STRUCTURAL CHARACTERIZATION OF A-ZEIN

Authors: Momany, Frank; Sessa, David; Lawton Jr, John; Selling, Gordon; Hamaker, Sharon; Willett, Julious

Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2005
Publication Date: 1/1/2006
Citation: Momany, F.A., Sessa, D.J., Lawton Jr, J.W., Selling, G.W., Hamaker, S.A., Willett, J.L. 2005. Structural characterization of a-zein. Journal of Agriculture and Food Chemistry. 54:p.543-547.

Interpretive Summary: The storage proteins of maize seeds are alcohol-soluble groups that are collectively known as zein and account for 50% or more of the total endosperm protein. Their only known function is to store nitrogen for the developing seed. In this work a 3-dimensional structure of alpha-zein is created through modeling and molecular mechanics methods. The model consists of coiled-coil segments combined into super-helical segments composed of three chains, and three different segments of super-helices are attached to form an elongated rod-like structure which is consistent with known experimentally derived properties. Knowledge of the structure of zeins is important since this protein is extracted during the conversion of starch to ethanol and the creation of value added products from the remaining protein is desirable.

Technical Abstract: A variety of physical measurements, computational algorithms, and structural modeling methods, have been used to create a molecular model of 19 kDa a-zein. Zeins are water insoluble storage proteins found in corn protein bodies. Analysis of the protein sequence using probability algorithms, structural studies by circular dichroism, infrared spectroscopy, small angle x-ray scattering (SAXS), light scattering, proton exchange, NMR, and optical rotatory dispersion experiments, suggest that the 19 kDa a-Zein (Z19) has less than 35-60% helical character, made up of nine helical segments of about 20 amino acids with glutamine-rich 'turns' or 'loops'. SAXS and light scattering experiments suggest that in alcohol/water mixtures a-zein exists as an oblong structure with an axial ratio of less than 6:1. Further, ultracentifugation, birefringence, dielectric, and viscosity studies indicate that a-zein behaves as an asymmetric particle with axial ratio from 7:1 to 28:1. Published models of a-zein to date have not been consistent with the experimental data, and for this reason we reexamined the structure using molecular mechanics and dynamics simulations creating a new 3D-structure for Z19. From the amino acid sequence and probability algorithms our analysis suggested that a-zein has coiled tendencies resulting in a-helices with less than four residues per turn in the central helical sections with the nonpoplar residue side-chains forming in hydrophobic face inside a triple super-helix. The nine helical segments of the 19 kDa protein were modeled into three sets of three interacting coiled-coil helices with segments positioned end to end. The resulting structure lengthens with the addition of the N- and C-terminal sections, to give an axial ratio of less than 6 or 7:1 in agreement with recent 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 explicit methanol/water molecules as solvent have been carried out to refine the 3D-structure.