H-clusters

The Mosshauer data performed on the as-isolated D. vulgaris [Fe] hydrogenase are in agreement with the EPR data, and further support the presence of two ferredoxin-type [4Fe-4S] clusters and that the H-cluster is in the diamagnetic state in the as-purified enzyme (169). 

Tard C, Liu XM, Ibrahim SK, Bruschi M, Gioia LD, Davies SC, Yang X, Wang LS, Sawers G, Pickett CJ. 2005. Synthesis of the H-cluster framework of iron-only hydrogenase. Nature 433 610-613. 

The [FeFe]-hydrogenases have been known since the 1930s [305], and Warburg recognized sulfur-coordinate iron as an essential element of the enzyme [306]. The [FeFe]-hydrogenase from Clostridium pasteurianum accommodates 20 iron atoms organized in one [2Fe-2S] cluster, three [4Fe S] clusters and the so-called H-cluster which is a [4Fe-4S] cluster covalently linked to a dinuclear [FeFe]... 

Figure 5-4. Structure of (a) I e.4S4 ("SCHj l2 unit and (b) hydrogenase-cluster (H-cluster)...

Figure 2 Structures of the actrive sites of metalloenzymes containing metal-sulfur cluster units, (a) Fe only hydrogenase, H-cluster (Hoxfarm) (b) Sulfite reductase (c) NiFe carbon monoxide dehydrogenase, C-cluster and (d) NiFe carbon monoxide dehydrogenase, A-cluster, which functions as acetyl-CoA synthase...

Kneussel, M. and Betz, H. Clustering of inhibitory neurotransmitter receptors at developing postsynaptic sites the membrane activation model. Trends Neurosci. 23 429-435, 2000. 

Within the confidently identified sequences of both proteins, all three of four cysteine residues which are involved in the structural integration of the H-cluster (active center consisting of one [4Fe4S] subcluster and a special [2Fe2S] subcluster) are present (Fig. 2). 

Furthermore, all highly conserved amino acid positions, that are essential for the formation of the H-cluster containing hydrophobic niche or essentially involved in the proton transfer process can be found at their expected position in the sequences of both HydA fragments. 

MORE COMPLEX COFACTORS MoCo, FeMoCo, P-CLUSTERS, H-CLUSTERS AND CuZ... 

More Complex Cofactors MoCo, FeMoCo, P-Clusters, H-Clusters and CuZ.36... 

Uneven open shell Pn clusters are easier to ionize than even closed shell ones and the stability of the closed shell uneven P cluster cations is higher. For very large Pn+ cations with n = 25 + 8x (x — 0.1.2. 8) islands of stability were observed in the time of flight mass spectrum (TOF-MS) obtained by laser ablation of red phosphorus, suggesting that the more stable P clusters have connections with units of eight P atoms [71d]. A lot of effort has been put into the calculation of the most stable Pn+ cation structures. The respective global minimum structures of the more stable uneven P3+, Ps+, P71 and P<)h cluster cations are shown in Figure 2.6-10 [73, 74],... 

Table 8 Fragment-ion abundance ratios from CID of proton-bound [S2-M-H]" " clusters... 

Differently from serine, ESI-MS analysis of homoserine (HSer) solutions reveals an unusually abundant diprotonated homoserine octamers [(HSer)g-2H], but not the expected monoprotonated [(HSer)g H]" one." A 3/1 mixture of L-serine and L-homoserine yields abundant mixed serine octamers with the incorporation of one or two homoserine molecules into the cluster. CID of the isolated [(Ser)6(HSer)2-H] cluster leads to the preferentially loss of two neutral serine molecules. Homoserine is always retained. The ESl-MS spectral patterns of threonine and allothreonine solutions is similar to that of homoserine. A 1/1 mixture of D-serine and D-threonine yields abundant mixed singly- and doubly-charged octamers incorporating from 2 to 6 threonine molecules. Their relative abundance indicates that threonine may incorporate freely into serine clusters because the additional methyl group does not interfere with the bonding of the cluster. 

Fig. 7.7. Comparison of (a) 70 eV El and (b) isobutane-d spectrum of glycerol. Instead of a molecular ion, an [M+H] quasimolecular ion is observed in El mode, too. In addition to [M+H], the Cl spectrum shows few fragment ions and a weak [2M+H] cluster ion signal.

FAB matrix spectra are generally characterized by a series of matrix (Ma) cluster ions accompanied by some more abundant fragment ions in the lower m/z range. In positive-ion FAB, [Ma +H] cluster ions predominate, while [Ma -H] cluster ions are preferably formed in negative-ion FAB (Fig. 9.7). The principal ion series... 

Site-site radial distribution functions for the CNWS system (C carbon P polymer backbones W water H cluster containing hydronium). (Reprinted from K. Malek et al. Journal of Physical Chemistry C 111 (2007) 13627. Copyright 2007, with permission from ACS.)... 

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