Kelly J A, Sielecki A R, Sykes B D, James M N, Phillips D C
Nature. 1979;282(5741):875-8. doi: 10.1038/282875a0.
Hen egg white lysozyme was the first enzyme whose structure was determined by X-ray crystallography. The proposed mechanism based on this structure involves the distortion of the saccharide residue (2-acetamido-2-deoxy-D-muramic acid, NAM) in the natural substrate (an alternating beta (1 leads to 4) linked oligomer of 2-acetamido-2-deoxy-D-glucose (NAG) and NAM residues) bound to site D in the binding cleft. The importance of substrate distortion has prompted numerous enzymatic, chemical, theoretical, and physical studies, but there is little direct crystallographic evidence on the conformation of a NAM residue bound at site D. We now present the X-ray structure of the non-hydrolysed trisaccharide NAM-NAG-NAM bound in subsites B, C, D. Our interpretation of the 2.5-A resolution difference map does not involve distortion of this residue in site D. Comparison with the structure of the delta-lactone derived from tetra N-acetylchitotetraose (NAG)3NAL) bound to lysozyme suggests we may be looking at a Michaelis complex.
鸡蛋清溶菌酶是首个其结构由X射线晶体学确定的酶。基于该结构提出的机制涉及天然底物（2-乙酰氨基-2-脱氧-D-葡萄糖（NAG）和2-乙酰氨基-2-脱氧-D- Muramic酸（NAM）残基以β(1→4)交替连接的寡聚物）中糖残基（2-乙酰氨基-2-脱氧-D- Muramic酸，NAM）在结合裂隙中与位点D结合时的扭曲。底物扭曲的重要性促使了大量的酶学、化学、理论和物理研究，但关于在位点D结合的NAM残基构象几乎没有直接的晶体学证据。我们现在展示了结合在亚位点B、C、D中的非水解三糖NAM-NAG-NAM的X射线结构。我们对2.5埃分辨率差异图的解释并不涉及该残基在位点D中的扭曲。与结合到溶菌酶的源自四-N-乙酰壳四糖（NAG）3NAL的δ-内酯结构的比较表明，我们可能看到的是一个米氏复合物。
