Bouyoub A, Barbier G, Forterre P, Labedan B
Institut de Génétique et Microbiologie, URA 1354, Université Paris-Sud, Orsay, France.
J Mol Biol. 1996 Aug 16;261(2):144-54. doi: 10.1006/jmbi.1996.0448.
The first example of a hyperthermophilic adenylosuccinate synthetase is reported, which is an enzyme that must maintain its folded structure at temperatures as high as 102 degrees C. The amino acid sequence of this key enzyme has been determined after cloning and sequencing the purA-like gene from the archaeal Pyrococcus sp. strain ST700. The corresponding protein displays two unexpected features: (1) it is 21% shorter than the homologous mesophilic enzymes and this shortening corresponds to the loss of two alpha-helices and three beta-strands present in the Escherichia coli enzyme; (2) surprisingly, the archaeal adenylosuccinate synthetase has a significant number of substitutions in residues that are conserved in all other homologous enzymes from bacteria to man. In E. coli, the conserved residues have been described as essential for catalytic activity and/or for maintaining the folded structure of the homodimer. Despite these drastic differences, the purA-like archaeal gene seems to be normally expressed and its product functions in vivo in bacteria, since it complemented an E. coli purA auxotroph. The archaeal adenylosuccinate synthetase appears to be a good example of a bona fide orthologous protein. Reconstruction of phylogenetic trees showed that the archaeal gene is equally distantly related to both eukaryotes and bacteria, independently of the numerous substitutions observed at critical positions.
报道了嗜热腺苷酸琥珀酸合成酶的首个实例，该酶必须在高达102摄氏度的温度下维持其折叠结构。从嗜热栖热菌属菌株ST700克隆并测序类purA基因后，确定了这种关键酶的氨基酸序列。相应的蛋白质呈现出两个意外特征：（1）它比同源的嗜温酶短21%，这种缩短对应于大肠杆菌酶中存在的两个α螺旋和三条β链的缺失；（2）令人惊讶的是，古菌腺苷酸琥珀酸合成酶在所有其他从细菌到人类的同源酶中保守的残基上有大量取代。在大肠杆菌中，保守残基被描述为对催化活性和/或维持同型二聚体的折叠结构至关重要。尽管存在这些巨大差异，但类purA古菌基因似乎正常表达，其产物在细菌体内发挥功能，因为它补充了大肠杆菌purA营养缺陷型。古菌腺苷酸琥珀酸合成酶似乎是真正直系同源蛋白的一个很好的例子。系统发育树的重建表明，古菌基因与真核生物和细菌的亲缘关系同样遥远，与在关键位置观察到的大量取代无关。
