Redundancies in Protein Sequences

Abstract

At one point in the history of protein chemistry it was thought that—because of the vast number of possible sequences—proteins ought to exist in a countless number of forms befitting any conceivable function or structure. In fact, as the number of known amino acid sequences continues to mount, it is becoming abudantly clear that there is a practical limit to the number of types of protein structures that exist in living systems on Earth. Thus, although the number of possible sequences for 20 amino acids arranged randomly in strings of 350 units is a superastronomical 20350, certainly nowhere near that number of protein sequences has ever or will ever exist. Instead, a small number of genetically encoded protein structures has been expanded by the general route of “duplication and modification.” The duplications come in various degrees, from the very short to the supragenic or chromosomal. Postduplication modification mostly takes the form of base substitutions leading to amino acid replacement, and, theoretically, the history of any protein ought to be evident by appropriate comparison of the diverging sequences.