Why was the cow insulin an effective substitute for the human insulin?

The insulin of the cows is very similar to the human one, differing in the composition three amino acids.

Explanation:

Insulin is a peptide hormone, with a very complex structure. Its structure is formed by 51 amino acids distributed in two chains, linked by disulfide bonds between the cysteine amino acids. Cow insulin is different in three amino acids from human insulin. This difference is quite significant to produce allergy. To solve the problem, a process known as transpeptidization must be applied, which allows the substitution of the three different amino acids with the amino acids of the human insulin sequence.