Schininà M E, Bossa F, Lania A, Capo C R, Carlini P, Calabrese L
Department of Biochemical Sciences, University of Rome La Sapienza, Italy.
Eur J Biochem. 1993 Feb 1;211(3):843-9. doi: 10.1111/j.1432-1033.1993.tb17617.x.
A copper,zinc superoxide dismutase, has been isolated from the marine turtle Caretta caretta and the complete amino acid sequence obtained. The sequence was determined by isolation and analysis of peptides obtained after cleavage of the carboxymethylated apoenzyme with trypsin or Staphylococcus aureus protease. Turtle superoxide dismutase consists of 166 amino acid residues, which represents the largest number to date for a cytosolic copper,zinc superoxide dismutase. The comparison of this amino acid sequence with that of bovine superoxide dismutase revealed a one-residue C-terminal extension, two single residue insertions and a 12-residue insertion in the N-terminal region, in turtle superoxide dismutase. The new segment consists of a threefold repeating sequence and was found to be the site for selective proteolytic attack by trypsin under native conditions. The biochemical characteristics, the spectroscopic and catalytic properties as well as the thermal stability and the resistance to irreversible denaturation, were carefully examined and were very similar to those of other superoxide dismutases. These results indicate that the presence of the new polypeptide segment does not affect the main folding of the chain and the quaternary structure, nor the functional parameters of turtle superoxide dismutase. The possibility that the new insert constitutes a loop excluded from the protein scaffold providing the framework of the active site is also discussed.
已从蠵龟（Caretta caretta）中分离出一种铜锌超氧化物歧化酶，并获得了其完整的氨基酸序列。该序列是通过对羧甲基化脱辅基酶用胰蛋白酶或金黄色葡萄球菌蛋白酶切割后得到的肽段进行分离和分析来确定的。蠵龟超氧化物歧化酶由166个氨基酸残基组成，这是迄今为止胞质铜锌超氧化物歧化酶中数量最多的。将该氨基酸序列与牛超氧化物歧化酶的序列进行比较发现，在蠵龟超氧化物歧化酶中，其C末端有一个残基的延伸，N末端区域有两个单残基插入和一个12残基的插入。新的片段由一个三重重复序列组成，并且发现在天然条件下是胰蛋白酶选择性蛋白水解攻击的位点。对其生化特性、光谱和催化性质以及热稳定性和对不可逆变性的抗性进行了仔细研究，发现它们与其他超氧化物歧化酶非常相似。这些结果表明，新的多肽片段的存在并不影响链的主要折叠和四级结构，也不影响蠵龟超氧化物歧化酶的功能参数。还讨论了新插入片段构成一个从提供活性位点框架的蛋白质支架中排除的环的可能性。
