Convergent evolution of similar function in two structurally divergent enzymes

An example of two related enzymes that catalyse similar reactions but possess different active sites is provided by comparing the structure of Escherichia coli thioredoxin reductase with glutathione reductase. Both are dimeric enzymes that catalyse the reduction of disulphides by pyridine nucleotides through an enzyme disulphide and a flavin. Human glutathione reductase contains four structural domains within each molecule: the flavin-adenine dinucleotide (FAD)- and nicotinamide-adenine dinucleotide phosphate (NADPH)-binding domains, the 'central' domain and the C-terminal domain that provides the dimer interface and part of the active site. Although both enzymes share the same catalytic mechanism and similar tertiary structures, their active sites do not resemble each other. We have determined the crystal structure of E. coli thioredoxin reductase at 2 A resolution, and show that thioredoxin reductase lacks the domain that provides the dimer interface in glutathione reductase, and forms a completely different dimeric structure. The catalytically active disulphides are located in different domains on opposite sides of the flavin ring system. This suggests that these enzymes diverged from an ancestral nucleotide-binding protein and acquired their disulphide reductase activities independently.     

机械设计课程的CAI

计算机辅助教学(CAI)是将计算机引入教学过程而产生的教学方式,它能辅助教师的教学工作.本文论述了CAI在机械设计课程教学中所起的重要作用.     

Interactive graphics mechanical optimum design

Virtually all conventional optimizations are Performed in a batch computer environment. No graphic information during the optimization process is provided. The research tactics and implementation procedure of interactive graphics in mechanical optimum design are presented. An interactive Graphics Mechanical Optimum Design Program(IGMODP) for microcomputers is developed. The example of wheeled loader's working device optimum design using IGMODP is carried out.     

机械运动仿真中的反向运动设计

根据3D Studio MAX的应用实践,探讨灵活运用反向运动设计,模拟多种形式的机械运动,以使仿真效果更贴近于现实。     

概念设计及机械产品计算机辅助概念设计过程分析

介绍了概念设计的定义、特点，并在其研究状况的基础上，建立了计算机辅助机械产品概念设计的过程，为概念设计的进一步研究打下了基础．     

Solid solution evolution during mechanical alloying in Cu-Nb-Al compounds

This work concerns the structural evolution of Cu₇₀Nb₂₀Al₁₀ (at%) alloy processed by mechanical alloying using a planetary ball mill in air atmosphere for different times (4 to 200 h). The morphological, structural, microstructural, and thermal behaviors of the alloy were investigated by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and differential scanning calorimetry. X-ray diffraction patterns were examined using the Rietveld refinement technique with the help of the MAUD software. A disordered FCC-Cu(Nb,Al) solid solution was formed after 8 h of milling. The crystallite size, microstrain, and lattice parameter were determined by the Rietveld method. With increasing milling time, the crystallite size of the final product-ternary-phase FCC-Cu(Nb,Al)-is refined to the nanometer scale, reaching 12 nm after 200 h. This crystallographic structure combines good mechanical strength and good ductility. An increase in microstrain and partial oxidation were also observed with increasing milling time.     

Microstructure evolution and mechanical properties of a hot-rolled Ti alloy

The microstructure evolution and mechanical properties of a hot-rolled Ti-5.1 Al-2.5 Cr-0.5 Fe-4.5 Mo-1.1 Sn-1.8 Zr-2.9 Zn titanium alloy sheet along the thickness direction were investigated.The results indicated that the hotrolled titanium alloy sheet presented different microstructures along the thickness direction owing to the uneven distribution of stress and temperature during the hot rolling.The grains in central region underwent a larger deformation,leading to relative complete grain fra...     

机械设计节能基本原理的分析与应用

为了得到机械设计时可遵循的节能基本原则,采用能量分析方法对功率方程中以周期函数表示的系统势能变化率、系统动能变化率、其他输出力消耗功率、无用功率进行逐项和综合分析,求证得到引起机器输入功率匹配较高和功率没有得到充分利用的原因与机械设计节能基本原理。分析表明,在设计机械系统时,只要使各种能量和功率分别为常量或在最小范围内变化,或者使系统能量和系统功率为常量或在最小范围内变化,则维持系统正常工作所需输入功率将会降低,系统能量将得到充分利用。通过工程实际应用,证明该原理具有较好的节能效果,适用于机电类节能产品设计。     

三维动画在机械设计中的应用

本文介绍了一种利用计算机多媒体三维动画技术(3DS MAX软件包)进行机械系统运动机构动态设计的方法.这种方法可使各组成构件的运动规律一目了然,它缩短了设计周期,可节省大量的人力、物力和财力.     

机械工程专业毕业设计改革思考

在分析了机械工程专业毕业设计中存在师资不足、过程管理繁琐等同题的基础上,提出引入毕业设计信息管理系统、青年教师培养、对学生强化培训等方法和手段来解决上述问题.     

