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首页> 外文期刊>Acta crystallographica. Section D, Structural biology >Biochemical and structural insights into an unusual, alkali-metal-independent S-adenosyl-L-homocysteine hydrolase from Synechocystis sp. PCC 6803
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Biochemical and structural insights into an unusual, alkali-metal-independent S-adenosyl-L-homocysteine hydrolase from Synechocystis sp. PCC 6803

机译：生化和结构的见解不寻常,alkali-metal-independentS-adenosyl-L-homocysteine水解酶的

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摘要

The mesophilic cyanobacterium Synechocystis sp. PCC 6803 encodes an S-adenosyl- l-homocysteine hydrolase (SAHase) of archaeal origin in its genome. SAHases are essential enzymes involved in the regulation of cellular S-adenosyl- l-methionine (SAM)-dependent methylation reactions. They are usually active as homotetramers or, less commonly, as homodimers. A SAHase subunit is composed of two major domains: a cofactor (NAD(+))-binding domain and a substrate (S-adenosyl-l-homocysteine)-binding domain. These are connected by a hinge element that is also a coordination site for an alkali-metal cation that influences domain movement during the catalytic cycle. Typically, the highest activity and strongest substrate binding of bacterial SAHases are observed in the presence of K+ ions. The SAHase from Synechocystis (SynSAHase) is an exception in this respect. Enzymatic and isothermal titration calorimetry studies demonstrated that in contrast to K+-dependent SAHases, the activity and ligand binding of SynSAHase are not affected by the presence of any particular alkali ion. Moreover, in contrast to other SAHases, the cyanobacterial enzyme is in an equilibrium of two distinct oligomeric states corresponding to its dimeric and tetrameric forms in solution. To explain these phenomena, crystal structures of SynSAHase were determined for the enzyme crystallized in the presence of adenosine (a reaction byproduct or substrate) and sodium or rubidium cations. The structural data confirm that while SynSAHase shares common structural features with other SAHases, no alkali metal is coordinated by the cyanobacterial enzyme as a result of a different organization of the macromolecular environment of the site that is normally supposed to coordinate the metal cation. This inspired the generation of SynSAHase mutants that bind alkali-metal cations analogously to K+-dependent SAHases, as confirmed by crystallographic studies. Structural comparisons of the crystal structure of SynSAHase with other experimental models of SAHases suggest a possible explanation for the occurrence of the cyanobacterial enzyme in the tetrameric state. On the other hand, the reason for the existence of SynSAHase in the dimeric state in solution remains elusive.
机译：嗜中温的藻青菌集胞藻属sp。PCC 6803编码一个S-adenosyl - l-homocysteine水解酶(SAHase)的热点在其起源基因组。调节细胞S-adenosyl -l-methionine (SAM)端依赖甲基化反应。一般homotetramers或者更少,为。SAHase单元是由两个主要领域:代数余子式(NAD(+))绑定域和一个衬底(S-adenosyl-l-homocysteine)绑定域。这也是一个协调为一个网站碱金属阳离子影响域在催化循环运动。最高的活动和最强的衬底绑定的细菌SAHases中观察到K +离子的存在。集胞藻属(SynSAHase)是一个例外尊重。量热法研究表明,相比之下K +端依赖SAHases,活动和配体绑定SynSAHase不受影响的任何特定的碱离子的存在。相比其他SAHases,蓝藻酶在两个截然不同的一个平衡低聚物的状态对应于二聚的和四聚物的形式的解决方案。这些现象,SynSAHase的晶体结构测定酶结晶的腺苷酸(一种反应副产物的存在或基质)和钠或铷阳离子。结构数据证实,尽管SynSAHase与其他共享共同的结构特点SAHases,不协调的碱金属蓝藻酶作为一个不同的结果大分子的环境的组织通常的网站应该协调金属阳离子。SynSAHase绑定碱金属阳离子的突变体类似地,K +端依赖SAHases,如确认通过晶体研究。比较SynSAHase的晶体结构与其他SAHases建议的实验模型出现的一个可能的解释蓝藻酶四聚物的状态。另一方面,存在的原因SynSAHase二聚的状态的解决方案仍然是难以捉摸的。

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