是什么使得一些類型的細(xì)菌對(duì)一種抗生素產(chǎn)生了耐藥性,？美國圣·猶大兒童醫(yī)院的一項(xiàng)新研究表明,，這個(gè)現(xiàn)象背后的一個(gè)令人驚訝的研究結(jié)果為人們了解兒童所患的一種罕見的大腦退化疾病——泛酸激酶相關(guān)神經(jīng)退化癥（PKAN）提供了重要線索。通過對(duì)泛酸激酶的三維結(jié)構(gòu)進(jìn)行研究,，研究人員將抗生素耐藥性與這種大腦疾病聯(lián)系在一起,。該研究的結(jié)果刊登在8月16日出版的《結(jié)構(gòu)》（Structure）雜志上。
泛酸激酶能觸發(fā)輔酶A（CoA）產(chǎn)生過程的第一步,，而CoA則是所有生命體不可或缺的一種重要分子,。CoA在細(xì)胞從脂肪酸和糖獲得能量的能力中起到關(guān)鍵作用。細(xì)菌也需要CoA來形成它們的細(xì)胞壁,。泛酸激酶的職責(zé)就是分別“抓”住一種泛酸（維生素B5）和另外一種含有磷酸基團(tuán)的分子,，接著該酶將磷酸基團(tuán)移走并連接到泛酸上。在人類中,，這種酶產(chǎn)生的某些突變能妨礙將磷酸連接到泛酸上的能力,，從而減少CoA的制造并導(dǎo)致泛酸激酶相關(guān)神經(jīng)退化癥。
研究人員已經(jīng)知道一類叫做泛酸氨基化物（pantothenamides）的抗生素能夠模擬維生素B5并進(jìn)入這種酶中,，從而抑制細(xì)菌產(chǎn)生脂肪酸的能力,。不同類型的細(xì)菌會(huì)形成自己獨(dú)特版本的泛酸激酶，如I,、II,、III型。
這個(gè)研究組此前還確定出I型泛酸激酶的結(jié)構(gòu)和功能,。他們想知道結(jié)構(gòu)差異很大的II型和III型泛酸激酶如何行使相同的功能,，為什么具有III型酶的細(xì)菌會(huì)對(duì)pantothenamide抗生素產(chǎn)生耐藥性。此外,，研究組還希望通過比較細(xì)菌的泛酸激酶和人類的泛酸激酶來更好地了解人類患PKAN的病因,。研究人員利用X射線晶體學(xué)技術(shù)獲得了II型和III型泛酸激酶以及它們與泛酸和ATP（提供磷酸基團(tuán)的能量分子）相互作用時(shí)的三維圖像。
首先,，研究人員結(jié)晶了這種酶的樣品并用X射線轟擊該結(jié)晶晶體,；然后，他們利用產(chǎn)生的光衍射模式構(gòu)建出不同類型泛酸激酶扭轉(zhuǎn),、折疊氨基酸鏈的計(jì)算機(jī)三維模型,，以及它們與其他分子相互作用的模型。
當(dāng)研究這些圖像時(shí)，研究人員意識(shí)到構(gòu)成每種類型酶的單體（亞基）是由差異很大的氨基酸鏈構(gòu)成的,，但是它們卻折疊成幾乎相同結(jié)構(gòu)的三維單體構(gòu)型,。這一發(fā)現(xiàn)令研究人員非常驚訝，因?yàn)槊總€(gè)鏈上的不同氨基酸具有不同的尺寸和生化特征,，因此它們應(yīng)該是不可能形成相同的三維結(jié)構(gòu)的,。
這個(gè)現(xiàn)象還不止于此。接下來的分析顯示,，每對(duì)相同形狀的單體能以一種新穎的方式相互結(jié)合,，從而形成同一個(gè)酶的兩個(gè)版本，雖然它們看起來不像,，但卻行使相同的功能。研究人員分析說,，兩種類型酶的編碼基因是由一個(gè)共同的祖先基因進(jìn)化而來,。這個(gè)共同的基因發(fā)生進(jìn)化并形成最終的II型和III酶結(jié)構(gòu)，雖然外形和工作方式不同,，但起到的功能卻相同,。
這些三維圖像揭示出兩種類型的泛酸激酶如何以不同的方式執(zhí)行相同的任務(wù)，而且還表明金黃葡萄球菌中的II型泛酸酶在它的氨基酸鏈的環(huán)和扭轉(zhuǎn)處有一個(gè)空出來的“洞穴”——正是這個(gè)“洞”讓pantothenamide抗生素滑進(jìn)酶的內(nèi)部,。但是,，假單胞菌的III型酶則沒有這個(gè)洞，因此抗生素不能進(jìn)入酶中,，正是這一結(jié)構(gòu)使假單胞菌對(duì)這類抗生素產(chǎn)生了抗藥性,。
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.
