This is a ball-and-stick model of ubiquitin, an important protein in many cellular processes (https://pdb101.rcsb.org/motm/60). I have included the PDB file which used to create this model, a complete model (1ubq.stl; including main chain hydrogen bonds), and individual amino acids. The individual amino acids are "pinned" to allow them to be printed separately and assembled. You will need to print one of each amino acid and one set of bonds (ubq_bonds.stl) that are the backbone H-bonds that will help hold the model together.The pins in the amino acids are squared-off to make it easier to get the proper orientation during assembly. For the biochemist/structural biologists, they are pinned at alpha carbons to avoid having multiple holes in a single atom.
You'll need to scale. They print nicely at ~700%
The most difficult part about printing ball-and-stick models is getting the model oriented correctly. Maximizing the number of spheres on the build plate is important. Also, using brims that directly contact the spheres helps significantly in preventing lift-off during printing.
Some cleanup of parts will be necessary after printing to get pins/holes fitting together.