是什么使得一些类型的细菌对一种抗生素产生了耐药性？美国圣·犹大儿童医院的一项新研究表明，这个现象背后的一个令人惊讶的研究结果为人们了解儿童所患的一种罕见的大脑退化疾病——泛酸激酶相关神经退化症（PKAN）提供了重要线索。通过对泛酸激酶的三维结构进行研究，研究人员将抗生素耐药性与这种大脑疾病联系在一起。该研究的结果刊登在8月16日出版的《结构》（Structure）杂志上。
泛酸激酶能触发辅酶A（CoA）产生过程的第一步，而CoA则是所有生命体不可或缺的一种重要分子。CoA在细胞从脂肪酸和糖获得能量的能力中起到关键作用。细菌也需要CoA来形成它们的细胞壁。泛酸激酶的职责就是分别“抓”住一种泛酸（维生素B5）和另外一种含有磷酸基团的分子，接着该酶将磷酸基团移走并连接到泛酸上。在人类中，这种酶产生的某些突变能妨碍将磷酸连接到泛酸上的能力，从而减少CoA的制造并导致泛酸激酶相关神经退化症。
研究人员已经知道一类叫做泛酸氨基化物（pantothenamides）的抗生素能够模拟维生素B5并进入这种酶中，从而抑制细菌产生脂肪酸的能力。不同类型的细菌会形成自己独特版本的泛酸激酶，如I、II、III型。
这个研究组此前还确定出I型泛酸激酶的结构和功能。他们想知道结构差异很大的II型和III型泛酸激酶如何行使相同的功能，为什么具有III型酶的细菌会对pantothenamide抗生素产生耐药性。此外，研究组还希望通过比较细菌的泛酸激酶和人类的泛酸激酶来更好地了解人类患PKAN的病因。研究人员利用X射线晶体学技术获得了II型和III型泛酸激酶以及它们与泛酸和ATP（提供磷酸基团的能量分子）相互作用时的三维图像。
首先，研究人员结晶了这种酶的样品并用X射线轰击该结晶晶体；然后，他们利用产生的光衍射模式构建出不同类型泛酸激酶扭转、折叠氨基酸链的计算机三维模型，以及它们与其他分子相互作用的模型。
当研究这些图像时，研究人员意识到构成每种类型酶的单体（亚基）是由差异很大的氨基酸链构成的，但是它们却折叠成几乎相同结构的三维单体构型。这一发现令研究人员非常惊讶，因为每个链上的不同氨基酸具有不同的尺寸和生化特征，因此它们应该是不可能形成相同的三维结构的。
这个现象还不止于此。接下来的分析显示，每对相同形状的单体能以一种新颖的方式相互结合，从而形成同一个酶的两个版本，虽然它们看起来不像，但却行使相同的功能。研究人员分析说，两种类型酶的编码基因是由一个共同的祖先基因进化而来。这个共同的基因发生进化并形成最终的II型和III酶结构，虽然外形和工作方式不同，但起到的功能却相同。
这些三维图像揭示出两种类型的泛酸激酶如何以不同的方式执行相同的任务，而且还表明金黄葡萄球菌中的II型泛酸酶在它的氨基酸链的环和扭转处有一个空出来的“洞穴”——正是这个“洞”让pantothenamide抗生素滑进酶的内部。但是，假单胞菌的III型酶则没有这个洞，因此抗生素不能进入酶中，正是这一结构使假单胞菌对这类抗生素产生了抗药性。
英文原文：
A 3-D Link between Antibiotic Resistance and Brain Disease
The story of what makes certain types of bacteria resistant to a specific antibiotic has a sub-plot that gives insight into the cause of a rare form of brain degeneration among children, according to investigators at St. Jude Children's Research Hospital. The story takes a twist as key differences among the structures of its main molecular characters disappear and reappear as they are assembled in the cell.
The story is based on a study of the three-dimensional (3-D) structure of an enzyme called pantothenate kinase, which triggers the first step in the production coenzyme A (CoA), a molecule that is indispensable to all forms of life. Enzymes are proteins that speed up biochemical reactions.
CoA plays a pivotal role in the cells' ability to extract energy from fatty acids and carbohydrates; bacteria need CoA to make their cell walls. The job of pantothenate kinase is to grab a molecule of pantothenic acid (vitamin B-5) and another molecule that contains a chemical group called "phosphate." The enzyme then removes the phosphate group from that molecule and sticks it onto pantothenic acid.
In humans, certain mutations in this enzyme block its ability to put the phosphate group onto pantothenic acid. That diminishes the production of CoA by this route and causes the neurodegenerative disease called pantothenate kinase associated neurodegeneration (PKAN). Certain antibiotics, called pantothenamides, work by impersonating vitamin B-5 and slipping into the enzyme. This blocks the bacteria's ability to produce fatty acids.
The researchers already knew that different types of bacteria build their own versions of the enzyme pantothenate kinase, which are called Types I, II and III. For example, bacteria called Escherichia coli, found in the intestines and polluted water, produce Type I; Staphylococcus aureus, which causes skin infections and serious blood infections, makes type II; and Pseudomonas aeruginosa, which is an important cause of hospital-based infections, especially in burn patients, makes Type III. Types I, II and III each consist of two identical molecules called monomers, which bind together to form the enzyme.
The groups had previously identified the structure and role of the Type I enzymes in pantothenamide inhibition of bacterial growth. What intrigued the St. Jude investigators now was the mystery of how Types II and III manage to do the same job even though they are constructed so differently; and why bacteria with the Type III enzyme are resistant to pantothenamide antibiotics. They also wanted to better understand the cause of PKAN in humans by comparing bacterial pantothenate kinase with the various types found in humans.
Like all proteins, these enzymes are made up of long chains of amino acids, like beads on a string, and each type of amino acid has a unique shape and size. The pantothenate kinase enzymes consist of two strands of amino acids that fold into various twists and turns to make a complex 3-D structure. These modules, called monomers, snap together to form the enzyme. The researchers used x-ray crystallography to produce 3-D images of Types II and III and their interactions with panthothenic acid and ATP, a molecule that supplies the phosphate that the enzyme puts onto pantothenic acid.
First, the researchers crystallized a sample of the enzyme and bombarded it with x-rays using the Advanced Photon Source beamline facilities at Argonne. Then they used the pattern formed by the beams as they bounced off the crystals to create computer-generated, 3-D images of the patterns of twisting and folding amino acid chains that make up the different types of pantothenate kinase and their interactions with the other molecules.
The images added a fascinating twist to the story of the enzymes, according to the researchers. When they studied the images, the St. Jude team realized that the monomers making up each type of enzyme were made from quite different "strings" of amino acids. But they fold up into virtually identical looking 3-D monomers. It was as if the uniqueness of each structure disappeared--each string folded up into the same shape as the other ones. The group found this to be very surprising, because the different amino acids on each string have different sizes and different biochemical characteristics. So it would usually be impossible for them to form the same three-dimensional shapes.
