The catalytic metal atoms of cobalt substituted liver alcohol dehydrogenase.

The researchers replaced some metal atoms in a type of enzyme found in horse liver with other metal types, triggering changes in the enzyme’s activity and characteristics. They used this approach to distinguish between catalytic and non-catalytic metal atoms in complex metalloenzymes.

Replacing Zn Atoms with Cobalt and 65Zn

• The study revolves around experiments with horse liver alcohol dehydrogenase (LADH), a type of enzyme. Specifically, the researchers manipulated the metallic elements of the enzyme by swapping the existing zinc (Zn) atoms with either cobalt (Co) or zinc-65 (65Zn), a radioactive isotope of zinc.
• The Zn atoms replaced were classified into two categories based on their roles: catalytic and non-catalytic. Catalytic atoms actively contribute to the chemical reactions facilitated by the enzyme, whereas non-catalytic atoms play a more passive, structural role.

Changes in Enzyme Activity and Spectra

• The replacement of Zn atoms with Co led to visible changes. The modified enzymes displayed unique spectra, which are graphic representations depicting the absorption, emission, or distribution of light.
• When Co was used to replace the Zn atoms that were categorized as “exchanging secondly” or those making secondary contributions to chemical reactions, there were notable alterations in the enzyme’s activity.

Using Imidazole and 1,10-Phenanthroline

• The researchers added two compounds, imidazole and 1,10-phenanthroline (OP), to the mix. These compounds interacted with the modified enzymes, causing additional changes in their spectra. This shows that these molecules influence the behavior of the catalytic cobalt atoms within the enzymes.
• Furthermore, the removal of Co had substantial implications for the enzyme’s behavior with OP. Without Co, the OP’s inhibitory effect on the enzymes, which had originally occurred immediately and reversibly, became time-dependent and irreversible.

Implications and Applications

• Collectively, the findings of these experiments point out the pair of Zn atoms in LADH that exchange in secondary roles as the catalytic ones, revealing the critical role these atoms play in ensuring the enzyme’s activities.
• This method of metal atom replacement thereby provides a new approach to distinguish between the catalytic and non-catalytic metal atoms in metalloenzymes, which are complex proteins that contain metal ions. This could lead to further insight into the workings of such enzymes and potential applications in biochemistry and medicine.