Cleaving the N,N Triple Bond: The Transformation of Dinitrogen to Ammonia by Nitrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 144

4 THE DISTINCT STRUCTURAL AND CATALYTIC FEATURES
OF V-NITROGENASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

4.1 The Structural Features . . . . . ........................................................ 166

4.1.1 The Fe Protein ............................................................... 166

4.1.2 The VFe Protein ............................................................. 167

4.2 The Catalytic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

4.2.1 The Reduction of Dinitrogen . . . . . . . . . . . .................................... 169

4.2.2 The Reduction of Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

5 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

Abstract Biological nitrogen fixation is a natural process that converts atmospheric
nitrogen (N 2 ) to bioavailable ammonia (NH 3 ). This reaction not only plays a key role
in supplying bio-accessible nitrogen to all life forms on Earth, but also embodies the
powerful chemistry of cleaving the inert N,N triple bond under ambient conditions.
The group of enzymes that carry out this reaction are called nitrogenases and
typically consist of two redox active protein components, each containing metal
cluster(s) that are crucial for catalysis. In the past decade, a number of crystal
structures, including several at high resolutions, have been solved. However, the
catalytic mechanism of nitrogenase, namely, how the N,N triple bond is cleaved by
this enzyme under ambient conditions, has remained elusive. Nevertheless, recent
biochemical and spectroscopic studies have led to a better understanding of the
potential intermediates of N 2 reduction by the molybdenum (Mo)-nitrogenase.
In addition, it has been demonstrated that carbon monoxide (CO), which was thought
to be an inhibitor of N 2 reduction, could also be reduced by the vanadium (V)-
nitrogenase to small alkanes and alkenes. This chapter will begin with an introduction
to biological nitrogen fixation and Mo-nitrogenase, continue with a discussion of the
catalytic mechanism of N 2 reduction by Mo-nitrogenase, and conclude with a survey
of the current knowledge of N 2 - and CO-reduction by V-nitrogenase and how
V-nitrogenase compares to its Mo-counterpart in these catalytic activities.

Keywords ammonia ￿ ATP hydrolysis ￿ carbon monoxide ￿ crystallography
￿ dinitrogen ￿ ENDOR spectroscopy ￿ FeMoco ￿ Fe protein ￿ [Fe 4 S 4 ] cluster
￿ MoFe protein ￿ P-cluster ￿ Thorneley-Lowe model ￿ VFe protein

Please cite as: Met. Ions Life Sci . 14 (2014) 147-176

1 Introduction

Biological nitrogen fixation refers to the natural process in which the atmospheric
nitrogen (N 2 ) is converted to a bio-accessible form, ammonia (NH 3 )[ 1 - 16 ].
This process represents a critical step in the global nitrogen cycle. Since nitrogen,