Xu Changliang, Thomas G Spiro
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.
J Biol Inorg Chem. 2008 May;13(4):613-21. doi: 10.1007/s00775-008-0349-8.
Resonance Raman studies have uncovered puzzling complexities in the structures of NO adducts of heme proteins. Although CO adducts of heme proteins obey well-behaved anti-correlations between Fe-C and C-O stretching frequencies, which reflect changes in backbonding induced by distal H-bonding residues, the corresponding NO data are scattered. This scatter can be traced to distal influences, since protein-free NO-hemes do show well-behaved anti-correlations. Why do distal effects produce irregularities in vFeN/ vNO plots but not in vFeC/vCO plots? We show via density functional theory (DFT) computations on model systems that the response to distal H-bonding differs markedly when the NO acceptor atom is N versus O. Backbonding is augmented by H-bonding to O, but the effect of H-bonding to N is to weaken both N-O and N-Fe bonds. The resulting downward deviation from the vFeN/vNO backbonding line increases with increasing H-bond strength. This effect explains the deviations observed for a series of myoglobin variants, in which the strength of distal H-bonding is modulated by distal pocket residue substitutions. Most of the data follow a positive vFeN/vNO correlation with the same slope as that calculated for H-bonding to N. Such deviations are not observed for CO adducts, because the CO pi* orbital is unoccupied, and serves as a delocalized acceptor of H-bonds. H-bonding to N primes NO-heme for reduction to the HNO adduct, a putative intermediate in NO-reducing enzymes.
共振拉曼研究揭示了血红素蛋白的NO加合物结构中令人困惑的复杂性。尽管血红素蛋白的CO加合物在Fe-C和C-O伸缩频率之间呈现出良好的反相关关系，这反映了由远端氢键残基引起的反馈键变化，但相应的NO数据却很分散。这种分散可以追溯到远端影响，因为无蛋白的NO-血红素确实呈现出良好的反相关关系。为什么远端效应在vFeN/vNO图中产生不规则性，而在vFeC/vCO图中却没有？我们通过对模型系统的密度泛函理论（DFT）计算表明，当NO受体原子为N和O时，对远端氢键的响应有显著差异。与O形成氢键会增强反馈键，但与N形成氢键的效果是削弱N-O和N-Fe键。由此导致的与vFeN/vNO反馈键线的向下偏差随着氢键强度的增加而增大。这种效应解释了在一系列肌红蛋白变体中观察到的偏差，其中远端口袋残基取代调节了远端氢键的强度。大多数数据呈现出正的vFeN/vNO相关性，斜率与计算得到的与N形成氢键的斜率相同。对于CO加合物没有观察到这种偏差，因为CO的π*轨道未被占据，并且作为氢键的离域受体。与N形成氢键使NO-血红素易于还原为HNO加合物，这是NO还原酶中的一个假定中间体。
