Approaches to enzyme stabilization.

much less stable in both cases (Wiseman et al., 1975). Reduced forms contain Fe", which is a high-spin-state form, and this may explain the lower stability, although there is a possibility of a conformational difference for the Fel* form associated with its mechanistic role in oxygen binding. This is another possible case, therefore, of a mechanistic advantage and stability being mutually exclusive. It is clear that, for a cytochrome P-450 acting on a Type I substrate, the enzyme will be more unstable in use (i.e. in the presence of substrate if no compensating stabilization occurs on its binding) than in storage, because the substrate converts the enzyme into the high-spin form. Type I1 substrates, however, have lower rates, but higher stability in use. Several Type I1 ligands, including enzyme inhibitors, should be tested as stabilizers for storage of these enzymes. Consideration of enzyme mechanism may therefore be of general interest in relation to stability. Induced fit and other likely mechanisms of increasing rates of enzyme reactions may also diminish the stability of the enzyme. Therefore, although the efficiency of the enzyme, expressed as kCat.lK,, may be higher, lowered stability may also result in some cases (especially in highly evolved enzymes) from adverse conformational or electronic changes. These factors, where opposing, must be considered in selecting enzymes-and in the future in designing them!