued that no matter what you purified, you could not completely disprove the notion that the life force was somehow carried along with what you purified.

It was the famous work of Moses Kunitz and John Howard Northrop at the Rockefeller Institute in New York City in the 1930s that finally convinced the world. There they studied three different chemical reactions that seems to come from three different proteins. Each protein/reaction examined by three different techniques.

1. Gel electrophoresis, which separates molecules based on their charge in solution.
2. Centrifugation, which separates molecules based on their size.
3. Acid treatment of protein, which breaks apart the protein in solution.

In the first two techniques, the chemical activity was found to completely follow the protein being purified. The last technique showed that the chemical activity disappeared proportionally with the amount of protein destroyed.

In all of these experiments, the fact that the physical materials moves or is inactivated in the same way as the biological activity led to the conclusion that the catalytic power was an intrinsic property of the protein, thus ending the great debate.

One final quote for the road. Looking back on the protein catalytic ability controversy, Northrop wrote:

The history of biochemistry is a chronicle of a series of controversies. These controversies exhibit a common pattern. There is a complicated hypothesis, which usually entails an element of mystery and several unnecessary assumptions. This is opposed by a more simple explanation, which contains no unnecessary assumptions. The complicated one is always the more popular one at first, but the simpler one, as a rule, eventually is found to be correct. This process frequently requires ten to twenty years. The reason for this long time lag was explained by Max Planck (a physicist). He remarked that scientists never changed their minds, but eventually they die.

If you want to read more about both of these stories, I highly recommend a book called, "Discovering Enzymes," by David Dressler and Huntington Potter. It is a wonderfull book on the history of proteins and contains everything I mentioned here and a lot more. The book is no longer in print, but you probably can find it at the library or out-of-print book stores.

I hope that wasn't too long of an answer. Thanks for asking!

-Daniel