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B. Protein <strong>Structure</strong><br />

Page 467<br />

The 6 β sheets of the subunit, although topologically identical, vary in size and detailed structure.<br />

However, the six sheets can be considered as maintaining an approximate 6-fold symmetry relationship<br />

to each other about an axis parallel to the mean direction of the first strand of each sheet and through the<br />

centroid of these directions. Four disulfide bridges are formed between adjacent sheets as well as two<br />

between the sheets, which distort the sheet structure. Similar pairing has now been found in one of the<br />

domains of CD4 [57]. Also short β bulges occur on the inner and outer strands of most of the sheets. The<br />

packing of the β sheets is stabilized <strong>by</strong> hydrophobic interactions at the sheet interfaces and hydrogen<br />

bonding at the periphery of the sheets. In addition the intersheet disulfide bonds and ion pairs stabilize<br />

the structure.<br />

In general the loops connectioning the β sheets are shorter and tighter on the bottom of the subunit<br />

compared to the outer loops, which tends to be more elaborate. The intersheet loop connecting the fourth<br />

and fifth loop is the most extensive in the structure and is stabilized primarily <strong>by</strong> a disulfide bridge<br />

between Cys318 (sequence numbering of A/Tokyo/3/67 will be used throughout) and Cys337, a<br />

conserved ion pair Asp330—Arg364, and a putative Ca 2+ ion binding site. This calcium binding site has<br />

approximate octahedral coordination with the main-chain oxygens of residues 293, 297, 345, and 348, a<br />

carboxylate oxygen of Asp324, and a water molecule in N2 and type B neuraminidases. In N9, the mainchain<br />

oxygen at 343 is replaced <strong>by</strong> a water molecule. Calcium has been shown to be necessary for<br />

neuraminidase activity [58] and this site is connected to the active site via conserved residues; however,<br />

the functional role of calcium in the structure is unknown, although it has been shown that calcium is<br />

essential for the thermostability of the molecule [59].<br />

C. Carbohydrate <strong>Structure</strong><br />

Carbohydrate at four N-linked glycosylation sites were observed in N2 neuraminidase at residues 86,<br />

146, 200, and 234 in the x-ray structure. Two N-acetylglucosamines were resolved at Asp86 and<br />

Asp234, both at the bottom surface of the monomer. The carbohydrate at Asp200 consists of eight sugar<br />

residues with linkages consistent with known mannose-rich simple N-linked carbohydrates [60]. This<br />

oligosaccharide emerges from the side of the monomer and covers a neighboring subunit (see Figure 4).<br />

The oligosaccharide site at Asn146 is the most conserved of all neuraminidase glucosylation sites,<br />

except that of the neurovirulent virus A/WSN/33 [61]. The absence of this glycosylation site in<br />

A/WSN/33 has been shown to confer neurovirulence in mice [62]. It is a complex sugar containing Nacetylgalactosamine<br />

[63] that is not found in any other of the known oligosaccharides of influenza virus<br />

glycoproteins and is the only glycopeptide antigenically related to chick embryo “host antigen”<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_467.html [4/9/2004 12:16:34 AM]

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