Solid Edge在机械产品设计开发中的应用

本文指出了二维CAD软件在机械产品设计开发中的不足,介绍了SolidEdge的基本组成及各功能模块;阐述了个功能模块在机械产品设计中用途;指出SolidEdge的应用,能有效提高机械产品设计开发的质量和效率。     

空间相机主支撑结构设计与分析

本文针对某离轴三反空间相机研制任务,基于铝基复合材料——硼铝合金（TiB2/Al）,采用了适合中、小型空间相机的薄壁框架式主结构形式,进行了相机主支撑结构的设计。通过对整机进行有限元建模,以及固有模态、静力学、动力学、光机热集成分析,优化了相机结构设计,使得相机整机基频>85Hz,在?0℃均匀温度变化和1g重力耦合作用下,光学元件的面形精度仍符合光学设计要求。最终工程分析结果表明：相机的薄壁框架式主结构设计是合理的,整机结构一阶频率高,抗重力变形能力、抗振性能及抗热变形能力均能满足相机总体设计指标要求,与卫星平台的双脚架柔节机械接口能够有效抑制卫星平台的结构变形对相机主结构的扭曲作用。     

现代商业照明的产品设计初探

从现代商业照明的界定、特征、种类、产品现状等几个方面进行了阐述,进而探讨了现代商业照明对产品开发设计的要求,并进一步归结成现代商业照明的产品设计理念,最后指出了商业照明产品设计理念上的发展方向以及现代商业照明的产品发展趋势.现代商业照明的产品设计突破传统,简洁合理,动感美观,经典浪漫,前卫个性彰显,强调文化内涵,融合不同风格,追求品质灯具设计,营造健康光环境.     

现代商业照明的产品设计初探

从现代商业照明的界定、特征、种类、产品现状等几个方面进行了阐述,进而探讨了现代商业照明对产品开发设计的要求,并进一步归结成现代商业照明的产品设计理念,最后指出了商业照明产品设计理念上的发展方向以及现代商业照明的产品发展趋势.现代商业照明的产品设计突破传统,简洁合理,动感美观,经典浪漫,前卫个性彰显,强调文化内涵,融合不同风格,追求品质灯具设计,营造健康光环境.     

时变不确定性机械设计方法

针对机械零件的应力和强度在不确定因素作用下的时变性,基于含有时变和随机因素的时变可靠度计算模型,提出了包含时变和随机因素的时变不确定性机械设计方法. 该方法以当前时刻为时间分析起点,不但考虑应力和强度受到随机因素的影响,而且考虑了在某一时刻t应力和强度的分布,讨论了其求解方法. 该方法是基于时变可靠度计算模型的时变概率设计方法,它同样可以应用于其他领域如结构工程领域.     

Microstructural evolution and mechanical properties of nanostructured Cu-Al-Ni shape memory alloys

The melt spinning technique, with an applied cooling rate of about 106K/s, was used to produce a nanostructured Cu+13.2Al+ 5.1Ni (in wt%) shape memory alloy. The properties of nanostructured ribbons were then compared with those of conventional coarse structure. The microstructural evolution was characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. Microhardness measurements indicate a two-fold increase in hardness because of the produced nanostructure. Comparing to its coarse structure, the nanostructured Cu-Al-Ni shape memory alloy exhibited the enhanced mechanical properties including a ductility of 6.5% and a pronounced plateau in the stress-strain curve.     

MgO-C材料在氮气气氛下的物相演化及其力学性能研究

以电熔镁砂、天然鳞片石墨、煤沥青、Al粉及Si粉为主要原料,以热固性酚醛树脂为结合剂,混匀后压制成MgO-C材料试样。将试样在氮气气氛下分别经1000℃×3h、1200℃×3h、1400℃×3h热处理,研究热处理温度对材料物相组成、显微结构及力学性能的影响。结果表明,1000℃热处理后,试样中Al消失,反应生成了柱状AlN和八面体状MgAl2O4,此温度下Si尚未参与反应;1200℃热处理后,Si开始反应生成六边形板状的SiC,镶嵌在镁砂基体中,提高了试样的高温抗折强度和热震后残余抗折强度;1400℃热处理后,试样中除有柱状AlN和八面体状MgAl2O4生成外,还有较多晶须状SiC和针状β-Si3N4生成,形成了良好的非氧化物结合,使得材料具有优良的高温力学性能和抗热震性能。     

机械零件的可靠性优化设计

在分析传统设计方法与可靠性设计方法在设计思想、设计原理上的区别的基础上,将可靠性设计与优化设计结合起来,对机械零件的可靠性优化设计方法进行研究。在满足可靠度约束及结构几何约束的条件下,以机械零件具体情况最小作为优化目标函数进行可靠性优化设计,得到了比常规设计更为合理的设计结果。     

股骨近端髓内钉结构设计与试件力学检测研究

为解决髓内钉手术存在骨质丢失和螺钉旋转问题,本文基于人体髓腔解剖形态,进行髓内钉三维构型设计,建立髓内钉仿真模型,采用ANSYS进行应力应变仿真分析,对股骨近端髓内钉试制件进行静态压缩试验,分析载荷-位移曲线。有限元分析结果表明最大应力主要集中在螺旋刀片钉与主钉交界面处;静态压缩试验得到的应力最大位置与有限元分析结果相同,髓内钉承受的最大极限载荷高于股骨头承受的关节力。分析结果表明本文设计的股骨近端髓内钉的力学性能满足股骨生物学固定物的要求,可为髓内钉设计提供理论指导。