ARS | Publication request: A New Look at An Old Protein; Alpha-Zeins

Submitted to: United States Japan Natural Resources Protein Panel
Publication Type: Abstract
Publication Acceptance Date: May 30, 2002
Publication Date: December 1, 2002
Citation: MOMANY, F.A., SESSA, D.J., LAWTON JR, J.W., WILLETT, J.L. A NEW LOOK AT AN OLD PROTEIN; ALPHA-ZEINS. UNITED STATES JAPAN NATURAL RESOURCES PROTEIN PANEL. 2002. Abstract p. WW1-WW9.

Technical Abstract: Alpha-Zeins are water insoluble storage proteins found in corn granules. Analysis of the protein sequence using probability algorithms, circular dichroism and optical rotary dispersion results, and other published data, suggest that the 19kDa alpha-Zein has approximately 50 percent helical character, made up of nine helical segments of about 20 amino acids with glutamine-rich 'turns' or 'loops'. Small angle X-ray scattering and light scattering experiments have suggested that in alcohol/water mixtures alpha-zein exists as an oblong structure with an axial ration of approximately 6:1. Ultracentrifugation, birefringence, dielectric, and viscosity studies indicated that alpha-zein behaves as an asymmetric particle with axial ratios of from 7:1 to 28:1. The amino acid sequence of the 19kDa alpha-zein was re-examined and a new model found that fits the experimental data. Our analysis of the sequence data suggests that alpha-zein has coiled-coil tendencies resulting in alpha-helices with approximately 4 residues/turn in their central helical section. Coiled-coil helices usually create amphiphilic profiles with a polar face and a non-polar face, and are generally curved as one looks down the helix axis. In alpha-zein, there are no strong polar residues and one must consider only the non-polar residue side-chains as forming a hydrophobic face. The tendency for curvature of the helix segments allows these helices to fit together as triple helices with the non-polar faces interacting inside the core of the three chain complex. We have modeled the nine helical segments of the 19kDa protein into three sets of three interacting helices, with these segments positioned end to end. The resulting 3D-structure is rod like with dimensions of approximately 110 long and approximately 20 across. This complex structure lengthens with the addition of the N- and C-terminal sections, to give an axial ration of approximately 6 or 7:1 in agreement with recent experimental light scattering experiments. The natural carotenoid, lutein, is found to fit into the core of the triple helical segments and help stabilize the configuration. Molecular dynamics simulations with methanol molecules as solvent have been carried out to refine the 3D-structure.