P-cluster pair

Fig. 6. View of the nitrogenase MoFe protein P-cluster pair where ( ) represents Fe, (O) S, and (Q) C as modeled (153). The side chain of one of the...

The VFe protein also has the equivalent of P-cluster pairs which have similar properties to those found in the MoFe protein (159). No information is available on whether P-cluster pairs exist in the FeFe protein, but because of the relatively high sequence identity and the similar genetic basis of its biosynthesis, the occurrence seems highly likely. The catalytic role assigned to the P-cluster pair involves accepting electrons from the Fe protein for storage and future deUvery to the substrate via the FeMo-cofactor centers. As of this writing (ca early 1995), this role has yet to be proved. 

P-cluster pairs in, 17 307-308 structure of, 17 307-310 Mo-Fe-S clusters, 17 315 Mohavite, 4 133t Mohs hardness scale, 1 3-4 selected materials, l 3t Moire interferometry equations in, 17 426-428 experimental setup for, 17 428-429 in nondestructive evaluation,... 

P-cluster pair role, 40 108-111 physiological pathway, flavocytochrome f 2> 36 275-276... 

The MoFe-protein is an a2 2 tetramer (with the subunits coded by the nifD and nifK genes, respectively), with a total molecular weight of 240,000. The two subunits are of similar size for example, the isolated a and /3 subunits of A. vinelandii MoFe-protein have 491 and 522 amino acids, respectively (46). In general, the amino acid sequences of MoFe-proteins are less well conserved than are Fe-protein sequences, so that the MoFe-protein sequences from A. vinelandii and C. pasteuria-num are only 36% identical (47). Associated with the MoFe-protein tetramer are approximately 2 molybdenum atoms, 30 iron atoms, and 30 sulfur atoms that are organized into two types of metal centers the FeMo-cofactor and the P-cluster pair. The structures and properties of these centers have been extensively probed by a wide variety of techniques. 

Fig. 3. Oxidation-reduction properties of the P-cluster pair. P represents the form of the P-cluster pair found in dithionite isolated MoFe-protein, whereas P represents more oxidized forms of the center (65).

Recent EPR and ENDOR studies on the binding of manganese substituted adenosine nucleotides have been interpreted to suggest that these nucleotides may bind near the P-cluster pair (69). 

The P-cluster pair is formed by two 4Fe 4S clusters that are bridged by two cysteine thiol ligands (residues a88 and j895), which are further connected by a disulfide bond between two of the cluster sulfurs (Fig. 7) (16-18). The disulfide bond is particularly intriguing and it is located on the side of the P-cluster pair closest to the proposed (27) binding... 

As had been proposed from sequence comparisons and mutagenesis experiments (73, 74, 76), the P-cluster pair is located at the interface between the a and the f3 subunits. The approximate twofold S)mimetry... 

Fig. 9. A proposed P-cluster pair cycle (P-cycle) illustrating a possible sequence of electron transfer events at the P-cluster pair during substrate reduction by nitrogenase. See text for details.

The preceding discussion has been based on the assumption that the P-cluster pair functions as an intermediate electron carrier between... 

Fig. 5. Ribbon diagram (81) of the polypeptide chain fold of the A. vinelandii MoFe protein (17). The two a subunits in the MoFe-protein tetramer are colored red and lavender, while the two )3 subunits are colored dark blue and light blue. The FeMo cofactor and P-cluster pair are represented by atomic models.

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