COMPARISON OF PROTEINS FROM THERMOPHILIC AND NONTHERMOPHILIC SOURCES IN TERMS OF STRUCTURAL PARAMETERS INFERRED FROM AMINO ACID COMPOSITION*

Abstract

The amino acid composition of 14 different proteins from thermophilic bacteria were compiled along with the amino acid compositions of 56 corresponding proteins from nonthermophilic sources [plants and animals]. A comparison was made between proteins serving the same catalytic function, and significant differences in composition were noted for those proteins from thermophilic bacteria. No consistent pattern was evident and the differences were often small. The 2 data pools were treated as 2 distinct classes and a thermophilic vs. nonthermophilic comparison of amino acid composition was made using the Student''s t-test. Significant differences in composition were found for Asx (sum of Asp and Asn, if known), Ser and Arg. Both classes of data have similar SD for the mean of any single amino acid, suggesting a similar tolerance of variation in the 2 classes of proteins. This would argue against the hypothesis that thermophiles exhibit a greater frequency of errors in protein synthesis. The amino acid compositions were used to calculate structural parameters (% helis, % beta, % turn, hydrophobicity and melting temperatures) for the 2 classes of proteins. Only the predicted % beta content was significantly lower for proteins of thermophilic origin. No differences in hydorphobicity or predicted melting temperature were observed for the 2 classes of proteins. The hypothesis that while small differences may occur in the amino acid composition of thermophilic proteins, they are quite varied and often are very subtle is supported.