硫葉菌硫酸磷酸腺苷還原酶與其基質及產物複合體提供催化機制的結構基礎層面探討

硫酸磷酸腺苷還原酶 (sulfonucleotide reductase) ，為一種將外界取得的 sulfate (SO42-) 進行還原代謝 (reductive sulfate assimilation) 之酵素。細菌、真菌及植物進行生合成 cysteine 的系列反應中，此酵素為第一個關鍵反應步驟，然而其反應機制仍然有許多未知的部份尚待釐清。此外，哺乳動物並不會進行此類的還原途徑，這也使得此酵素被視為新穎抗病菌藥物開發的潛力目標。為了進一步闡明此類酵素的催化機制，本研究針對硫葉菌的硫酸磷酸腺苷還原酶 (sulfonucleotide reductase from Sulfolobus solfataricus, ssSNRase) 進行研究分析。經由表現純化到的重組蛋白呈現棕色，暗示了蛋白中可能含有金屬錯化物基團在其中，再藉由 UV-vis 吸光光譜和 X 射線螢光掃描分析，我們可以推測 [4Fe-4S] 簇輔基團的存在。此外，利用圓二色光譜儀 (circular dichroism) 測定可以得知此種蛋白的 Tm 在 82°C 以上，而將 [4Fe-4S] 簇移除則會降低此酵素的結構穩定性。為獲得更詳盡的結構資訊，我們進行 ssSNRase 與其產物 (adenosine 5’-monophosphate, AMP) 和基質 (adenosine 5’-phosphosulfate, APS) 複合物的共結晶實驗，並且成功解出其晶體結構，解析度分別為 1.97 及 2.03 Å。在這兩種不同催化階段的結構中，可以看到含有活性必要殘基 (residue) cysteine 231 的羧基端尾巴片段，被一個架構完整的氫鍵網絡固定在其酵素活性區上方的溝槽之中。藉由比較 ssSNRase-APS 與 ssSNRase-AMP 兩種不同複合物結構，我們發現到數個參與催化活性過程中的重要殘基並且推測出其功能。根據本篇實驗的結果，我們提供更多藉由酵素結構變化所調控的 SNRase 活性的相關訊息，也進一步的對其羧基端尾巴開闔的動態變化與調控機制提出更完整的註解。

關鍵字

硫葉菌；硫酸磷酸腺苷還原酶催化機制；蛋白質結晶學；複合體結構

並列摘要

The first committed step of reductive sulfate assimilation in the biosynthesis of cysteine in pathogenic bacteria is catalyzed by sulfonucleotide reductases (SNRases). However, mammals do not possess the sulfate reduction pathway, which makes sulfonucleotide reductases being a promising target for drug development against human pathogens. To elucidate the catalytic mechanism, ssSNRase (sulfonucleotide reductase from Sulfolobus solfataricus) was studied by a combination of biochemical, spectroscopic, and crystallographic approaches. Purified ssSNRase protein in solution is shown brownish in color and proposed it should contain one [4Fe-4S] cluster per polypeptide chain. Data from ultraviolet-visible absorption spectroscopy and X-ray fluorescence scan were collected to elucidate the nature of the prosthetic group containing property. CD experiments showed that ssSNRase possesses high melting temperature around 82°C, which consistent with the physiological growth environment of Sulfolobus solfataricus. Crystal structure of ssSNRases in complex with its product AMP and its substrate APS were solved to 1.97 and 2.03 Å, respectively. Residues that are essential for catalytic activity could be elucidated by comparing substrate-product substituted active sites and the possible function of these residues were also proposed. C-terminus tail containing active Cys231 residue in an organized H-bonding network with the channel of enzyme were both observed. By these two snapshots of ssSNRase catalytic states, we can provide more information about the C-terminus tail displacement and the mobility model of the C-terminal tail during APS reduction can be modified.

並列關鍵字

Sulfolobus solfataricus ； sulfonucleotide reductases ； catalytic mechanism ； protein crystallography ； crystal structure

參考文獻

3. Osslund, T., C. Chandler, and I.H. Segel, ATP sulfurylase from higher plants : purification and preliminary kinetics studies on the cabbage leaf enzyme. Plant Physiol, 1982. 70(1): p. 39-45.

4. Seubert, P.A., et al., ATP sulfurylase from Penicillium chrysogenum: measurements of the true specific activity of an enzyme subject to potent product inhibition and a reassessment of the kinetic mechanism. Arch Biochem Biophys, 1983. 225(2): p. 679-91.

5. Renosto, F., P.A. Seubert, and I.H. Segel, Adenosine 5'-phosphosulfate kinase from Penicillium chrysogenum. Purification and kinetic characterization. J Biol Chem, 1984. 259(4): p. 2113-23.

6. Segel, I.H., F. Renosto, and P.A. Seubert, Sulfate-activating enzymes. Methods Enzymol, 1987. 143: p. 334-49.

7. Bick, J.A., et al., Identification of a new class of 5'-adenylylsulfate (APS) reductases from sulfate-assimilating bacteria. J Bacteriol, 2000. 182(1): p. 135-42.

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硫葉菌硫酸磷酸腺苷還原酶與其基質及產物複合體提供催化機制的結構基礎層面探討

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