AB INITIO STRUCTURE DETERMINATION OF XYLAN ESTERASE

我们正在使用SP并行超级计算机来确定X射线 乙酰木聚糖酯酶（AXE）晶体结构的从头算研究 分阶段程序。 该过程是基于联合概率 测量的X射线强度与其未知值之间的分布 阶段。 总的问题是从全球角度来表达的， 约束最小的函数，其中未知的相位是 变量 该程序已作为计算机程序实施 一种名为Shake-and-Bake（SnB）的方法，最近在IBM 康奈尔大学的SP2。 尽管直接定相方法已经被常规地 用于解决100个或更少原子的结构，蛋白质结构 通常由其他方法确定。 然而，最近， 蝎毒素II的结构，其含有620 使用SnB程序常规地解析非氢原子。 斧子已经 一种含有1500个非氢原子的酶，其结构是 目前未知。 AXE的X射线强度数据测量为0.9 Cornell High Energy Synchrotron Source（CHESS） SnB在AXE结构测定中的成功应用 这将是高分子领域的一个重要进展。 结晶学 证明大的蛋白质结构可以 仅从强度数据确定，而不使用重原子 衍生品，将为进一步发展铺平道路， 计算机算法的优化。 当这种情况发生时，许多新 可以使用这些方法确定蛋白质结构 需要几天而不是几个月（或几年）才能产生结果。

We are using the SP parallel supercomputer to determine the X-ray crystal structure of acetyl xylan esterase (AXE) using a new ab initio phasing procedure. The procedure is based on the joint probability distribution between measured X-ray intensities and their unknown phases. The overall problem is expressed in terms of the global, constrained minimum for a function in which the unknown phases are variables. The procedure has been implemented as a computer program called Shake-and-Bake (SnB) which has recently been tested on the IBM SP2 at Cornell. Although direct phasing methods have been routinely used to solve structures with 100 or fewer atoms, protein structures are usually determine by other methods. Recently, however, the structure of androctonus scorpion toxin II, which contains 620 non-hydrogen atoms was routinely solved using the SnB program. AXE is an enzyme containing 1500 non-hydrogen atoms and its structure is currently unknown. X-ray intensity data for AXE were measured to 0.9 resolution at the Cornell High Energy Synchrotron Source (CHESS). Successful application of SnB in the structure determination of AXE will represent an important advance in the field of macromolecular crystallography. Proof that large protein structures can be determined from intensity data alone, without the use of heavy-atom derivatives, will pave the way for further development and optimization of the computer algorithms. When this happens, many new protein structures may be determined using these methods which will require days rather than months (or years) to produce results.