Guranowski A, Jakubowski H
Biochim Biophys Acta. 1983 Jan 12;742(1):250-6. doi: 10.1016/0167-4838(83)90383-7.
Substrate specificity of S-adenosylhomocysteinases (S-adenosyl-L-homocysteine hydrolase, EC 3.3.1.1) with respect to amino acid has been studied using homogeneous preparations of the enzymes from yellow lupin (Lupinus luteus) seeds and bovine liver. Both enzymes use cysteine, in addition to homocysteine, as a substrate. Homoserine, serine, pinicillamine, reduced glutathione and 2-mercaptoethanol are not substrates. In the presence of cysteine, the reaction of S-adenosylthio-amino acid synthesis is characterized by 20-40-fold lower kcat values (kcat = 0.23 s-1 or 0.11 s-1 in the presence of cysteine and either bovine or lupin enzyme) and 270-250-fold higher Km values (Km for cysteine is 15 mM and 35 mM with bovine and lupin enzyme, respectively) than the reaction in the presence of the normal substrate, homocysteine. In the reverse reaction, S-adenosylcysteine is hydrolyzed by the mammalian enzyme much faster than by the plant one. Specificity (kcat/Km) towards S-adenosylcysteine and S-adenosylhomocysteine is 0.9 M-1 . s-1 and 60 000 M-1 . s-1, respectively, with the plant enzyme and 15.3 M-1 . s-1 and 70 000 M-1 . s-1, respectively, with the mammalian enzyme. With plant enzyme, the reactions with cysteine and homocysteine are not competitive, i.e., cysteine does not inhibit the synthesis of S-adenosylhomocysteine, and homocysteine does not inhibit the synthesis of S-adenosylcysteine. This is consistent with independent binding of cysteine and homocysteine to both enzyme subunits. Using adenosine analogs and the mammalian S-adenosylhomocysteinase we were able to synthesize a number of novel S-adenosylcysteine analogs. These included: S-N6-hydroxyadenosyl-L-cysteine, S-2-aminoadenosyl-L-cysteine, S-nebularyl-L-cysteine, S-3-deazaadenosyl-L-cysteine, S-formycyl-L-cysteine, S-N6-methyladenosyl-L-cysteine and S-N1-oxideadenosyl-L-cysteine.
利用来自黄羽扇豆（Lupinus luteus）种子和牛肝的纯酶制剂，研究了S-腺苷同型半胱氨酸酶（S-腺苷-L-同型半胱氨酸水解酶，EC 3.3.1.1）对氨基酸的底物特异性。除同型半胱氨酸外，这两种酶都将半胱氨酸用作底物。高丝氨酸、丝氨酸、青霉胺、还原型谷胱甘肽和2-巯基乙醇不是底物。在存在半胱氨酸的情况下，S-腺苷硫代氨基酸合成反应的特征是，与存在正常底物同型半胱氨酸时相比，催化常数（kcat）值低20 - 40倍（在存在半胱氨酸以及牛或羽扇豆酶的情况下，kcat分别为0.23 s-1或0.11 s-1），米氏常数（Km）值高270 - 250倍（半胱氨酸的Km值，牛酶为15 mM，羽扇豆酶为35 mM）。在逆反应中，哺乳动物酶水解S-腺苷半胱氨酸的速度比植物酶快得多。植物酶对S-腺苷半胱氨酸和S-腺苷同型半胱氨酸的特异性（kcat/Km）分别为0.9 M-1·s-1和60000 M-1·s-1，哺乳动物酶的分别为15.3 M-1·s-1和70000 M-1·s-1。对于植物酶，与半胱氨酸和同型半胱氨酸的反应无竞争性，即半胱氨酸不抑制S-腺苷同型半胱氨酸的合成，同型半胱氨酸也不抑制S-腺苷半胱氨酸的合成。这与半胱氨酸和同型半胱氨酸独立结合到两个酶亚基上是一致的。使用腺苷类似物和哺乳动物S-腺苷同型半胱氨酸酶，我们能够合成许多新型的S-腺苷半胱氨酸类似物。这些包括：S-N6-羟基腺苷-L-半胱氨酸、S-2-氨基腺苷-L-半胱氨酸、S-星云基-L-半胱氨酸、S-3-脱氮腺苷-L-半胱氨酸、S-甲酰基-L-半胱氨酸、S-N6-甲基腺苷-L-半胱氨酸和S-N1-氧化腺苷-L-半胱氨酸。
