Mixed-Metal Proteins

Many enzymes perform chemically demanding tasks under very mild conditions of pH and temperature. To achieve sufficient activation multi-centre mixed-metal systems are often employed. Many include iron as one of the metals, such as many oxidases. The different metal centres in mixed-metal proteins may have different roles– substrate binding and electron transfer, for example or else several metals may be required to collectively bind a substrate so as to activate it sufficiently for reaction to occur. Whereas some mixed metal proteins have the metals participating cooperatively at a single active site, others have separate metalloproteins working in concert but playing quite distinct roles. The active site of a nickel-iron hydrogenase from an S-reducing bacterium shown in Figure 8.12 is a typical example of a mixed-metal centre where the metals perform a cooperative function as a single unit. The nitrogenase enzyme is an example of a system where separate metalloproteins play quite distinct roles although within each protein there may be several metals acting coop eratively. The task for nitrogenase is nitrogen fixation which is essential for plant life so it is one of Nature’s fundamental synthetic processes. Nitrogenase produces two ammonium ions from dinitrogen and water in the presence of oxygen at soil temperatures and normal atmospheric pressure– a chemically demanding task. It occurs only in prokaryotic cells (those without nuclear membrane or mitochondria) such as bacteria and blue-green algae. An important nitrogen fixer is rhizobium which invades legume roots. Nitrogenase is a

Figure 8.13

A cartoon representation of nitrogenase in operation, showing the component proteins. The active site of nitrogenase that resides in the Fe Mo protein determined from a crystal structure analysis and which contains two linked distorted M4S4 cubes is also shown.

complex association of three proteins: a Fe M protein (Fe-Mo Fe-V or Fe-Fe) a different Fe protein and a ferredoxin (Figure 8.13). The first two are required in combination to fix nitrogen. Ferredoxin is present as simply a reducing agent, and it can be replaced by other chemical reductants, even simple inorganic reducing agents. The overall reaction is given in (8.4).

N₂ + 16 MgATP + 16 H₂O + 8e → 2 NH₄+H₂ +16 MgADP + 16HPO^-2₄ +6H⁺

(8.4) This reaction is thermodynamically inefficient reflected in the complexity of the enzymic process.

The essential metalloproteins in this complex system contain Fe-S clusters somewhat like those we have met earlier in the ferredoxins. The Fe protein (MW ~60 000) mediates the energy transfer mechanism for the reaction as a specific one-electron donor. A dimer of two identical subunits bridged by one Fe₄S₄ is involved, and it is thought to bind two ATP in order to deliver one electron to the adjacent Fe-M protein. The Fe-M protein is a large protein (MW ~220 000) which exists as a a₂ß₂ tetramer, containing two Mo (or V. or extra Fe), ~32 Fe centres including four Fe4S4 units, and ~32 S2 ions. In recent years after much effort the crystal structure of the active site of a nitrogenase was revealed (Figure 8.13); note the linked cubic M4S4 clusters. It is believed that the dinitrogen binds near the centre of this linked cluster, which is the active site of the system. Nitrogenase is not a specific enzyme. It will reduce other small molecules of similar size and/or shape, as they can also bind to the active site. It promotes six-electron reductions such as its primary target N2 to NH⁴⁺ and HCN to CH₄ + NH⁴⁺, as well as two-electron reductions such as N3 to N²⁺ NH⁴⁺ and C₂H₂ to C₂H₄.

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