Damaschun G, Damaschun H, Gast K, Zirwer D
Humboldt-Universität zu Berlin, Institut für Biologie, c/o Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Strasse 10, Berlin, PF 740238, D-13092, Germany.
J Mol Biol. 1999 Aug 20;291(3):715-25. doi: 10.1006/jmbi.1999.3009.
The three-dimensional structure of a protein is determined by interactions between its amino acids and by interactions of the amino acids with molecules of the environment. The great influence of the latter interactions is demonstrated for the enzyme phosphoglycerate kinase from yeast (PGK). In the native state, PGK is a compact, bilobal molecule; 35% and 13% of its amino acids are organised in the form of alpha-helices and beta-sheets, respectively. The molecules unfold at acidic pH and low ionic strength forming random-walk structures with a persistence length of 3 nm. More than 90% of the amino acid residues of the ensemble have phi,psi-angles corresponding to those of a straight beta-chain. Upon addition of 50% (v/v) trifluoroethanol to the acid-unfolded protein, the entire molecule is transformed into a rod-like, flexible alpha-helix. Addition of anions, such as chloride or trichloroacetate, to the acid-unfolded protein leads to the formation of amyloid-like fibres over a period of many hours when the anion concentration exceeds a critical limit. Half of the amino acid residues are then organised in beta-sheets. Both of the non-natively folded states of PGK contain more regular secondary structure than the native one. The misfolding starts in both cases from the acid-unfolded state, in which the molecules are essentially more expanded than in other denatured states, e.g. those effected by temperature or guanidine hydrochloride.
蛋白质的三维结构由其氨基酸之间的相互作用以及氨基酸与周围环境分子的相互作用所决定。酵母磷酸甘油酸激酶（PGK）就证明了后一种相互作用的巨大影响。在天然状态下，PGK是一个紧密的双叶分子；其氨基酸分别有35%和13%以α螺旋和β折叠的形式排列。分子在酸性pH值和低离子强度下展开，形成持续长度为3纳米的随机游走结构。该集合中超过90%的氨基酸残基具有与直链β链相对应的φ、ψ角。向酸变性的蛋白质中加入50%（v/v）的三氟乙醇后，整个分子会转变为棒状的柔性α螺旋。当阴离子浓度超过临界限度时，向酸变性的蛋白质中加入氯离子或三氯乙酸根等阴离子，经过数小时会导致形成淀粉样纤维。此时一半的氨基酸残基会排列成β折叠。PGK的这两种非天然折叠状态都比天然状态含有更多规则的二级结构。这两种情况下的错误折叠都始于酸变性状态，在这种状态下，分子比在其他变性状态（如受温度或盐酸胍影响的状态）下本质上更加伸展。
