Evidence from FTIR difference spectroscopy of an extensive network of hydrogen bonds near the oxygen-evolving Mn(4)Ca cluster of photosystem II involving D1-Glu65, D2-Glu312, and D1-Glu329.
Evidence from FTIR difference spectroscopy of an extensive network of hydrogen bonds near the oxygen-evolving Mn(4)Ca cluster of photosystem II involving D1-Glu65, D2-Glu312, and D1-Glu329.
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来自光系统 II 的放氧 Mn(4)Ca 簇附近广泛氢键网络（涉及 D1-Glu65、D2-Glu312 和 D1-Glu329）的 FTIR 差异光谱的证据。
DOI：
10.1021/bi100730d
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发表时间：
2010
期刊：
影响因子：
2.9
通讯作者：
Debus,RichardJ
中科院分区：
文献类型：
--
作者：
Service,RachelJ;Hillier,Warwick;Debus,RichardJ
对光系统II（PSII）在2.9−3.5 nm处的精细X射线晶体结构的分析表明，在水裂解反应期间，存在从催化Mn 4Ca簇中去除质子的可能通道。作为实验验证这些通道的初步尝试，在对羧酸残基的质子化状态敏感的光谱区域中，用FTIR差光谱探测Mn 4Ca簇附近氢键网络的存在，特别是在1747 cm− 1处的负带，这在蓝细菌Synechocystissp的PSII的S2-minus-S1 FTIR差光谱中经常观察到。PCC 6803。根据其在D2 O中的4 cm− 1下移，该谱带被归属于质子化羧酸基团的羰基伸缩振动（C O），其pK在S1到S2跃迁期间降低。Mn 4Ca簇上形成的正电荷在S1到S2过渡期间可能会导致结构扰动，通过静电相互作用和/或扩展的氢键网络传输到该羧酸基团。为了鉴定产生该带的羧酸基团，检查了来自突变体D1-Asp 61 Ala、D1-Glu 65 Ala、D1-Glu 329 Gln和D2-Glu 312 Ala的PSII核心复合物的FTIR差光谱。在X射线晶体学模型中，这些是最接近Mn 4Ca簇的羧酸残基，其不连接Mn或Ca，并且都是高度保守的。1747 cm− 1谱带存在于D1-Asp 61 Ala的S2-minus-S1 FTIR差谱中，但不存在于D1-Glu 65 Ala、D2-Glu 312 Ala和D1-Glu 329 Gln的相应谱中。当样品保持在≤ 85%的相对湿度下时，野生型中的条带幅度也急剧减小。D1-Glu 65、D2-Glu 312和D1-Glu 329参与了一个共同的氢键网络，该网络包括水分子和产生1747 cm− 1谱带的羧酸基团。进一步提出，这三个残基中的任何一个的突变，或将样品保持在≤ 85%的相对湿度下引起的部分脱水，都足以破坏网络，使得与S1到S2转变相关的结构扰动不再传递到羧酸基团，从而产生1747 cm− 1带。由于D1-Glu 329位于距离D1-Glu 65和D2-Glu 312约20 nm处，因此假设的氢键网络必须在Mn 4Ca簇的内腔表面延伸至少20 nm。D1-Asp 61 Ala、D1-Glu 65 Ala和D2-Glu 312 Ala突变也明显降低了响应饱和闪光而经历S3到S 0转变的PSII反应中心的分数。这种行为与D1-Asp 61、D1-Glu 65和D2-Glu 312参与连接Mn 4Ca簇与类囊体腔的主导质子出口通道一致。
Analyses of the refined X-ray crystallographic structures of photosystem II (PSII) at 2.9−3.5 Å have revealed the presence of possible channels for the removal of protons from the catalytic Mn4Ca cluster during the water-splitting reaction. As an initial attempt to verify these channels experimentally, the presence of a network of hydrogen bonds near the Mn4Ca cluster was probed with FTIR difference spectroscopy in a spectral region sensitive to the protonation states of carboxylate residues and, in particular, with a negative band at 1747 cm−1that is often observed in the S2-minus-S1FTIR difference spectrum of PSII from the cyanobacteriumSynechocystissp. PCC 6803. On the basis of its 4 cm−1downshift in D2O, this band was assigned to the carbonyl stretching vibration (CO) of a protonated carboxylate group whose pKadecreases during the S1to S2transition. The positive charge that forms on the Mn4Ca cluster during the S1to S2transition presumably causes structural perturbations that are transmitted to this carboxylate group via electrostatic interactions and/or an extended network of hydrogen bonds. In an attempt to identify the carboxylate group that gives rise to this band, the FTIR difference spectra of PSII core complexes from the mutants D1-Asp61Ala, D1-Glu65Ala, D1-Glu329Gln, and D2-Glu312Ala were examined. In the X-ray crystallographic models, these are the closest carboxylate residues to the Mn4Ca cluster that do not ligate Mn or Ca and all are highly conserved. The 1747 cm−1band is present in the S2-minus-S1FTIR difference spectrum of D1-Asp61Ala but absent from the corresponding spectra of D1-Glu65Ala, D2-Glu312Ala, and D1-Glu329Gln. The band is also sharply diminished in magnitude in the wild type when samples are maintained at a relative humidity of ≤85%. It is proposed that D1-Glu65, D2-Glu312, and D1-Glu329 participate in a common network of hydrogen bonds that includes water molecules and the carboxylate group that gives rise to the 1747 cm−1band. It is further proposed that the mutation of any of these three residues, or partial dehydration caused by maintaining samples at a relative humidity of ≤85%, disrupts the network sufficiently that the structural perturbations associated with the S1to S2transition are no longer transmitted to the carboxylate group that gives rise to the 1747 cm−1band. Because D1-Glu329 is located approximately 20 Å from D1-Glu65 and D2-Glu312, the postulated network of hydrogen bonds must extend for at least 20 Å across the lumenal face of the Mn4Ca cluster. The D1-Asp61Ala, D1-Glu65Ala, and D2-Glu312Ala mutations also appear to substantially decrease the fraction of PSII reaction centers that undergo the S3to S0transition in response to a saturating flash. This behavior is consistent with D1-Asp61, D1-Glu65, and D2-Glu312 participating in a dominant proton egress channel that links the Mn4Ca cluster with the thylakoid lumen.