Cold inactivation of L-threonine deaminase from Rhodospirillum rubrum. Involvement of hydrophobic interactions.

Threonine deaminase from Rhodospirillum rubrum is subject to reversible cold inactivation a t low protein concentrations. The inactivation appears to involve a dissociation of the native tetrameric protein to dimers. The standard free energy of dissociation a t 0 "C was + 7.8 kcal/mole, while A H was 3.2 kcal/mole and AX was 41 entropy units. These values suggest that hydrophobic interactions are important in the maintenance of the tetrameric structure. The labilization of the protein by low concentrations of urea and the protection by high salt concentrations also indicate the importance of hydrophobic bonds in the protein. However, the effects of pH and low salt concentrations on the cold inactivation suggest that ionic bonds do play some role in subunit interactions. The normal allosteric modifier for threonine deaminase, L-isoleucine, protected the enzyme against cold inactivation. Although the R. rubrum enzyme is virtually insensitive to isoleucine inhibition, it is clear that this ligand can induce profound changes in the protein structure.