Kumar Sandeep, Nussinov Ruth
Basic Research Program, SAIC-Frederick, Inc., Laboratory of Experimental and Computational Biology, NCI-Frederick, Building 469, Room 151, Frederick, MD 21702, USA.
Chembiochem. 2004 Mar 5;5(3):280-90. doi: 10.1002/cbic.200300627.
Electrostatics plays a major role in heat adaptation by thermophilic proteins. Here we ask whether electrostatics similarly contributes to cold adaptation in psychrophilic proteins. We compare the sequences and structures of citrate synthases from the psychrophile Arthobacter Ds2-3R, from chicken, and from the hyperthermophile Pyrococcus furiosus. The three enzymes share similar packing, burial of nonpolar surface area, and main-chain hydrogen bonding. However, both psychrophilic and hyperthermophilic citrate synthases contain more charged residues, salt bridges, and salt-bridge networks than the mesophile. The electrostatic free-energy contributions toward protein stability by individual charged residues show greater variabilities in the psychrophilic citrate synthase than in the hyperthermophilic enzyme. The charged residues in the active-site regions of the psychrophile are more destabilizing than those in the active-site regions of the hyperthermophile. In the hyperthermophilic enzyme, salt bridges and their networks largely cluster in the active-site regions and at the dimer interface. In contrast, in the psychrophile, they are more dispersed throughout the structure. On average, salt bridges and their networks provide greater electrostatic stabilization to the thermophilic citrate synthase at 100 degrees C than to the psychrophilic enzyme at 0 degrees C. Electrostatics appears to play an important role in both heat and cold adaptation of citrate synthase. However, remarkably, the role may be different in the two types of enzyme: In the hyperthermophile, it may contribute to the integrity of both the protein dimer and the active site by possibly countering conformational disorder at high temperatures. On the other hand, in the psychrophile at low temperatures, electrostatics may contribute to enhance protein solvation and to ensure active-site flexibility.
静电作用在嗜热蛋白的热适应性中起着重要作用。在此，我们探讨静电作用是否同样有助于嗜冷蛋白的冷适应性。我们比较了嗜冷菌节杆菌Ds2 - 3R、鸡以及嗜热菌激烈火球菌中柠檬酸合酶的序列和结构。这三种酶在堆积、非极性表面积的埋藏以及主链氢键方面具有相似性。然而，嗜冷和嗜热的柠檬酸合酶比中温菌含有更多的带电残基、盐桥和盐桥网络。单个带电残基对蛋白质稳定性的静电自由能贡献在嗜冷柠檬酸合酶中比在嗜热酶中表现出更大的变异性。嗜冷菌活性位点区域的带电残基比嗜热菌活性位点区域的带电残基更具去稳定性。在嗜热酶中，盐桥及其网络主要聚集在活性位点区域和二聚体界面。相比之下，在嗜冷菌中，它们在整个结构中分布更分散。平均而言，盐桥及其网络在100℃时为嗜热柠檬酸合酶提供的静电稳定性比在0℃时为嗜冷酶提供的静电稳定性更大。静电作用似乎在柠檬酸合酶的热适应和冷适应中都起着重要作用。然而，值得注意的是，在这两种类型的酶中其作用可能不同：在嗜热菌中，它可能通过抵消高温下的构象无序来有助于蛋白质二聚体和活性位点的完整性。另一方面，在低温下的嗜冷菌中，静电作用可能有助于增强蛋白质的溶剂化作用并确保活性位点的灵活性。
