Electrocatalytic Studies of Sulfite Reduction by Heme Enzymes

Electron-transfer reactions play an important role in many biological systems. Nitrite reductase (Nir) and sulfite reductase (Sir) are widespread classes of heme enzymes, which perform integral steps in the biological cycling of nitrogen and sulfur. The overall assimilatory reduction process involves the 6-electron reduction from nitrite (bisulfite) to ammonium (hydrogen sulfide). In spite of the importance of these enzymes, there is little direct evidence for the steps in the reduction mechanism. Electrochemistry is a good tool to study the redox mechanism of proteins. However, it is difficult to obtain direct reduction of these enzymes( relatively large molecules) on the electrode. Direct electrochemical reduction of some smaller heme proteins, such as myoglobin and microperoxidase (MP-11 ), have been demonstrated on some electrodes. The prosthetic groups in the sulfite reductase are the same as in plant nitrite reductases (siroheme and 4Fe-4S cluster), and the enzymes can reduce nitrite as well as bisulfite. Because the active site of these enzymes (Nir and Sir) and hemoproteins are the same(heme ), investigating the electrocatalytic behavior of these hemoproteins (MP-11 and myoglobin) with nitrite (sulfite) can help us to get some understanding of the reduction mechanism of Nir and Sir