Sjöberg B M, Karlsson M, Jörnvall H
J Biol Chem. 1987 Jul 15;262(20):9736-43.
A C-terminally truncated form of protein B2, the homodimeric small subunit of ribonucleotide reductase from Escherichia coli, was found as the result of an apparently specific proteolysis. Truncated homodimers contain an intact binuclear iron center and a normal tyrosyl radical but have no binding capacity for the other ribonucleotide reductase subunit, protein B1, and are consequently enzymatically inactive. Heterodimers, consisting of one full-length and one truncated polypeptide, formed spontaneously during a chelation-reconstitution cycle and were easily separated from the two homodimeric variants. The heterodimeric form of B2 shows a weak interaction with the B1 subunit resulting in low enzyme activity. Using heterodimers containing deuterated tyrosine on the full-length side and protonated tyrosine on the truncated side, we could demonstrate that the tyrosyl radical was randomly generated in one or the other of the two polypeptide chains of the heterodimeric B2 subunit. The small subunit of ribonucleotide reductase thus conforms to a half-site reactivity.
蛋白质B2是来自大肠杆菌的核糖核苷酸还原酶的同二聚体小亚基，一种C末端截短形式是明显特异性蛋白水解的结果。截短的同二聚体含有完整的双核铁中心和正常的酪氨酸自由基，但对另一个核糖核苷酸还原酶亚基蛋白质B1没有结合能力，因此没有酶活性。由一条全长多肽和一条截短多肽组成的异二聚体在螯合-重组循环中自发形成，并且很容易与两种同二聚体变体分离。B2的异二聚体形式与B1亚基表现出弱相互作用，导致酶活性较低。使用在全长侧含有氘代酪氨酸而在截短侧含有质子化酪氨酸的异二聚体，我们可以证明酪氨酸自由基在异二聚体B2亚基的两条多肽链中的一条或另一条中随机产生。因此，核糖核苷酸还原酶的小亚基符合半位点反应性。
