Prediction of two- and three-amino-acid sequences of Citrobacter Freundii beta-lactamase from its amino acid composition.

The repeated amino-acid sequences in Citrobacter Freundii beta-lactamase may be indispensable for its function, because such repetitions cannot be simply attributed to a chance. In order to fully explore the functional units in Citrobacter Freundii beta-lactamase, it may need to analyse all the amino acid pairs, triplets, etc. along Citrobacter Freundii beta-lactamase from one terminal to the other terminal, to count their frequencies and calculate their probabilities. The amino-acid sequence of Citrobacter Freundii beta-lactamase was counted according to two-, three- and four-amino-acid sequences. The counted frequency and probability were compared with the predicted frequency and probability. The amino acid sequences, which appear in Citrobacter Freundii beta-lactamase and can be predicted from its amino acid composition according to a purely random mechanism, should not be deliberately evolved and conserved. By contrast, the amino acid sequences, which appear in Citrobacter Freundii beta-lactamase but cannot be predicted from its amino acid composition according to a purely random mechanism, should be deliberately evolved and conversed. Accordingly 99 (26.053%) and 33 (8.684%) of 380 two-amino-acid sequences can be predicted by the frequency and probability according to a purely random mechanism. Some kinds of amino acid sequences, which absent in Citrobacter Freundii beta-lactamase and can be predicted from its amino acid composition according to a purely random mechanism, should not be deliberately excluded from Citrobacter Freundii beta-lactamase. By contrast, some kinds of amino acid sequences, which absent in Citrobacter Freundii beta-lactamase and cannot be predicted from its amino acid composition according to a purely random mechanism, should be deliberately excluded from Citrobacter Freundii beta-lactamase. Accordingly 89 (48.370%) and 41 (22.283%) of 184 kinds of absent two-amino-acid sequences can be predicted by the frequency and probability according to a purely random mechanism, and 7236 (99.848%) of 7247 kinds of absent three-amino-acid sequences can be predicted by the frequency according to a purely random mechanism. The amino acids, whose probabilities in following certain preceding amino acids can be predicted from Citrobacter Freundii beta-lactamase amino acid composition according to a purely random mechanism, should not be deliberately evolved and conversed, accordingly 2 (0.526%) of 380 counted first order Markov transition probabilities for the second amino acid in two-amino-acid sequences match the predicted conditional probabilities.