Obligately Tungsten-Dependent Enzymes─Catalytic Mechanisms, Models and Applications

Tungsten-dependent enzymes incorporate a tungsten ion into their active site in the form of a complex with two pyranometallopterin (MPT) molecules, also known as tungsten cofactor (W-co). W-co-containing enzymes are found in several bacteria and archaea, predominantly in enzymes involved in anaerobic metabolism. While some enzymes occur with either molybdenum or tungsten in their active sites, we concentrate here on enzymes obligately depending on W-co, which are not functional as isoenzymes with Mo-co. These are represented by several subtypes of aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductase (BCRs) and acetylene hydratase (AHs). They catalyze either low-potential redox reactions or the unusual hydration reaction of acetylene. In this review, we analyze the catalytic and structural properties of these enzymes and focus on various mechanistic hypotheses proposed to describe their catalytic action, including hypothetical mechanistic patterns common to all of these enzymes. The biochemical characterization of the enzymes is supported by studies with functional inorganic models that help in the elucidation of their spectroscopic and catalytic features. Finally, we discuss a range of ongoing biotechnological applications utilizing obligately tungsten-dependent enzymes in producing value-added chemicals, indicating the expected advantages of incorporating these enzymes into biotechnological processes despite their intrinsic oxygen-sensitivity and the requirement of special recombinant expression platforms.