Stich Troy A, Yamanishi Mamoru, Banerjee Ruma, Brunold Thomas C
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Am Chem Soc. 2005 Jun 1;127(21):7660-1. doi: 10.1021/ja050546r.
The human adenosyltransferase hATR converts exogenous cobalamin into coenzyme B12 by transferring the adenosyl group from cosubstrate ATP to a transiently formed Co1+cobalamin (Co1+Cbl) species. A particularly puzzling aspect of hATR function is that the midpoint potential for Co2+Cbl --> Co1+Cbl reduction is below that of readily available biological reductants. Our magnetic circular dichroism and electron paramagnetic resonance spectroscopic studies reported here reveal that, in the absence of ATP, the interaction between Co2+Cbl and hATR promotes partial conversion of the cofactor to its "base-off" form in which a water molecule occupies the lower axial position. This interaction becomes much stronger in the presence of ATP, leading to the formation of an unprecedented Co2+Cbl species with spectroscopic signatures consistent with an essentially four-coordinate, square-planar Co2+ center. This unusual Co2+Cbl coordination is expected to raise the Co2+/1+ reduction potential well into the physiological range.
人类腺苷转移酶hATR通过将辅底物ATP上的腺苷基团转移到瞬时形成的Co1+钴胺素（Co1+Cbl）物种上，将外源性钴胺素转化为辅酶B12。hATR功能中一个特别令人困惑的方面是，Co2+Cbl→Co1+Cbl还原的中点电位低于易于获得的生物还原剂的中点电位。我们在此报告的磁圆二色性和电子顺磁共振光谱研究表明，在没有ATP的情况下，Co2+Cbl与hATR之间的相互作用促进了辅因子部分转化为其“碱基脱离”形式，其中一个水分子占据较低的轴向位置。在ATP存在的情况下，这种相互作用变得更强，导致形成一种前所未有的Co2+Cbl物种，其光谱特征与基本上四配位的平面正方形Co2+中心一致。这种不寻常的Co2+Cbl配位预计会将Co2+/1+还原电位提高到生理范围内。
