Genetically-engineered bacteria are used to produce insulin in industry, but as far as I know, the bacteria can produce only proinsulin. Why is that? What happens in the human body in order to make functional insulin, that can't happen in some microorganisms? In the body proteases cut some at amino acids, and cleave the polypeptide while other enzymes create disulfide bonds. Why is there no bacteria in the industry that can do the same? It's not possible for some reason or in other words — can such a bacterial strain not be engineered?
Insulin is processed from proinsulin by folding the peptide and forming 3 disulfide bonds before cleaving the peptide in 2 positions, leaving 2 peptide chains held in place by the disulfide bonds. This process is summarized in this image:
This only works because the peptide is packaged in a secretory vesicle during synthesis. This protects the peptide against the reductive environment of the cytoplasm, where glutathione is likely to reduce the disulfide bonds that hold insulin together.
When industrial insulin production moved from extracting the hormone from animal pancreases (pancreai?) to bacterial production of human recombinant insulin, glutathione was likely to cause a problem because simple protein production in bacteria just builds the protein in the cytoplasm, where glutathione is present. It is possible to export the protein into the periplasm during synthesis, similar to how proinsulin is moved into the endoplasmic reticulum during synthesis in human cells, but this can complicate protein purification (larger volumes of media, if protein is inside cells you can spin the cells down and work with a smaller volume).
Note that more recent methods use yeast to make insulin, potentially allowing production of actual insulin instead of proinsulin. And modern purification methods can extract insulin from animal sources that is just as good as recombinant sources.
I know from personal experience that it can be very hard to produce proteins with multiple disulfide bonds in bacteria. We attempted to produce mutants of bee venom PLA2 in E. coli, but the protein was misfolded and wound up in inclusion bodies. We purified the inclusion bodies, unfolded the proteins, then slowly refolded them by dialysing the protein in a glutathione solution. We aimed at making a protein with a free thiol for later chemical modification, but that free thiol just picked up a glutathione we couldn't remove without ruining enzyme activity.
Also please note that I am not fully aware of how insulin is made on the industrial scale and I could be wrong. That's why this was originally a comment.