Oligosaccharide Substrate and Inhibitor Interactions with beta-1,4-Gal-T1

我们之前对b4Gal-T1和突变体Met340His-b4Gal-T1与壳聚糖和各种三糖复合物的晶体学研究，以及酶动力学分析和MD模拟确定了b4Gal-T1的寡糖结合位点（见Project #s Z01 BC 009304）。为了更好地了解b4Gal-T1对n -聚糖中GlcNAc残基的分支特异性，我们将野生型人b4Gal-T1 （h-b4Gal-T1）和突变体met340hs -b4Gal-T1 （h-M340H-b4Gal-T1）与n -聚糖中含有五糖的GlcNAc和几种含三糖的GlcNAc配合物进行动力学和晶体学研究。结果表明，b4Gal-T1优先与1,2-1,6臂三糖相互作用，而不是与n -聚糖的双天线或三天线低聚糖链的1,2-1,3臂或1,4-1,3臂相互作用（见2006年项目# Z01 BC 010041）。使用合成三糖作为受体模型系统，我们发现受体1,2,1,3-臂（GlcNAc-b1,2-Man-a1,3-Man）和1,4,1,3-臂（GlcNAc-b1,4-Man-a1,3-Man）三糖对b4Gal-T1的Km比1,2,1,6-臂三糖（GlcNAc-b1,2-Man-a1,6-Man）高10- 60倍，后者对底物的抑制浓度远低于其他受体底物。b4Gal-T1对IgG双触角聚糖1-3或1-6臂的转移偏好：我们确定了b4Gal-T1对IgG双触角聚糖1-3或1-6臂的转移偏好。IgG的每个重链在fc区Asn 297处携带一个复杂的双天线n -链聚糖链。这些聚糖表现出不同的糖基化模式；G0糖型末端有两个GlcNAc残基，G1和G2糖型末端分别含有一个或两个半乳糖的低聚糖。为了研究b4Gal-T1对半乳糖的转移，我们建立了完全脱半乳糖基化（100%）到G0形式的条件。为了确定IgG的n -链聚糖的哪一个天线，1-3臂或1-6臂，或两者都是半乳糖化，我们与Timothy Weybright博士（来自Timothy Veenstras博士小组，来自SAIC-Frederick， Inc. BPP质谱中心）合作，通过MS/MS分析了PNGaseF处理单克隆抗体后释放的寡糖产物。我们发现野生型b4al-T1将半乳糖转移到IgG的G0糖型（MW 1485） 6小时后，半乳糖主要转移到一侧（G1糖型，MW 1647）。进一步的双天线聚糖链的MS/MS分析表明，野生型b4gal - t1以更快的速度将半乳糖转移到附着在Man 1-3臂上的GlcNAc上。相比之下，突变体b4Gal-T1-Y289L在孵育6小时后将GalNAc主要转移到双臂（G2糖型MW 1891）。PNGasesF处理单克隆抗体后G1糖型的MS/MS分析显示，突变体Y289L- b4Gal-T1以更快的速度将GalNAc转移到附着在Man 1-6臂上的GlcNAc上。h-M340H-Gal-T1与双糖glcnac - b3gal -b- o -萘二烯乙醇复合物的晶体结构：加州大学圣地亚哥分校的Jeff Eskos博士实验室已经证明，一类合成的双糖是肿瘤转移的潜在抑制剂。它们是b4GalT1的高亲和力底物，因此可以作为sialyl Lewis X （sLeX）合成的诱饵，sLeX是细胞粘附表位，在转移细胞中表达水平升高。迄今为止，他们发现的最有效的化合物是glcnac - b3gal -b- o -萘二烯乙醇。乙酰化的化合物被细胞吸收，o -去乙酰化，然后双糖在细胞表面糖缀合物上诱骗合成含slex的聚糖。在一些模型系统中，这导致肿瘤形成的抑制。正如Eskos博士的实验室所观察到的，这种化合物的所有活性都取决于细胞中b4GalT(s)的作用，因为利用它的第一步涉及半乳糖基化。在他们用glcnac - b3gal -b- o -萘二烯乙醇作为b4Gal-T1的受体底物的体外研究中，他们显示出10mum的Km。这种对酶的高亲和力可能是它与b4Gal-T1结合的独特模式，类似于我们观察到的1,2,2,6臂三糖（见上文）。我们用Eskos博士实验室提供的glcnac - b3gal -b- o -萘二烯乙醇携带了h-M340H-Gal-T1的晶体结构，并确定了双糖与酶之间的相互作用模式。GnGl-NP与Met344His-Cys342Thr-Gal-T1分子的整体结合与之前观察到的三糖GlcNAcb1-2Mana1-6Mana的结合非常相似（见2006项目# Z01 BC 010041），表明其对酶的Km将与1-6臂三糖相似，Km为60 muM。这种三糖来源于双触角n-聚糖的1-6臂，从核心甘露糖到非还原末端的游离GlcNAc。在酶结合的1-6臂三糖中，中间甘露糖与蛋白质分子的相互作用最小，而末端甘露糖（即双触角n -聚糖的核心甘露糖残基）与Tyr282残基的芳香侧链进行广泛的堆叠相互作用。相比之下，双糖GnGl-NP中的β连接的Gal残基与Tyr282形成了广泛的堆叠相互作用，而末端的芳香萘残基在b4Gal-T1分子的低聚糖结合位点上产生了额外的相互作用，尽管很弱。因此，由于萘部分产生的额外相互作用，预计GnGl-NP双糖的Km值将低于1-6臂三糖，从而使其成为已知的对b4Gal-T1具有最高亲和力的受体底物。此外，b4Gal-T家族成员T5和T6具有与b4Gal-T1的Phe356残基对应的保守残基Tyr。这些家族成员中的Tyr残基有望通过Tyr残基的侧链羟基进行额外的氢键相互作用，这有望进一步降低b4Gal-T5和b4Gal-T6的GnGl-NP双糖的Km。通过晶体结构分析，我们能够理解双糖GnGl-NP对b4Gal-T1的高亲和力。此外，这些分析表明，未来的化学修饰，如在受体设计中加入结构水分子，在Gal的第2位上加入氨基或甲基化的氨基，甚至在萘环上适当取代极性基团，都可能提高受体底物的亲和力，从而更好地设计肿瘤转移的双糖抑制剂。这些研究结果发表在《生物学杂志》上。化学学报，2009,28(4):452 - 459。h-M340H-Gal-T1与1-硫-n -丁基gln - β -（2-萘基）配合物的晶体结构：Inka Brockhausen博士（加拿大安大略省金斯顿皇后大学）发现了几种具有双环结构的化合物，即2-萘基苷元，可以结合并抑制b4Gal-T1的活性。他们发现的最好的小疏水glcnac类似物抑制剂是Ki为40 muM的1-thio- n - butyal - glcn - β -（2-萘基）。这种对酶的高亲和力可能是它与b4Gal-T1结合的独特模式，类似于我们观察到的1,2-1,6臂三糖或glcnac - b3gal -b- o -萘thalenemeiethanol，由Eskos博士实验室提供。我们正在研究该抑制剂与Met344His-Cys342Thr-Gal-T1分子配合物的晶体结构，以确定抑制剂与酶的相互作用模式。

Our previous crystallographic studies on b4Gal-T1 and of the mutant Met340His-b4Gal-T1 in complex with chitobiose and various trisaccharides, together with the enzyme kinetic analysis and MD simulations defined the oligosaccharide binding site of b4Gal-T1 (see Project #s Z01 BC 009304). For a better understanding of the branch specificity of b4Gal-T1 towards the GlcNAc residues of N-glycans, the kinetic and crystallographic studies with the wild-type human b4Gal-T1 (h-b4Gal-T1) and the mutant Met340His-b4Gal-T1 (h-M340H-b4Gal-T1), in complex with a GlcNAc containing pentasaccharide