Wednesday, October 8, 2008

Nobel Prize in Chemistry for Green Fluorescent Protein

Green fluorescent protein was isolated from the green fluorescent jellyfish, Aequorea victoria. Seems like hardly a big deal, except that today the scientists who discovered this interesting jellyfish received the 2008 Nobel Prize in chemistry for their work. What's all the fuss?

When I was a grad student at WSU in 1996 for biochemistry, I was studying telomeres and solving the structure of the ACCCT DNA motif. I was asked to present my work at the 5th annual Protein Crystallography conference at the University of Oregon that year, and was pretty nervous. But when I got there, I realized that, much like today, I was nobody. The buzz was all around the presenter after me, who had solved the structure of green fluorescent protein. It was quite a scene, and I remember asking myself what all the commotion was about.

A Glowing Jellyfish, a Nobel Prize? First, the jellyfish was found to fluoresce green, and scientists were perplexed as to why. Think about it, we see something that doesn't make sense, and we investigate as to why the jellyfish glows green. Probably some evolutionary favorable trait that allows a deep water organism to illuminate its surroundings to find food, or scare its predators.

The Structure of GFP The structure is quite elegant. Proteins are made up of amino acids that are linked together via an amide bond (N-C). There are 20 amino acids, and they are linked sequentially in different and distinct combinations to produce a long peptide polymer. Each amino acid has a different side chain, which has a certain chemistry, if you will, to it that makes each amino acid distinct. GFP is made up of 238 amino acids. If you're still with me, here's where nature's beauty "unfolds" (no pun intended.)

The 238 amino acids are linked together to form the basis for GFP protein. This "sequence" is called the protein's primary structure. Guess what? There is a secondary structure, dictated by the chemistry of the protein's amino acids' amide bonds. Since proteins are hydrophilic, or water loving, the amino acids arrange themselves to hydrogen bond in the most energetically favorable state with water molecules. That is to say, hydrogen bonds are formed with water molecules to form what is called a "beta sheet". This beta sheet can be thought of as ribbons of amino acids that stablize themselves, via structure, in water, through hydrogen bonding.

Stay with me The sequence of the 238 amino acids is GFP's primary structure. The hydrogen bonding by these amino acids with water molecules is GFP's secondary structure. So most every protein has a tertiary structure. The beta sheet of GFP folds back upon itself to form what is called a "Beta Barrel" or " Beta Can." This formation is again driven by the interaction of the protein with "itself" and the water environment. Remember, all those amino acids have distinct side chains, and these side chains have distinct chemistries. So in the tertiary structure, the protein "folds" to form a structure to make sure all of the amino acid side chains that like water face outwardly and interact with water, and side chains that don't like water are shielded from the water. So for GFP, looking at the beta barrel cartoon, the water hatin' stuff is shielded because it's located between "ribbons" and thus, shielded from water. The water loving stuff is hanging off the ribbons and interacting with the water. (I'm overly simplifying things, but you get the point I hope.) As a final note, the ribbons could have folded up into any structure, but the beta barrel is the most energy efficient structure given GFP's primary structure (sequence of amino acids). Nature is elegant, we are anything but. (Great link with pictures on protein structure here.)

Back to the GFP's Structure and its Applications If you remember nothing else, just know that the GFP is a beta barrel with a chromophore protected in the center of the barrel. The chromophore in this case is formed via an interaction between Amino acid #65 (Serine 65), Tyrosine #66, and Glycine #67 that absorbs blue light and emits green, giving GFP its green glow. Neat, but again, what's all the commotion?

Well you can create different chromophores to GFP and have the protein emit different colors of light. Think red, blue, green, yellow, etc., any wavelength that is visible. So the big picture goes like this:

  1. You can create any "fluorescence" you want, given changing the chromophore by changing amino acids 65, 66 67 or some combo of the three...
  2. You can link green fluorescent protein to other molecules....
  3. Therefore you can link green fluorescent protein variant that emits color x,y,z, etc. to any molecule...
  4. Build an detector that looks for molecule via "colored" light, and now you can do some targeted science...
A simple example is DNA sequencers. DNA is made up of A,C,T,G base pairs. Tag each molecule with blue, green, red, or yellow, and then you can determine the sequence of any DNA molecule based upon the color pattern. Link GFP to a drug that targets cancer cells, and then inject drug into biopsy matter. If drug lights up in cancerous cells, but not normal cells, bingo. The drug might or might not work, but at least you know the drug is targeting the bad cells and not the good cells. The applications are limitless.

The Nobel Prize and Basic Research So maybe my talk on ACCCT wasn't earth shattering, but compared to Mr. Yang and the others after me, it made good sense why GFP's structure was a huge deal. BTW, the "discovery" by Mr. Yang at Rice University in 1996 probably generates that university tons in royalty streams from all the chemistry kits/tools that license the technology for use.

This is a classic example of why basic research is so critical, and maybe why pharma has lost its way. Seriously, a green glowing jellyfish might someday lead to a cure for cancer. Think about that, that's awesome, and certainly worth the Nobel Prize.

1 Comment:

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