A study of the structure of human complement component factor H by Fourier transform infrared spectroscopy and secondary structure averaging methods

Abstract

Fourier transform infrared spectroscopy was used to investigate the secondary structure of human complement component factor H in H2O and 2H2O buffers. The spectra show a broad amide I band which after second-derivative calculations is shown to be composed of three components at 1645, 1663, and 1685 cm-1 in H2O and at 1638, 1661, and 1680 cm-1 in 2H2O. The frequencies of these components are consistent with the existence of an extensive antiparallel .beta.-strand secondary structure. The exchange properties of the amide protons of factor H as measured in 2H2O buffers are rapid and lead to an estimate of NH proton nonexchange that is comparable with those for small globular proteins. Human factor H is constructed from a linear sequence of 20 short consensus repeats with a mean of 61 residues in each one. To investigate the secondary structure further, secondary structure predictions were carried out on the basis of an alignment scheme for 101 sequences for these repeats as found in human factor H and 12 other proteins. These predictions were averaged in order to improve the reliability of the calculations. Both the Robson and the Chou-Fasman methods indicate significant .beta.-structural contents. Residues 21-51 in the 61-residue repeat show a clear prediction of four strands of .beta.-structure and four .beta.-turns. A structural model based on antiparallel .beta.-strands in the secondary structure is proposed and discussed. This is able to account for the pattern of hydrophobic and hydrophilic residues in the repeats, the location of strongly conserved residues and some insertion sites, and the length of 4.5 nm of the 61-residue repeat as determined from previous electron microscopy and synchrotron X-ray scattering studies.