Weichsel A, Andersen J F, Champagne D E, Walker F A, Montfort W R
Department of Biochemistry, University of Arizona, Tucson 85721, USA.
Nat Struct Biol. 1998 Apr;5(4):304-9. doi: 10.1038/nsb0498-304.
The nitrophorins are heme-based proteins from the salivary glands of the blood-sucking insect Rhodnius prolixus that deliver nitric oxide gas (NO) to the victim while feeding, resulting in vasodilation and inhibition of platelet aggregation. The nitrophorins also bind tightly to histamine, which is released by the host to induce wound healing. Here we present three crystal structures of nitrophorin 1 (NP1): bound to cyanide, which binds in a manner similar to NO (2.3 A resolution); bound to histamine (2.0 A resolution); and bound to what appears to be NH3 from the crystallization solution (2.0 A resolution). The NP1 structures reveal heme to be sandwiched between strands of a lipocalin-like beta-barrel, and in an arrangement unlike any other gas-transport protein discovered to date. The heme is six-coordinate with a histidine (His 59) on the proximal side, and ligand in a spacious pocket on the distal side. The structures confirm that NO and histamine compete for the same binding pocket and become buried on binding. The dissociation constant for histamine binding was found to be 19 nM, approximately 100-fold lower than that for NO.
亲硝蛋白是吸血昆虫长红猎蝽唾液腺中基于血红素的蛋白质，在进食时将一氧化氮气体（NO）传递给受害者，导致血管舒张并抑制血小板聚集。亲硝蛋白还能紧密结合组胺，组胺由宿主释放以促进伤口愈合。在此，我们展示了亲硝蛋白1（NP1）的三种晶体结构：与氰化物结合（分辨率为2.3 Å），氰化物的结合方式与NO相似；与组胺结合（分辨率为2.0 Å）；以及与结晶溶液中看似NH3的物质结合（分辨率为2.0 Å）。NP1的结构显示血红素夹在类脂钙蛋白β桶的链之间，其排列方式与迄今发现的任何其他气体运输蛋白都不同。血红素在近端与一个组氨酸（His 59）形成六配位，在远端的一个宽敞口袋中有配体。这些结构证实NO和组胺竞争同一个结合口袋，并且在结合时会被埋入其中。发现组胺结合的解离常数为19 nM，大约比NO的解离常数低100倍。
