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In the hyperthermophilic bacterium Thermotoga maritima, the two glycolytic enzymes phosphoglycerate kinase (PGK) and triosephosphate isomerase (TIM) are covalently connected forming a tetrameric single-chain PGK-TIM fusion protein. A frameshift allows the translation of PGK alone, whereas TIM activity exclusively resides in the fusion protein (Schurig et al., 1995). Cloning the pgk-tim gene from Thermotoga maritima in Escherichia coli, yields monomeric PGK and tetrameric PGK-TIM fusion protein as authentic recombinant proteins (Beaucamp et al., 1995). Both exhibit high intrinsic stability. The thermal transitions at approximately 80 degrees C are irreversible, rendering determination of thermodynamic data impossible. The half-concentrations, (cGdmCl)1/2, of the guanidinium-chloride induced unfolding transitions are 3.0 and 3.9 M GdmCl for PGK and the PGK-TIM fusion protein, respectively. Monitoring denaturation by activity, fluorescence emission and circular dichroism, deactivation and unfolding of the two-domain PGK is found to precede the transitions of the TIM domain. With increasing temperature, (cGdmCl)1/2 is shifted to lower denaturant concentrations; at the same time, the transitions change from bimodal to unimodal. As indicated by the incomplete reversibility of the deactivation/unfolding/dissociation transitions, misfolding, as well as wrong domain interactions seem to interfere with the correct folding and association of the bienzyme complex.

The PGK-TIM Fusion Protein from Thermotoga maritima and Its Constituent Parts Are Intrinsically Stable and Fold Independently