Precise structure and double substrate recognition of pyridoxal protein

为了阐明双底物识别的机制，对plp依赖性转氨酶进行了x射线晶体学研究。酶似乎有各种复杂的方法从许多其他小分子中识别两种氨基酸，这两种氨基酸的侧链在大小和性质上都不同。然而，有两个基本机制。芳香和支链氨基酸转氨酶（分别为AroAT和BCAT）的基本机制已经被观察到。前者表现出活性位点氢键网络的大规模重排（开关机制），后者在不改变主链和侧链构象折叠的情况下识别两种底物（原样机制）。AroAT或BCAT在活性位点的同一位置结合底物的疏水性和酸性侧链。在AroAT中，Arg的胍基与底物的羧酸盐（谷氨酸）形成盐桥。当疏水侧链与羧酸盐的侧链结合在同一位置时，胍基随着活性位点口袋的重建而移动。在BCAT中，亲水侧链与溶剂侧附近的袋状结构的亲水位点形成氢键。该袋整体疏水，可将疏水侧链包封在袋内而不重建活性位点袋。组氨酸二酚磷酸转氨酶的双底物识别机制与BCAT的基本相同，尽管这种识别与小尺度诱导拟合有关。活性位点被设计为锁，酸性和碱性侧链作为钥匙。正如在BCAT的例子中所观察到的那样，Glu的侧链羧酸盐结合在与溶剂区相邻的地方。谷氨酰胺转氨酶的识别方法与BCAT相同。疏水袋，其内部有亲水性位点，可以容纳疏水和亲水侧链。乙酰鸟氨酸转氨酶是不寻常的，因为这种酶缺乏羧酸盐。在这种情况下，使用原状和开关的混合机制来识别乙氨酸和谷氨酸。少

X-ray crystallographic studies of PLP-dependent aminotransferases have been performed in order to elucidate the mechanism for double substrate recognition. The enzymes seem to have various and complex methods to recognize two kinds of amino acids, the side chains of which are different in size and property, from many other small molecules. However, there are two fundamental mechanisms. The fundamental mechanisms have been observed in aromatic and branched-chain amino acid aminotransferases (AroAT and BCAT, respectively). The former shows the large-scale rearrangement of hydrogen-bonding network at the active site (switch mechanism), while the latter recognizes two kinds of substrate without changing the folding of the main-chain and side-chain conformation (as-is mechanism). Either of AroAT or BCAT binds hydrophobic and acidic side chains of the substrates at the same place of the active site. In AroAT, the guanidino group of Arg makes a salt bridge with the carboxylate of the substrat … More e (glutamate). When the hydrophobic side chain is bound to the same place as that of the carboxylate, the guanidino group moves away with the concomitant reconstruction of the active-site pocket. In BCAT, the hydrophilic side chain is hydrogen bonded to the hydrophilic sites of the pocket which are in the vicinity of the solvent side. The pocket is hydrophobic as a whole and may encapsulate the hydrophobic side chain inside the pocket without reconstruction of the active site pocket.The double substrate recognition mechanism in histidinol phosphate aminotransferase is essentially the same as that of BCAT, although small-scale induced-fit is associated with the recognition. The active site is designed as the lock for both acidic and basic side chains as keys. As is observed in the case for BCAT, the side-chain carboxylate of Glu binds to the place adjacent to the solvent region. The recognition method in glutamine aminotransferase is the same as that in BCAT. The hydrophobic pocket, the inside of which has hydrophiilic sites, can accommodate hydrophobic and hydrophilic side chains. Acetylornithine aminotransferase is unusual in that the enzyme lacks ct-carboxylate. In this case, a hybrid of as-is and switch mechanism is used to recognize acetylornithine and glutamate. Less