T.G.V. vs T-G-E
Published on 12/04/2007
Since I published an email address (if you hasn't noticed it, it's right below the recent posts), some people have sent me mails, after April 3, about the French train which smashed the speed record on conventional rails.
In their letters, people asked me if the TGV V150 is faster than the TGE sequence of the Sarco/Endoplasmic Reticulum Ca-ATPase (SERCA1a).
Now, this is obviously the most pointless of all the questions, since the TGV ran on rails among stations in France, while TGE hydrolyzes a mixed anhydride, which results from the transphosphorilation of a molecule of ATP on a molecule of aspartic acid (D351).
Nevertheless, I can't let my readers down so, there we are: here comes the V150, pride of French engineering.
This has two engines, which develop a mind-blowing 25,000 horsepower. As a result this monster does 356 mph! Much more than any F1 car or Bugatti and pretty much exactly the same of a "World War II Spitfire fighter at top speed".
On the other hand, there's a protein. Welcome everyone to this spectacular P-class ATPase: it uses the hydrolysis of ATP to carry two calcium ions from the cytoplasm to the sarcoplasmic reticulum.
Actually, it doesn't hydrolyze ATP but phosphorilates an amino acid (D351), which yields a very negative deltaG. So, to sum up, it's still a proper primary active transport.
In a not-so-recent article, turnover rates of the different types of SERCAs have been determined, as well as those of the different steps.
I'm not interested in sorting out if SERCA is faster than TGV, because this would be even more meaningless (there's not even any similarity in the names), but I want to focus on the said motif.
The picture below shows how the protein changes during a complete catalytic cycle.
Briefly, the phosphorilated protein has the perfect structure to release the ions in the SR. Then, the anhydride is hydrolyzed and the whole cycle starts again.
The TGE is on the A-domain which, ehm..., just rotates on its axis, opening the pore for the ions and brining the three amino acids close to the phosphate.
Predictably, the glutamate is the most important group: using fluoroaluminate and related compounds to simulate the transition state, it has been proved that the acid activates a molecule of water (present in the crystals too) which removes the phosphate.
So, from the beginning of the cycle, the A-domain (and so the TGE motif) rotates anticlockwise of 30°, 110° on the minor axis of this cylindrical subunit, 30° up (towards the anhydride) and, finally, 110° on its minor axis again, to reach its start-position.
As you can see, the turnover rate for the dephosphorilation is 0.003 per second.
So, can we compare a train to a group of amino acids? Well, why not?
