The Evolution of Biochemical Complexity

[Antifreeze glycoproteins, or AFGPs] consist of a repeating three-amino acid sequence consisting of threonine alanine-alanine. With two sugars attached to each threonine, AFGPs are the fish's version of ethylene glycol. But how did such unusual proteins arise? [...] Cheng's group discovered that AFGP genes evolved from an ancestral gene encoding trypsinogen [...] What clinched the story was Cheng's finding that trypsinogen contains a three-amino acid sequence with no known function in the enzyme. You guessed it: threonine alanine-alanine. In constructing AFGP, the tripeptide reiterated again and again, probably because the repetition had antifreeze properties strongly selected by ice cold water. Most of the rest of the trypsinogen gene was discarded. By deleting parts of the trypsinogen gene and recruiting and amplifying others, evolution did its borrowing act.

Barry A. Palevitz, "Missing Links and the Origin of Biochemical Complexity," The Scientist, November 22, 1999.