Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions
Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions
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DOI：
10.1007/s11064-015-1590-5
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发表时间：
2015-06-01
影响因子：
4.4
通讯作者：
Karuso, Peter
中科院分区：
文献类型：
--
作者：
Hallen, Andre;Cooper, Arthur J. L.;Karuso, Peter
哺乳动物的酮胺还原酶与结晶素(CRYM)完全相同，CRYM也是一种重要的甲状腺激素结合蛋白。这种双重功能意味着甲状腺激素在酮胺还原酶调节中的作用，以及酶催化在甲状腺激素生物利用度中的相互作用。在这项研究中，我们证明了在中性pH下，甲状腺激素L-甲状腺素和3，5，3‘-三碘甲状腺原氨酸对酶催化的亚纳摩尔抑制作用，而其他甲状腺激素类似物的抑制作用要弱得多。我们还研究了底物类似物2-吡咯-2-羧酸酯、4，5-二溴-2-吡咯-2-羧酸酯和吡啶甲酸酯对酶的抑制作用，以及(B)环酮胺S-(2-氨基乙基)-L-半胱氨酸酮胺(根据作者的描述，含硫环酮胺命名繁琐)(Aeck)、Delta(1)-吡啶-2-羧酸酯(P_2C)、Delta(1)-吡咯啉-2-羧酸酯(P_2C)和Delta(2)-噻唑-2-羧酸酯的中性pH催化作用。在中性pH下获得的动力学数据表明，酮胺还原酶/CRYM作为一种P2C/Pyr2C还原酶起主要作用，而Aeck不是该pH下的主要底物。因此，酮胺还原酶是脑部赖氨酸降解的主要途径--哌替酸途径中的一个关键酶。在电子对接中，不同的配体进入活性部位的X射线结构的酶暗示了一种不同寻常的催化机制，涉及精氨酸残基作为质子供体。鉴于甲状腺激素在大脑功能中的关键重要性，这项研究进一步扩展了我们对氨基酸新陈代谢和甲状腺激素水平调节之间的联系的认识。
Mammalian ketimine reductase is identical to mu-crystallin (CRYM)-a protein that is also an important thyroid hormone binding protein. This dual functionality implies a role for thyroid hormones in ketimine reductase regulation and also a reciprocal role for enzyme catalysis in thyroid hormone bioavailability. In this research we demonstrate potent sub-nanomolar inhibition of enzyme catalysis at neutral pH by the thyroid hormones l-thyroxine and 3,5,3'-triiodothyronine, whereas other thyroid hormone analogues were shown to be far weaker inhibitors. We also investigated (a) enzyme inhibition by the substrate analogues pyrrole-2-carboxylate, 4,5-dibromopyrrole-2-carboxylate and picolinate, and (b) enzyme catalysis at neutral pH of the cyclic ketimines S-(2-aminoethyl)-l-cysteine ketimine (owing to the complex nomenclature trivial names are used for the sulfur-containing cyclic ketimines as per the original authors' descriptions) (AECK), Delta(1)-piperideine-2-carboxylate (P2C), Delta(1)-pyrroline-2-carboxylate (Pyr2C) and Delta(2)-thiazoline-2-carboxylate. Kinetic data obtained at neutral pH suggests that ketimine reductase/CRYM plays a major role as a P2C/Pyr2C reductase and that AECK is not a major substrate at this pH. Thus, ketimine reductase is a key enzyme in the pipecolate pathway, which is the main lysine degradation pathway in the brain. In silico docking of various ligands into the active site of the X-ray structure of the enzyme suggests an unusual catalytic mechanism involving an arginine residue as a proton donor. Given the critical importance of thyroid hormones in brain function this research further expands on our knowledge of the connection between amino acid metabolism and regulation of thyroid hormone levels.