Q.Describe how Eli Lilly's genetically engineered human insulin, marketed as Humulin, is produced. Explain why the A and B polypeptide chains are produced separately and then joined by disulphide bonds.

[!TLDR] Humulin: the A-chain and B-chain genes are expressed separately in two E. coli cultures, then the two chains are joined by disulphide bonds in the lab to form active insulin.

[!ANSWER] Eli Lilly's Humulin is produced by inserting the DNA sequences for human insulin's A chain and B chain into two separate plasmid vectors, introducing each into its own culture of E. coli, and letting each culture express one chain independently. The two chains are harvested separately and then joined together by forming disulphide bonds between them in the laboratory, producing complete, biologically active human insulin. They are produced separately (rather than attempting the full natural proinsulin-to-insulin maturation pathway in bacteria) because this two-chain approach is simpler to engineer reliably in a bacterial host, which lacks the complex processing machinery of a human pancreatic cell.

Natural human insulin is a small, two-chain protein -- chain A and chain B -- linked together by disulphide bonds, and within the human body it is originally produced as a single, longer precursor molecule called proinsulin, which is then processed (cut) to remove a connecting segment and yield the mature two-chain hormone.

Rather than attempting to replicate this entire natural proinsulin-processing pathway inside a bacterium -- which would require the bacterium to correctly perform the same maturation steps human pancreatic cells naturally carry out -- Eli Lilly's Humulin process instead chemically synthesised the DNA sequences coding for the insulin A chain and the insulin B chain as two separate genes. Each gene was inserted into its own plasmid vector, and each recombinant plasmid was introduced into its own, separate culture of E. coli bacteria. Each bacterial culture then expressed and accumulated only its one assigned chain (either A or B), entirely independently of the other culture.

The two chains, once produced and purified separately from their respective bacterial cultures, were then combined together in the laboratory, where controlled oxidation conditions were used to form the two disulphide bonds that link chain A and chain B together (plus the one internal disulphide bond within chain A itself), assembling them into the complete, correctly folded, biologically active human insulin molecule.

[!ANSWER] The A and B chains are produced separately because engineering two independent, relatively simple single-chain proteins in bacteria is far more straightforward and reliable than trying to reproduce the human pancreas's full proinsulin-processing pathway inside a bacterial cell; the separately produced chains are then chemically joined by disulphide bonds outside the cell to yield finished, active insulin.

Contrast this two-chain approach against the natural human proinsulin pathway to explain WHY the separate-chain method was chosen.

Do not describe Humulin as being produced from a single gene coding for the whole mature two-chain hormone directly -- the A and B chains are engineered and expressed as two SEPARATE gene products, joined only afterward in the lab.