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== Protein structure ==

Within vertebrates, the similarity of insulins is extremely close. [[Cow|Bovine]] insulin differs from human in only three [[amino acid]] residues, and [[Pig|porcine]] insulin in one. Even insulin from some species of fish is similar enough to human to be clinically effective in humans. Insulin in some invertebrates (eg, the ''Caenorhabditis elegans'' [[nematode]]) is quite close to human insulin, has similar effects inside cells, and is produced very similarly. Insulin has been strongly preserved over evolutionary time, suggesting its centrality in animal metabolic control. The C-peptide of [[proinsulin]] (discussed later), however, differs much more amongst species; it is also a hormone, but a secondary one.