Force in physics is very different from the way we think of it

I’m very lucky (and honored) that a friend asked me to read and comment on the galleys of a his book. He’s trying to explain some very advanced physics to laypeople (e.g. me). So he starts with force fields, gravitational, magnetic etc. etc. The physicist’s idea of force is so far from the way we usually think of it. Exert enough force long enough and you get tired, but the gravitational force never does, despite moving planets stars and whole galaxies around.

Then there’s the idea that the force is there all the time whether or not it’s doing something a la Star Wars. Even worse is the fact that force can push things around despite going through empty space where there’s nothing to push on, action at a distance if you will.

You’ve in good company if the idea bothers you. It bothered Isaac Newton who basically invented action at a distance. Here he is in a letter to a friend.

“That gravity should be innate inherent & {essential} to matter so that one body may act upon another at a distance through a vacuum without the mediation of any thing else by & through which their action or force {may} be conveyed from one to another is to me so great an absurdity that I beleive no man who has in philosophical matters any competent faculty of thinking can ever fall into it. “

So physicists invented the ether which was physical, and allowed objects to push each other around by pushing on the ether between them. 

But action at a distance without one atom pushing on the next etc. etc. is exactly what an incredible paper found [ Proc. Natl. Acad. Sci. vol. 117 pp. 25445 – 25454 ’20 ].

Allostery is an abstract concept in protein chemistry, far removed from everyday life. Far removed except if you like to breathe, or have ever used a benzodiazepine (Valium, Librium, Halcion, Ativan, Klonopin, Xanax) for anything. Breathing? Really? Yes — Hemoglobin, the red in red blood cells is really 4 separate proteins bound to each other. Each of the four can bind one oxygen molecule. Binding of oxygen to one of the 4 proteins produces a subtle change in the structure of the other 3, making it easier for another oxygen to bind. This produces another subtle change in structure of the other making it easier for a third oxygen to bind. Etc. 

This is what allostery is, binding of molecule to one part of a protein causing changes in structure all over the protein. 

Neurologists are familiar with the benzodiazepines, using them to stop continuous seizure activity (status epilepticus), treat anxiety (Xanax), or seizures (Klonopin). They all work the same way, binding to a complex of 5 proteins called the GABA receptor, which when it binds Gamma Amino Butyric Acid (GABA) in one place causes negative ions to flow into the neuron, inhibiting it from firing. The benzodiazepines bind to a completely different site, making the receptor more likely to open when it binds GABA. 

The assumption about all allostery is that something binds in one place, pushing the atoms around, which push on other atoms which push on other atoms, until the desired effect is produced. This is the opposite of action at a distance, where an effect is produced without the necessity of physical contact.

The paper studied TetR, a protein containing 203 amino acids. If you’ve ever thought about it, almost all the antibiotics we have come from bacteria, which they use on other bacteria. Since we still have bacteria around, the survivors must have developed a way to resist antibiotics, and they’ve been doing this long before we appeared on the scene. 

TetR helps bacteria resist tetracycline, an antibiotic produced by bacteria. When tetracycline binds to TetR it causes other parts of the protein to change so it binds DNA causing the bacterium, among other things, to make a pump which moves tetracyline out of the cell. Notice that site where tetracycline binds on TetR is not the business end where TetR binds DNA, just as where the benzodiazepines bind the GABA receptor is not where the ion channel is. 

This post is long enough already without describing the cleverness which allowed the authors to do the following. They were able to make TetRs containing every possible mutation of all 203 positions. How many is that — 203 x 19 = 3838 different proteins. Why 19? Because we have 20 amino acids, so there are 19 possible distinct changes at each of the 203 positions in TetR.  

Some of the mutants didn’t bind to DNA, implying they were non-functional. The 3 dimensional structure of TetR is known, and they chose 5 of nonfunctional mutants. Interestingly these were distributed all over the protein. 

Then, for each of the 5 mutants they made another 3838 mutants, to see if a mutation in another position would make the mutant functional again. You can see what a tremendous amount of work this was. 

Here is where it gets really interesting. The restoring mutant (revertants if you want to get fancy) were all over the protein and up to 40 – 50 Angstroms away from the site of the dead mutation. Recall that 1 Angstrom is the size of a hydrogen atom, a turn of the alpha helix is 5.4 Angstroms and contains 3.5 amino acids per turn.The revertant mutants weren’t close to the part of the protein binding tetracycline or the part binding to DNA. 

Even worse the authors couldn’t find a contiguous path of atom pushing atom pushing atom, to explain why TetR was able to bind DNA again. So there you have it — allosteric action at a distance.

There is much more in the paper, but after all the work they did it’s time to let the authors speak for themselves. “Several important insights emerged from these results. First, TetR exhibits a high degree of allosteric plasticity evidenced by the ease of disrupting and restoring function through several mutational paths. This suggests the functional landscape of al- lostery is dense with fitness peaks, unlike binding or catalysis where fitness peaks are sparse. Second, allosterically coupled residues may not lie along the shortest path linking allosteric and active sites but can occur over long distances “

But there is still more to think about, particularly for drug development. Normally, in developing a drug for X, we have a particular site on a particular protein as a target, say the site on a neurotransmitter receptor where a neurotransmitter binds. But the work shows that sites far removed from the actual target might have the same effect

Yet another mechanism of gene regulation

A snippet of RNA from an intron in a gene can bind to an upstream regulatory element forming a triple helix and shut off transcription of the gene.  Rather amazing don’t you think?  Yet exactly was found in a far from obscure gene, the beta globin gene of hemoglobin on chromosome #11 [ Proc. Natl. Acad. Sci. vol. 116 pp. 6130 – 6139 ’19 ].

