Heap Caroline J, Wang Yuqin, Pinheiro Teresa J T, Reading Steven A, Jennings Keith R, Dimmock Nigel J
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK.
J Gen Virol. 2005 Jun;86(Pt 6):1791-1800. doi: 10.1099/vir.0.80812-0.
The antibody-binding site, through which an antibody binds to its epitope, is a complex structure formed by the folding together of six complementarity-determining regions (CDRs). However, certain peptides derived from CDR sequences retain antibody specificity and function; these are know as microantibodies (MicroAbs). For example, the F58 MicroAb is a 17 residue, cyclized peptide (CDLIYYDYEEDYYFDYC) derived from CDR-H3 of F58, an IgG1 specific for the gp120 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). Both MicroAb and IgG recognize the same epitope in the V3 loop and, despite its small size, the MicroAb neutralizes the infectivity of HIV-1 IIIB only 32-fold less efficiently on a molar basis. The advantage of MicroAbs is that their small size facilitates structure-function analysis. Here, the F58 MicroAb was investigated using alanine scanning, mass spectroscopy and surface plasmon resonance. Neutralization of infectious IIIB was generally more sensitive to alanine substitution than binding to soluble gp120. There appeared to be a division of function within the MicroAb, with some residues involved in antigen binding (alanine substitution of 11D, 12Y or 13Y abrogated both binding and neutralization), whereas others were concerned solely with neutralization (substitution of 3L, 8Y or 14F abrogated neutralization, but not binding). The MicroAb is predominantly beta-sheet and has strong conformational constraints that are probably essential for activity. The MicroAb and soluble gp120 formed a 1 : 1 complex, with an association rate that was threefold greater than that with IgG and a faster dissociation rate. Its equilibrium dissociation constant is 37.5-fold greater than that of IgG, in line with neutralization data. This study demonstrates how MicroAbs can make a useful contribution to the understanding of antigen-antibody interactions.
抗体结合位点是抗体与抗原表位结合的部位，它是由六个互补决定区（CDR）折叠在一起形成的复杂结构。然而，某些源自CDR序列的肽段仍保留抗体的特异性和功能；这些被称为微型抗体（MicroAbs）。例如，F58微型抗体是一种由17个残基组成的环化肽（CDLIYYDYEEDYYFDYC），它源自F58的CDR-H3，F58是一种IgG1，特异性针对人类免疫缺陷病毒1型（HIV-1）的gp120包膜糖蛋白。微型抗体和IgG都识别V3环中的相同抗原表位，尽管其尺寸较小，但在摩尔基础上，微型抗体中和HIV-1 IIIB感染性的效率仅比IgG低32倍。微型抗体的优势在于其小尺寸便于进行结构-功能分析。在此，利用丙氨酸扫描、质谱和表面等离子体共振对F58微型抗体进行了研究。感染性IIIB的中和作用通常比与可溶性gp120的结合对丙氨酸取代更敏感。微型抗体内部似乎存在功能分工，一些残基参与抗原结合（11D、12Y或13Y的丙氨酸取代会消除结合和中和作用），而其他残基仅与中和作用有关（3L、8Y或14F的取代会消除中和作用，但不影响结合）。微型抗体主要由β-折叠组成，具有很强的构象限制，这可能对其活性至关重要。微型抗体和可溶性gp120形成1:1复合物，其结合速率比与IgG的结合速率高3倍，解离速率更快。其平衡解离常数比IgG的高37.5倍，与中和数据一致。这项研究证明了微型抗体如何能为理解抗原-抗体相互作用做出有益贡献。