and several GlcNAc containing trisaccharides present in N-glycans, showed that b4Gal-T1 preferentially interacts with the 1,2-1,6-arm trisaccharide rather than with the 1,2-1,3-arm or 1,4-1,3-arm of a bi- or tri-antennary oligosaccharide chain of N-glycan (see 2006 project # Z01 BC 010041). Using synthetic trisaccharides as acceptor model systems, we showed that the acceptor 1,2-1,3-arm (GlcNAc-b1,2-Man-a1,3-Man) and 1,4-1,3-arm (GlcNAc-b1,4-Man-a1,3-Man) trisaccharides have a 10- to 60-fold higher Km for b4Gal-T1 than the 1,2-1,6-arm trisaccharide (GlcNAc-b1,2-Man-a1,6-Man), latter shows substrate inhibition at concentrations that are much lower than for other acceptor substrates. Transfer preferences of b4Gal-T1 to the 1-3 or 1-6 arm of a biantennary glycan of IgG: : We determined the transfer preferences of b4Gal-T1 to 1-3 or 1-6-arm of the biantennary glycan of IgG. Each heavy chain of IgG carries a complex bi-antennary N-linked glycan chain at Asn 297 in the Fc-region. These glycans show various glycosylated patterns; G0 glycoform having two terminal GlcNAc residues, and G1 and G2 glycoforms which bear oligosaccharides with one or two terminal galactose, respectively. We established the condition for the complete de-galactosylation (100 %) to G0 form for the studies on the transfer of galactose by b4Gal-T1. To establish which antenna, 1-3-arm or 1-6-arm, or both, of the N-linked glycan of IgG is galactosylated, we, in collaboration with Dr. Timothy Weybright (from Dr. Timothy Veenstras group, of the Mass Spectrometry Center, BPP, SAIC-Frederick, Inc.), analyzed the oligosaccharide products released after PNGaseF treatment of the monoclonal antibodies by MS/MS analysis. We established that after 6 hr transfer of galactose by the wild type b4al-T1 to G0 glycoform (MW 1485) of IgG, galactose is mainly transferred to one arm (G1 glycoform, MW 1647). Further MS/MS analysis of the bi-antennary glycan chain shows that the wild type b4Gal-T1transfers galactose at a faster rate to the GlcNAc attached to the Man 1-3 arm. In contrast, the mutant b4Gal-T1-Y289L at 6 hours incubation transfers GalNAc mainly to both arms (G2 glycoform MW 1891). MS/MS analysis of the G1 glycoform after PNGasesF treatment of the monoclonal antibodies show that the mutant Y289L- b4Gal-T1 transfers GalNAc at a faster rate to the GlcNAc attached to the Man 1-6 arm. Crystal structure of the h-M340H-Gal-T1 in complex with the disaccharide GlcNAc-b3Gal-b-O-napthalenemethanol : A class of synthetic disaccharides has been shown as potential inhibitors of tumor metastasis by Dr. Jeff Eskos lab in UC, San Diego. They are high affinity substrates for b4GalT1 and thus act as decoys for the synthesis of sialyl Lewis X (sLeX ), the cell adhesion epitope, which is expressed at elevated levels in metastatic cells. The most effective compound they have identified to date is GlcNAc-b3Gal-b-O-napthalenemethanol. The acetylated compound is taken up by the cells, O-deacetylated, and then the disaccharide decoys the synthesis of sLeX-containing glycans on cell surface glycoconjugates. In several model systems this results in an inhibition of tumor formation. As Dr. Eskos lab has observed, all the activity of the compound depends on the action of b4GalT(s) in the cell, since the first step in its utilization involves galactosylation. In their in vitro studies with GlcNAc-b3Gal-b-O-napthalenemethanol as an acceptor substrate for b4Gal-T1, they show a Km of 10 muM. This high affinity for