Musick W D, Rossmann M G
J Biol Chem. 1979 Aug 25;254(16):7611-20. doi: 10.2210/pdb1ldx/pdb.
The structure of lactate dehydrogenase isoenzyme C4 from mouse testes was solved at 2.9 A resolution using the technique of molecular replacement. The electron density map revealed a ternary-like configuration of the flexible loop peptide although density corresponding to the coenzyme and substrate molecules was not present. Apparently the apo-lactate dehydrogenase molecule in solution is in a dynamic equilibrium between the O (loop open as found in dogfish apo-lactate dehydrogenase M4) and C (loop closed as found in a variety of ternary complexes) conformations. During crystallization of the apoenzyme one or the other conformers is selected. The apparent stability of the closed conformation for the apo-lactate dehydrogenase C4 molecule may in part explain the low catalytic turnover number of the C isoenzyme. A possible substitution of an arginine residue at position 30 may also be a contributing factor as well as allowing NADP to act as coenzyme.
利用分子置换技术，以2.9埃的分辨率解析了来自小鼠睾丸的乳酸脱氢酶同工酶C4的结构。电子密度图显示了柔性环肽的类三元结构，尽管不存在与辅酶和底物分子相对应的密度。显然，溶液中的脱辅基乳酸脱氢酶分子处于O（如在角鲨脱辅基乳酸脱氢酶M4中发现的环开放）和C（如在各种三元复合物中发现的环闭合）构象之间的动态平衡中。在脱辅基酶结晶过程中，选择了其中一种构象。脱辅基乳酸脱氢酶C4分子闭合构象的明显稳定性可能部分解释了C同工酶低催化周转率的原因。30位精氨酸残基的可能替代也可能是一个促成因素，同时允许NADP作为辅酶起作用。
