Antiparallel model complexes

Oarter has reviewed the comparative crystallography of oxidized and reduced C. vinosum HiPIP (1), and the dimensional changes of the iron-sulfur cube following oxidation or reduction have also been extensively tabulated and discussed for both model complexes and protein-bound clusters (118). In spite of the low sequence homology in HiPIPs, there is a remarkable similarity in tertiary structure, especially around the cluster (114). No significant secondary structure is observed in the HiPIPs, with only two short a-helical segments, three strands of antiparallel /3-pleated sheet, and one small helix near the N terminus (Fig. 1). The 4Fe-4S cluster is buried in the protein interior and is inaccessible to solvent (Fig. 2). This feature has been pro-... 

Further refinement of the electron crystallographic structure of tubulin-paclitaxel at 3.5A resolution delivered a similar result. Nevertheless, the T-Taxol model has not been completely accepted as the actual bioactive conformation [78], It is evident that the low 3.5-3.7A resolution of the complex limits the precision of the resulting model. In addition, the Zn2+-stabilized tubulin preparation employed in the electron crystallographic study involves antiparallel protofilaments organized in sheets, which strongly differ from genuine microtubules. Consequently, concern has been expressed that the sheets may not be representative of cellular microtubules and that sheet-bound paclitaxel geometry may differ from its bioactive form in microtubules. 

Fig. 7. (A) A detailed model of the amino-terminal al(IV) and a2(IV) chains containing the 7 S domain and an adjacent segment of the main triple helix. The terminal sequence of both chains is nonhelical and contains lysines (K), hydroxylysines (K), and cysteines (C) that participate in intra- and intermolecular cross-linking. A helical cross-linking site is located about 30 nm from the amino terminus (N) of the molecule and contains cysteines and a hydroxylysine in the al(IV) chain involved in cross-linking plus a complex carbohydrate attachment site (CHO). The cap site within the main triple helix identifies a series of four triplets containing proline (P) and hydroxyproline (P), a composition that would be expected to form a very stable helical structure, [Data from Glanville et al. (1985) Siebold et al. (1987)]. (B) The antiparallel arrangement of type IV molecules with alignment of cross-linking sites.

Pi-pi complex between a model for a polar component of a herbicide (uracil) and a model for a salt binding site (guanidinium bicarbonate) is shown in Figure 2. The geometry was computed from initio calculations (36). It should be noted that in the lowest energy geometry shown in Figure 2, the dipole moment of uracil is antiparallel to the dipole moment of the salt. 

This being a complex of cellulose II, one would expect these two chains to be antiparallel. We did consider parallel chain models but these always gave Inferior agreement with the intensity data and could be ruled out. 

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