Narváez A J, Kálmán L, LoBrutto R, Allen J P, Williams J C
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA.
Biochemistry. 2002 Dec 24;41(51):15253-8. doi: 10.1021/bi0264566.
The influence of the local environment on the formation of a tyrosyl radical was investigated in modified photosynthetic reaction centers from Rhodobacter sphaeroides. The reaction centers contain a tyrosine residue placed approximately 10 A from a highly oxidizing bacteriochlorophyll dimer. Measurements by both optical and electron paramagnetic resonance spectroscopy revealed spectral features that are assigned as arising primarily from an oxidized bacteriochlorophyll dimer at low pH values and from a tyrosyl radical at high pH values, with a well-defined transition that occurred with a pK(a) of 6.9. A model based on the wild-type structure indicated that the Tyr at M164 is likely to form a hydrogen bond with His M193 and to interact weakly with Glu M173. Substitution of Tyr or Glu for His at M193 increased the pK(a) for the transition from 6.9 to 8.9, while substitution of Gln for His M193 resulted in a higher pK(a) value. Substitution of Glu M173 with Gln resulted in loss of the partial formation of the tyrosyl that occurs in the other mutants at low pH values. The results are interpreted in terms of the ability of the residues to act as proton acceptors for the oxidized tyrosine, with the pK(a) values reflecting those of either the putative proton acceptor or the tyrosine, in accord with general models of amino acid radicals.
在球形红细菌的修饰光合反应中心中，研究了局部环境对酪氨酸自由基形成的影响。这些反应中心含有一个酪氨酸残基，它距离一个高氧化性的细菌叶绿素二聚体约10埃。通过光学和电子顺磁共振光谱测量揭示了光谱特征，这些特征主要被归因于低pH值下氧化的细菌叶绿素二聚体以及高pH值下的酪氨酸自由基，在pK(a)为6.9时发生了明确的转变。基于野生型结构的模型表明，M164位的酪氨酸可能与M193位的组氨酸形成氢键，并与M173位的谷氨酸弱相互作用。在M193位用酪氨酸或谷氨酸取代组氨酸，使转变的pK(a)从6.9增加到8.9，而用谷氨酰胺取代M193位的组氨酸则导致更高的pK(a)值。用谷氨酰胺取代M173位的谷氨酸导致在低pH值下其他突变体中发生的酪氨酸部分形成的丧失。根据这些残基作为氧化酪氨酸的质子受体的能力来解释结果，pK(a)值反映了假定的质子受体或酪氨酸的pK(a)值，这与氨基酸自由基的一般模型一致。
