On a whim over last winter break, I decided to google “Top Ten Coolest Proteins” to see what the biological world’s top ten coolest proteins are. Alas, I was a disappointed, because I only got directed a bunch of sites on nutrition. It seems (unbelievably) that no one has compiled such a list. Therefore, I am taking it upon myself to remedy this fact.
Hemoglobin is an iron-containing, oxygen-carrying protein found in the blood of almost all vertebrates. It is incredibly abundant, making up about 97% of dry weight of a red blood cell. Essentially, it is the iron in hemoglobin that is responsible for the redness of blood. The protein itself consists of four polypeptides, each in an alpha helix. Each polypeptide also has a “Heme group”, a complex made of four pyrrule molecules and an iron ion. Hemoglobin carries oxygen when one of the oxygen (in O2, that is) molecules binds to the iron ion. Though there is naturally some variation between the hemoglobins of humans, malfunctions of hemoglobins can be pathogenic. These diseases are called hemoglobinopathies. Sickle-cell is the most famous.
There seems to be protein for every imaginable function in the cell. Perhaps the most obscure protein to make the list, scramblases specialize in flipping phospholipid molecules from one side of the bilayer to the other. The point of this? Keeping the sides of the bilayer neutrally charged. If there are too many positively charged or negatively charged phospholipids on one side of the bilayer, an unintentional voltage may form. Scramblases consist of a beta barrel that surrounds a series of alpha helices called the importin. The importin helps the flipped phospholipids back into the membrane.
Another fairly obscure but interesting protein. This elegant beast of a protein contains a whopping 24 polypeptides. Its primary function is to store iron, which is toxic if floating freely in the blood yet is vital for the functions of other proteins (like hemoglobin.) Those 24 polypeptides form a roughly spherical shape, and the inside is lined with phosphate and hydroxide. These bind to the iron, thus storing it. Ferritin is also interesting from a medical and industrial standpoint. Elevated or diminished levels of ferritin can signal anemia or iron overload disorders. The shells of ferritin also promise to be an ideal place for the development of nanoparticles.
7. Reverse Transcriptase
This is a small protein (just two polypeptides) but it is a sinister protein, the protégé of the RNA-using retroviruses. Reverse transcriptase can take an RNA genome, and transcribe DNA. The host’s own DNA polymerases and integrases will mistake the DNA provirus for a primer and seal into the genome. (Though, to be fair, reverse transcriptase cannot make a provirus all by itself. Ribonucleases and tRNA are also involved.) Reverse transcriptase is notoriously sloppy, and its frequent mistakes lead to frequent mutations among retroviruses. Because the HIV virus uses reverse transcriptase, it is of great interest in the development of anti-viral medicines.
6. Topoisomerases (All Types)
Without topoisomerases, DNA replication would result in a tangled, supercoiled mess. If you unravel a rope, the coil goes down the fibers, getting tighter and tighter until it’s impossible to unravel it any further. This same things happens when DNA unravels to replicate, and topoisomerases relieve the tension. They are large, diverse group (found in both prokaryotes and eukaryotes) and their structures and mechanisms vary (some use ATP, some do not, and the number of polypeptides per protein depends on the type) but all involve cutting through one or two strands of DNA and then passing the whole strand through the cut so as to relieve the tension.
Probably the best known and most frequently used of all restriction enzymes, EcoRI consists of just two identical polypeptides. This protein recognizes the DNA sequence GAATTC, and cleaves, forming two “sticky ends” with four overhanging nucleotides each. In nature, this protein is meant to protect E. coli from viruses, but the sticky ends are extremely useful in genetic engineering. New genes can be engineering to came compatible sticky ends with those produced by EcoRI, and then the two strands of DNA can be sealed with DNA ligase.
Another gigantic complex, chaperonin consists of twenty-one polypeptides that make two proteins that work together. One of these looks like two doughnuts stacked on top of each other, and the other forms a small dome. Together, they act as container and lid. Chaperonins help other proteins fold into the correct shape by swallowing them up inside this container. The inside of the container is hydrophilic until the unfolded polypeptide enters, and then the inside of the container switches to hydrophobic, which makes a perfect environment for protein folding. Chaperonins are not nessicary for protein folding (a protein’s shape depends on its amino acid sequence alone) and they do not make proteins fold any faster, but they do make folding more accurate.
3. The fluorescent proteins (Red, Yellow, Green, Blue and others)
Probably the most familiar of the proteins on this list, Green Fluorescent Protein (and its companion proteins in other colors) is not only commercially available but also familiar to nonscientists. These proteins are used in everything from a familiar classroom-setting vector transformation to cutting-edge research to the charming GloFish. Originally a bluish protein extracted from jellyfish, this beta-barrel, eleven-beta-sheet protein was mutated in a laboratory to increase its fluorescence. Green Fluorescent Protein is mostly commonly used as a visual indicator to see whether or not a genetic transformation has occurred.
Obscure but fascinating enough to deserve such a high spot on the list, the kinesins are a family of motor proteins known for their “walking” motion. Think back to the protein towing a vesicle in the Inner Life of a Cell video. That’s a kinesin. Kinesins consists of two main parts. The first is the head, which contains the kinesin’s mobile “legs” and the other is a stalk, which is long and flexible, for pulling the vesicle. Each step a kinesin takes in powered by an ATP. Kinesins are fairly small, with most having around four polypeptides.
1. Photosystem II
The little protein that powers the world…. It makes plants grow. It contributed to the buildup of fossil fuel. It can extract electrons from water. There was very little contest when it came time to determine the coolest protein. Found in essentially everything that photosynthesizes, this 20-polypedptide, alpha helix-heavy protein remains mysterious. It uses a complex of four manganese atoms and a calcium ion, but the exact structure is still under debate. The exact method by which photosystem II strips electrons from water is also not well understood, though it is known that four electrons are take from two molecules of water each time. As this water-splitting device appears promising with regards to sunlight as a practical source of alternative energy, photosystem II is currently under scrutiny.