We’re talking large segments of DNA.  There are five genes for the beta subunit of hemoglobin located from 5′ to 3′ as epsilon, gammaG, gammaA, delta and beta.  The first 4 are expressed during fetal development.  Beta globin is the one found in our red blood cells.  The regulatory element controlling all 5 is found FIFTY kiloBases upstream from the beginning (5′ end) of beta globin.

The regulatory region is called the locus control region (LCR)and stretches over 20+ kiloBases.  It has 7 sites where transcription factors bind (called hypersensitive sites HS1 — HS7).  The hypersensitivity comes from the fact the chromosome is relative ‘open’ at these places and not compacted, so that an enzyme (DNAase I) can break the chromosome.

So after the beta globin gene is transcribed, the introns are spliced out, and the RNA from the second intron binds to HS2 forming a triple helix and displacing transcription factors bound there (USF2, GATA1, TAL1) which recruit RNA polymerase II (Pol II)  In the normal course of events the whole mess would then march around the genome and eventually hit the promoter of beta globin (at least 50 kiloBases away) and turn on transcription.

This seems to be yet another mechanism of gene regulation.  Just how widespread this is, isn’t known, but most protein coding genes have introns.  Stay tuned.

Molecular biology is fascinating

The uses of disorder

There was a lot of shock and awe about a report showing how seemingly minor changes in an aliphatic group on benzene led to markedly different conformations in its protein target (lysozyme from bacteriophage T4) http://pipeline.corante.com/archives/2015/06/18/tiny_and_not_so_tiny_changes.php.

Our noses are being rubbed in just how floppy proteins are, in contrast to the first glimpses of protein structure obtained by Xray crystallography. Back then we knew so little about proteins, that seeing all the atoms laid out in alpha helices and beta sheets was incredibly compelling. We talked about the structure of a protein rather than a structure. Even back then, with hemoglobin (one of the first solved proteins) it was obvious that proteins had to have more than one structure. The porphyrin ring in heme that oxygen binds to is buried deep in hemoglobin, and the initial structure had to move in some way to allow oxygen to find its way in (because the initial structure showed no obvious channel for oxygen). So hemoglobin had to breathe.

We now know that many proteins have intrinsically disordered segments. Amazingly, the most recent estimate I could find in my notes (or in Wikipedia) is this — It is estimated that over 30% of eukaryotic proteins have stretches of over 30 amino acids that are intrinsically disordered [ J. Mol. Biol. vol. 337 pp. 635 – 645 ’04 ]. Does anyone out there know of more recent data?

We’re a lot smarter now — here’s a comment on Derek’s post — “I have always thought crystal structures of proteins/enzymes are more a guide than actually useful. You are crystallizing a protein first-proteins don’t pack like that in vivo. Then you are settling on the conformation that freezes out- is this the lowest energy form? Then you are ignoring hte fact that these are highly dynamic structures that are constantly moving, sliding, shaking, adjusting. Then if you put a ligand in there you get the lowest energy form-which is what it would look like after reaction and before ligand dissociation- this is quite different from what it can look like at other stages of the reaction.”

Here is an interesting example of the uses of protein disorder going on right now in just about every neuron in your body. Most neurons have long processes, far too long for diffusion to move a needed protein to their ends. For that purpose we have microtubules (aka neurotubules in neurons) stretching the length of the processes, onto which two types of motors attach (dyneins which moves things to negative end of the microtubule and kinesins which move things to the positive end).

The microtubule is built from a heterodimer of two proteins (alpha and beta tubulin). Each contains about 450 amino acids and forms a globule 40 Angstroms (4 nanoMeters) in diameter. The heterodimers pack end to end to form a protofilament. 13 protofilaments line up side by side to form the microtubule, a hollow structure about 250 Angstroms in diameter. In cells microtubules are 1 to 10 microns long, but in nerve process they can be ‘up to’ 100 microns in length. Even at 1 micron (1,000 nanoMeters) that’s 13 * 250 heterodimers in a microtubule.

Any protein structure this important has a lot of modifications imposed on it to alter structure and function. Examples include phosphorylation and the addition of glutamic acid chains (polyglutamylation). The carboxy terminal tails of alpha and beta tubulin are flexible and stick out from the tubulin rod (which is why they aren’t seen on Xray crystallography). The carboxy terminal tail is the site of post-translational glutamylation. The enzyme polyglutamylating the carboxy terminal tail of beta tubular is TTLL7 (you don’t want to know what the acronym stands for). It binds to the alpha/beta tubular heterodimer by an intrinsically disordered region of its own (becoming structured in the process), then it binds to the intrinsically disordered carboxyl terminal tails, structuring them and modifying them. It’s basically a mating dance. There is a precedent for this — see https://luysii.wordpress.com/2013/12/29/the-mating-dance-of-a-promiscuous-protein/

So disordered regions of proteins although structureless are far from functionless