Lam K F, Logan D M
Can J Biochem. 1977 Jul;55(7):671-7. doi: 10.1139/o77-097.
Oligonucleotide chains consisting of adenosine residues and ranging from 1 to 70 residues in length have been tested as substrates or inhibitors with Lactobacillus plantarum exoribonuclease (EC3.1.4.20). The kinetic constants V, Km, and Ki are all chain-length dependent. Ki decreases with increasing chain length to a minimum for oligonucleotides seven residues in length and then begins to increase slightly. Kinetic plots indicate that the oligonucleotides are almost all competitive inhibitors of poly A hydrolysis. However, the oligonucleotide (Ap)3A greater than p probably leads to mixed inhibition. The enzyme is unable to retain its processivity when it hydrolyzes short oligonucleotides such as (Ap)2A and (Ap)3A. It is proposed that L. plantarum exoribonuclease possesses seven binding sites for the polynucleotide. When the enzyme is bound to a long-chain-length substrate the complex is stabilized by a binding energy of about 8 Kcal/mol. After cleavage of the terminal nucleotide the remaining binding energy is still sufficient to maintain an enzyme-substrate complex. The shortened nucleotide chain is moved relative to the enzyme to re-form the seven-bond association by a gradient of energy of about 1.7 Kcal/mol for the change from six to seven bonds.
由腺苷残基组成、长度在1至70个残基之间的寡核苷酸链已作为底物或抑制剂，用植物乳杆菌外切核糖核酸酶（EC3.1.4.20）进行了测试。动力学常数V、Km和Ki均与链长有关。Ki随着链长增加而降低，对于长度为7个残基的寡核苷酸达到最小值，然后开始略有增加。动力学曲线表明，寡核苷酸几乎都是多聚A水解的竞争性抑制剂。然而，寡核苷酸(Ap)3A大于p可能导致混合型抑制。当该酶水解短寡核苷酸如(Ap)2A和(Ap)3A时，它无法保持其持续合成能力。有人提出，植物乳杆菌外切核糖核酸酶拥有七个多核苷酸结合位点。当该酶与长链长度的底物结合时，复合物通过约8千卡/摩尔的结合能得以稳定。末端核苷酸裂解后，剩余的结合能仍足以维持酶-底物复合物。缩短的核苷酸链相对于酶移动，以通过约1.7千卡/摩尔的能量梯度重新形成七键结合，实现从六个键到七个键的转变。
