Bach Robert D, Dmitrenko Olga
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA.
J Am Chem Soc. 2004 Jan 14;126(1):127-42. doi: 10.1021/ja036310+.
A model C-(4a)-flavinhydroperoxide (FlHOOH) is described that contains the tricyclic isoalloxazine moiety, the C-4a-hydroperoxide functionality, and a beta-hydroxyethyl group to model the effect of the 2'-OH group of the ribityl side chain of native FADHOOH. The electronic structures of this reduced flavin (H(3)()Fl(red)()), its N1 anion (H(2)()Fl(red)()(-)()), oxidized flavin (HFl(ox)()), and FlHOOH have been fully optimized at the B3LYP/ 6-31+G(d,p) level of theory. This model C-4a-flavinhydroperoxide is used to describe the transition state for the key step in the paradigm aromatic hydroxylase, p-hydroxybenzoate hydroxylase (PHBH): the oxidation of p-hydroxybenzoate (p-OHB). The Tyrosine-201 residue in PHBH is modeled by phenol, and Arginine-214 is modeled by guanidine. Electrophilic aromatic substitution proceeds by an S(N)2-like attack of the aromatic sextet of p-OHB phenolate anion on the distal oxygen of FlHOOH 3. The transition structure for oxygen atom transfer is fully optimized [B3LYP/6-31+G(d,p)] and has a classical activation barrier of 24.9 kcal/mol. These data suggest that the role of the Tyr-201 is to orient the p-OHB substrate and to properly align it for the oxygen transfer step. Although the negatively charged phenolate oxygen does activate the C-3 carbon of p-OHB phenolate anion toward oxidation relative to ortho oxidation of the carboxylate anion, it appears that H-bonding the Tyr-201 residue to this phenolic oxygen stabilizes both the ground state (GS) and the transition state (TS) approximately equally and therefore plays only a minor role, if any, in lowering the activation barrier. Complexation of p-OHB with guanidine has only a modest effect upon the oxidation barriers. When the complex is in the form of a salt-bridge (10a), the barrier is only slightly reduced. When the TSs are placed in THF solvent (COSMO) with full geometry optimization, salt-bridge TS-A is slightly favored (DeltaDeltaE() = 2.3 kcal/mol).
描述了一种C-(4a)-黄素氢过氧化物（FlHOOH）模型，其包含三环异咯嗪部分、C-4a-氢过氧化物官能团和一个β-羟乙基，以模拟天然FADHOOH核糖醇侧链2'-OH基团的作用。这种还原型黄素（H₃OFl(red)）、其N1阴离子（H₂OFl(red)(-)）、氧化型黄素（HFl(ox)）和FlHOOH的电子结构已在B3LYP/6-31+G(d,p)理论水平上进行了完全优化。该模型C-4a-黄素氢过氧化物用于描述典型芳香羟化酶对羟基苯甲酸羟化酶（PHBH）关键步骤的过渡态：对羟基苯甲酸（p-OHB）的氧化。PHBH中的酪氨酸-201残基由苯酚模拟，精氨酸-214由胍模拟。亲电芳香取代通过p-OHB酚盐阴离子的芳香六隅体对FlHOOH 3的远端氧进行类似SN2的攻击来进行。氧原子转移的过渡结构已完全优化[B3LYP/6-31+G(d,p)]，具有24.9 kcal/mol的经典活化能垒。这些数据表明，酪氨酸-201的作用是使p-OHB底物定向并使其在氧转移步骤中正确对齐。尽管带负电荷的酚盐氧相对于羧酸盐阴离子的邻位氧化确实使p-OHB酚盐阴离子的C-3碳更易于氧化，但酪氨酸-201残基与该酚氧形成的氢键似乎对基态（GS）和过渡态（TS）的稳定作用大致相同，因此在降低活化能垒方面即使有作用也很小。p-OHB与胍的络合对氧化能垒只有适度影响。当络合物为盐桥形式（10a）时，能垒仅略有降低。当将过渡态置于四氢呋喃溶剂（COSMO）中并进行完全几何优化时，盐桥过渡态-A略占优势（ΔΔE = 2.3 kcal/mol）。
