Schiffer M, Stevens F J, Westholm F A, Kim S S, Carlson R D
Biochemistry. 1982 Jun 8;21(12):2874-8. doi: 10.1021/bi00541a011.
Immunoglobulins fragments are composed of globular domains linked by extended polypeptide segments. The molecular flexibility inherent in this arrangement allows for significant potential differences between structures observed in the crystalline state and those attained in solution. Small-angle neutron scattering measurements in dilute solution were performed on the Mcg Bence-Jones protein dimer, for which performed on the Mcg Bence-Jones protein dimer, for which accurate atomic coordinates have been determined by crystallographic methods [Edmundson, A. B., Ely, K. R., Abola, E. E., Schiffer, M., & Panagiotopoulos, N. (1975) Biochemistry 14, 3953-3961; Schiffer, M. (1980) Biophys. J. 32, 230-232]. The measured radius of gyration (Rg) in H2O buffer is 24.0 +/- 0.4 A and in D2O buffer is 23.3 +/- 0.1 A; the calculated value of Rv (Rg in vacuo) is 24.0 A. The above values compare well with the calculated Rg value of 23.6 A when refined coordinates of the trigonal crystal form of the Mcg Bence-Jones protein are used. On the basis of a match point of 44.2% D2O concentration, the experimental partial specific volume is 0.74 cm3/g. The experimentally derived molecular weight of 47 000 is in very good agreement with that (45 500) calculated from the amino acid composition. For comparison with different Fab's (antigen binding fragments) exhibiting various "elbow bends" due to the flexibility of the switch peptide between variable and constant domains of the immunoglobulin chains, calculation of the Rg value of the Mcg dimer was performed as a function of the elbow bend. The Rg varied from 22.8 to 26.0 A as the elbow bend was opened from 100 degrees to 180 degrees; the maximum radius of gyration of the particle was 26.5 A with the switch peptide stretched by separating the variable and constant domains by an additional 1.5 A at an elbow bend of 180 degrees.
免疫球蛋白片段由通过延伸多肽链段连接的球状结构域组成。这种排列所固有的分子柔韧性使得晶体状态下观察到的结构与溶液中达到的结构之间存在显著的潜在差异。对Mcg本斯-琼斯蛋白二聚体进行了稀溶液中的小角中子散射测量，其精确的原子坐标已通过晶体学方法确定[埃德蒙森，A.B.，伊利，K.R.，阿博拉，E.E.，希弗，M.，&帕纳吉奥opoulos，N.（1975年）《生物化学》14卷，3953 - 3961页；希弗，M.（1980年）《生物物理杂志》32卷，230 - 232页]。在H₂O缓冲液中测得的回转半径（Rg）为24.0 ± 0.4 Å，在D₂O缓冲液中为23.3 ± 0.1 Å；Rv（真空中的Rg）的计算值为24.0 Å。当使用Mcg本斯-琼斯蛋白三角晶体形式的精修坐标时，上述值与计算得到的23.6 Å的Rg值相当吻合。基于44.2% D₂O浓度的匹配点，实验偏比容为0.74 cm³/g。实验得出的分子量为47000，与根据氨基酸组成计算得到的分子量（45500）非常吻合。为了与由于免疫球蛋白链可变区和恒定区之间的开关肽柔韧性而表现出各种“肘部弯曲”的不同Fab（抗原结合片段）进行比较，作为肘部弯曲的函数对Mcg二聚体的Rg值进行了计算。随着肘部弯曲从100°打开到180°，Rg从22.8 Å变化到26.0 Å；在180°的肘部弯曲处，通过将可变区和恒定区分开额外的1.5 Å来拉伸开关肽时，粒子的最大回转半径为26.5 Å。
