Metal Enzymes That Multitask

Here is the heart of a metal-containing enzyme -- see how the two purple irons (marked Fe) are held in place by the network of dashed green bonds. Those iron ions are poised, waiting for a component of DNA (adenine) to bind the enzyme. When the adenine approaches, they will swiftly switch one of its protruding nitrogen atoms for an oxygen, changing the adenine into a hypoxanthine and producing nitrogen for nitrogen-needing processes.

But wait ... there's more. Some researchers (see reference below) had some trouble purifying this enzyme, and when they investigated why, they found out that it's because of an alternate activity; this enzyme does more than one thing with its iron atoms. If peroxide approaches the irons, they can neutralize it into oxygen or water by shuttling electrons around. So this enzyme converts nucleotides, and it also protects against reactive forms of oxygen. This means the enzyme has two names and two jobs: it is not just a "deaminase" (described in the first paragraph), but it's also a "catalase."

It's a useful reminder. When we ask an enzyme what its job is, we shouldn't stop when we get one answer. We should be like that kid in the Coke Zero commercial: "And ... ?" Metal ions in enzyme active sites are powerful enough to do several different things: even in the same enzyme. This is a good example of how the power of metal ions makes enzymes able to do more than we expected. What other hidden jobs are in other enzymes that we don't know yet because we haven't asked the right questions?

This also has important implications for the development of enzymes. If an enzyme can do two things at once, that gives it the freedom to tweak one of its activities while the other one keeps plugging away, earning its worth in the cell. In this way, the second activity might be able to change and do something else entirely. All because of the versatility of the iron ions at the heart of the enzyme.

Source: Kamat et al. (2011). "The catalase activity of diiron adenine deaminase." Protein Science 20:2080—2094.