The 'explanation' in the article is a little thin:

Why is the packing fraction 55% maximum (in globular proteins)? "The answer seems to be that the packing fraction stops increasing when the protein cores jam or rigidify." Ok, so ...

> "That is, the individual amino acids that make up the protein core couldn't compress any further when the protein folded"

So they can't pack any further because they 'jam'? Ohhh, from the abstract of the paper:

> "... However, important developments in the physics of jamming in particulate systems can shed light on the packing of protein cores. ... Then, we develop an all-atom model for proteins and find that, above ∼0.55, protein cores undergo a jamming-like transition"

Possibly this is related to the need for protein cores to remain relatively 'liquid', as enzymes (for example) need to be somewhat flexible when binding/releasing substrates. A fully 'jammed'/packed core would lead to an inflexible structure with lower ability to ... er... move, bind stuff (I'm handwaving here :) )

This is the kind of cool stuff I'm going to miss during the coming dark ages. When ever I read this in a paper:

"The authors acknowledge support from NIH Training Grant No. T32GM145452 (A.T.G., C.S.O., and Z.L.), NIH Training Grant No. T15LM007056-37 (J.A.L.), and the High Performance Computing facilities operated by Yale's Center for Research Computing."

All of these things (the NIH and Yale's Center for Research Computing) relied so heavily on government funding that they are no longer getting, especially if they don't sing ideologically pure songs for dear Leader.

"In living organisms, every protein—a type of biological polymer consisting of hundreds of amino acids—carries out specific functions, such as catalysis, molecule transport, or DNA repair. To perform these functions, they must fold up into specific shapes."

This is not true. There are some proteins that are intrinsically disordered, not folding into any preferred way, but they still perform biological functions.

https://en.wikipedia.org/wiki/Intrinsically_disordered_prote...

for perspective, monodisperse spheres max out at 74% (hexagonal close packing)

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