the enzyme may be its unique mode of binding to b4Gal-T1 similar to the one we have observed with the 1,2-1,6-arm trisaccharide (see above). We have carried the crystal structure of h-M340H-Gal-T1 with GlcNAc-b3Gal-b-O-napthalenemethanol, provided by Dr. Eskos lab, and determined the mode of interaction between the disaccharide and the enzyme. The overall binding of GnGl-NP to the Met344His-Cys342Thr-Gal-T1 molecule is quite similar to the binding of the tri-saccharide GlcNAcb1-2Mana1-6Mana, observed earlier (see 2006 project # Z01 BC 010041), suggesting its Km for the enzyme will be similar to that of the 1-6 arm tri-saccharide, a Km of 60 muM. This tri-saccharide is derived from the 1-6 arm of the biantennary N-glycan, starting from the core mannose to the free GlcNAc at the non-reducing end. In the enzyme-bound 1-6 arm tri-saccharide, the middle mannose exhibits the least interactions with the protein molecule, while the terminal mannose (i.e., the core mannose residue of the biantennary N-glycans) makes extensive stacking interactions with the aromatic side chain of the Tyr282 residue.In contrast, the beta-linked Gal residue in the disaccharide GnGl-NP forms extensive stacking interactions with Tyr282, and the terminal aromatic naphthalene residue makes additional interactions, although weak, in the oligosaccharide binding site of the b4Gal-T1 molecule. Therefore, due to the additional interactions that arise from the naphthalene moiety, it is expected that GnGl-NP disaccharide will have a lower Km than that of the 1-6 arm tri-saccharide, thus making it the best known acceptor substrate with the highest affinity for b4Gal-T1. Furthermore, the b4Gal-T family members T5 and T6 have the conserved residue Tyr corresponding to the Phe356 residue of the b4Gal-T1. The Tyr residue in these family members is expected to make additional hydrogen bonding interactions via the side-chain hydroxyl group of the Tyr residue, which is expected to further lower the Km of the GnGl-NP disaccharide for b4Gal-T5 and b4Gal-T6. From the crystal structure analysis we are able to understand the high affinity of the disaccharide GnGl-NP for b4Gal-T1. Furthermore, these analysis suggested that future chemical modifications, such as incorporating the structural water molecule in the acceptor design, amino or methylated amino group at the second position of Gal or even appropriate substitutions of polar groups at the naphthalene ring, may improve the affinity of the acceptor substrate and lead to better design of the disaccharide inhibitors for the tumor metastasis. These results have been published in J. Biol. Chem. 2009, 284, 4952-4959.Crystal structure of the h-M340H-Gal-T1 in complex with the 1-thio-N-butyryl GlcN-beta-(2-naphthyl):Several compounds having a bicyclic structures namely 2-naphthyl aglycones have been found by Dr. Inka Brockhausen (Queens University, Kingston, Ontario, Canada), to bind and inhibit the activity of b4Gal-T1. The best small hydrophobic GlcNAc-analog inhibitor that they have found is 1-thio-N-butyral-GlcN-beta-(2-naphthyl) with a Ki of 40 muM. This high affinity for the enzyme may be its unique mode of binding to b4Gal-T1 similar to the one we have observed with the 1,2-1,6-arm trisaccharide or GlcNAc-b3Gal-b-O-napthalenemethanol, provided by Dr. Eskos lab. The crystal structure of this inhibitor in complex with Met344His-Cys342Thr-Gal-T1 molecule is being carried out to determine the mode of interaction between the inhibitor and the enzyme.