Molybdenum Nitrogenase Lab Report

Subject Chemistry

Paper No and Title 15. Bioinorganic Chemistry

Module No and Title 35. Molybdenum nitrogenase

Module Tag

TABLE OF CONTENTS

1. Learning Outcomes

2. Introduction

3. Fe protein

4. MoFe-protein

4.1 MoFe-cofactor

4.2 Role of Mo

4.2 P cluster

5. Summary

1. Learning Outcomes

After studying this module, you shall be able to

• Identify the roles of the Fe protein and MgATP hydrolysis.

• Learn the roles of the two metal clusters contained in the MoFe protein in catalysis

• Gain insights from recent success in trapping substrates and inhibitors at the active site metal cluster FeMo-cofactor

• Know the mechanism of N2 reduction catalyzed by nitrogenase

2. Introduction

N-fixing bacteria catalyze the reduction of dinitrogen

It is the site of substrate binding and reduction. A [4Fe-3S] subcluster is connected to a [3Fe-Mo-3S] subcluster by an atom X at one corner and three bridging inorganic sulfides. (R)- Homocitrate is coordinated to the Mo atom through its 2-hydroxy and 2-carboxyl groups. At the centre of FeMo-cofactor, the presence of the light atom (X) was detected in a high-resolution (1.16 Å) structure of the MoFe protein. The electron density for X is mostly consistent with N, C, or O, but efforts to identify this atom have yet to be successful. FeMo-cofactor is anchored to the MoFe protein by α-275Cys to an iron atom at one end and α-442His to the Mo atom at the other end (Figure

The subscript denotes the number of electrons (and protons) added to the MoFe protein (E) (Figure 5, bottom cycle). Thus, MoFe protein proceeds through states from E0 to E8 during N2 fixation before it returns back to the resting state (E0). The 1-electron Fe protein cycle and 8-electron MoFe-protein cycle can be conceived of as interlocking, with the Fe protein cycle (Figure 5, top cycle) driving the MoFe protein (Figure 5, bottom cycle) to successively reduced states. This model for the nitrogenase mechanism depicts several important observations regarding the mechanics of catalysis. For instance, it is known that three or four electrons must accumulate within the MoFe protein before N2 binds (E3 or E4 states). Moreover, a stoichiometric quantity of H2 is evolved, when N2 binds to the MoFe protein. Besides, reducing N2 and protons, nitrogenase has the ability to reduce number of small compounds with double or triple bonds. The reduction of the acetylene (C2H2) to ethylene (C2H4) is the most commonly used method for monitoring nitrogenase activity. Even though both acetylene and N2 have potential to bind to the same site on FeMo-cofactor, it is worth mentioning that acetylene binds to a less-reduced E state (E2) than does N2 (E3, E4). Therefore, when these two compounds are present, acetylene appears to be a non-competitive