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The relevance of the interaction of the C-terminal [alpha]-helix (helix 5) of Escherichia coli thioredoxin (TRX) on the folding mechanism of TRX is evaluated by characterizing TRX1-93, a truncated variant of TRX devoid of the last stretch of 15 amino acid residues that includes helix 5. The results have shown that the binding of the C-terminal helix might highlight a late event in the consolidation of the overall TRX fold.

The importance of interactions involving residues L94, L99, E101, F102, L103 and L107 on the formation of the noncovalent complex between reduced thioredoxins (Trx1-93) and TRX94-108 is described. Specific interactions between peptide TRX94-108 and partially folded TRX1-93 have played a major role to stabilize the native-like tertiary structure of the complex.

The effects of C-terminal truncation on the equilibrium folding transitions and folding kinetics of B. licheniformis exo small Beta-lactamase (ES-BetaL) are measured. ES-betaL lacking 19 residues (ES-betaL (sub C delta 19)) have no enzymic activity.