Well, here’s an effect that I wouldn’t have thought of. This new paper investigates the effect that moving through small capillaries has on chemical reactions. Now, many organic chemists will hear that and think “Well, sure, flow chemistry, we know about that”. But that’s not where this is going. They’re talking about biological capillaries, blood vessels, and the fluid dynamics that are encountered there.
The behavior of fluids under various flow conditions is famously complex, at least when you hit the conditions where turbulence starts (which vary with viscosity, flow rate, and more). But in microcapillaries you’re generally looking at laminar flow conditions: the layer closest to the wall is moving the slowest, while the liquid in the very center is moving the fastest. There are shear stresses felt by (for example) blood cells as they navigate the circulatory system (and the vascular endothelial cells lining the walls), and these are picked up by cellular mechanosensors of the general type that one of the recent Nobel prizes was just awarded for. It’s been shown that these pathways seem to have an effect on (for example) the development of atherosclerosis and other aspects of cardiovascular health. Folks who have done cell culture will likely have encountered the weird effects that different sorts of shaking, stirring, and mixing can have on the health of their specimens and (for example) their yields of some desired protein – much of this is surely due to mechanosensing as well.
But these shear forces – one part being pulled one way and one part being pulled in another – go all the way down to protein molecules. To be honest, this shouldn’t come as a surprise, because that’s been shown to be one of the factors in how spider silk is produced, as a concentrated protein solution is forced through the spinnerets. But it is odd (at least to me) to think of a protein molecule being yanked on by the fluid around it, with different subunits being stretched or twisted. That’s exactly what this paper demonstrates, though. They don’t see really large-scale structural changes (as with spider silk), but they do see things like cysteine residues become more or less solvent-exposed under shear conditions.
This was shown with Cys34 of albumin, a well-studied residue on a well-studied protein. The rate of reactive trapping of its SH group (actually, the thiolate) by an electrophilic fluorescent dye increased linearly with increased shear in a series of microfluidic experiments, indicating that the solvent-accessible surface area of the protein was being increased (a result that also showed up in molecular dynamic simulations). Those same calculations suggested no real change for the protein’s lysine residues, though, and that was also confirmed by fluorescent labeling experiments. This would mean that some of the mechanosensing in cells may well be taking place at the level of individual proteins – as a cell is stretched and squished, important residues or protein surfaces may be hidden or revealed by the fluid forces around them.
The authors went on to study reactive Cys residues in a several proteins under these flow conditions, and found (remarkably) the same linear rate increases across seven orders of magnitude. This immediately suggests that chemical biologists and others who want to efficiently label such residues should switch to microfluidic flow conditions! There could be therapeutic implications as well, which might not be caught in assays before dosing in live animals (or humans): the team studied the monoclonal antibody trastuzumab (famous as Herceptin), and found that it can dissociate under reducing conditions as disulfide bonds are broken. But reducing conditions combined with shear increased this rate as well, and the molecular dynamics simulations suggest that the disulfides holding the heavy and light chains together (among others) become more solvent-accessible under shear, exposing them more to the reducing agent. So it might be useful to check therapeutic proteins with crucial disulfides and the like under capillary-shear-like conditions as well.
It’s also worth wondering if some other processes might be affected by these conditions – off the top of my head, I’m wondering about native protein ligation and other protein engineering techniques, about the reaction rates of enzymes whose active sites might become more or less accessible under shear conditions, and about the kinetics of protein-protein complex formation and dissociation. We might be able to understand what’s happening in the bloodstream and in cells, and at the same time we might be able to borrow these rate changes for our own purposes. . .
Note added in preparation: I hope that people will appreciate the effort needed to avoid using a title that puns on the word “shear”. I finally decided to leave that one to the hair salons!
