CHAPTER 6: Crystallographic Analyses of the Active Site Chemistry and Oxygen Sensitivity of [NiFe(Se)]-Hydrogenases

[NiFe(Se)]-hydrogenases, well-studied enzymes capable of catalyzing the H₂ ⇔ 2H⁺ + 2e⁻ reaction, constitute one of a few examples of the use of catalytic nickel in biology. These enzymes have attracted considerable attention for the last couple of decades because of their potential biotechnological use in both hydrogen generation and biofuel cells. One major stumbling block for the application of many highly active [NiFe(Se)]-hydrogenases in biotechnology is their catalytic inactivation by molecular oxygen, which operates through the modification of active site cysteine thiols. In this chapter we review these reactions by describing the observed oxygen-induced modifications in several hydrogenase crystal structures solved by others and us. Taking advantage of the availability of diffraction data from the Protein Data Bank, we have revisited some of these structures and found several often overlooked minor – but possibly relevant – features common to the various enzymes. We discuss the nature of these modifications and their relationships with existing spectroscopic data. We also discuss strategies both natural, like in the case of regulatory and O₂-tolerant [NiFe]-hydrogenases, and artificial, like in some of their engineered variants, that these enzymes can use to avoid oxygen-induced damage.