Composition, Secondary Structure, and Self-Assembly of Oat Protein Isolate

Abstract

The amino acid compositions, secondary structure, and self-assembly of oat protein isolate (OPI), which was purified from the high-protein Chinese oat, have been investigated by using a combination of amino acid analysis, Fourier transform infrared spectroscopy (FTIR), and tapping mode atomic force microscopy (TP-AFM). OPI, with molecular weights ranging from 14.0 kDa to 66.0 kDa, was rich in essential amino acids and contained 24.7% glutamic acid and 8.1% leucine. The amino acid contents of OPI are 4.5-8.7 times higher than those of oat flour. The secondary structures of OPI have been quantified by the deconvolution of the amide I band of the FTIR spectrum of OPI, which were found to contain approximately 7% beta-turn, 19% alpha-helix, and 74% beta-sheet. Tapping mode AFM results further suggest that the oat protein isolate has two major types of shapes, ellipsoidal and disk-like. At protein concentrations below 0.5 mg/mL, most of the OPI molecules are in the isolated form. However, when the concentration of OPI reaches 1.0 mg/mL, some of the OPI molecules self-assembled into large and heterogeneous protein aggregates.

Adsorption of oat proteins to air–water interface in relation to their colloidal state

“…At pH 9.0 however, the interface was covered more homogeneously with individual particles. The size of observed particles is in accordance with the reported diameter of 12S oat globulins which was also measured by AFM to be 12.1 nm in diameter (Liu et al, 2009). By using quasielastic light scattering, Zhao, Mine, and Ma (2004) were able to predict the shape of the 12S oat globulin hexamer as an oblate cylinder with a height of about 8.5 nm formed from two annular trimeric rings, with a diameter of 11.8 nm, placed on top of each other.…”

“…Oat protein isolate (OPI) was produced from oat protein concentrate (OPC) powder by alkaline extraction followed by isoelectric precipitation as described in Liu et al (2009). OPC was obtained from supercritical CO 2 extracted oat grits as described in Sibakov et al (2011).…”

“…Two main bands were detected at around 20 kDa and 30 kDa which belong to basic (b) and acidic (a) polypeptides of the oat globulins, respectively (Chang, Alli, Konishi, & Ziomek, 2011;Lasztity, 1996). Liu et al (2009) had previously shown that oat protein isolate prepared by alkaline extraction was mainly (~80%) comprised of globulins. The band around 15 kDa may belong to oat albumins and/or oat prolamins as both proteins were shown to have a fraction around 15 kDa together with fractions of higher molecular weights (Liu et al, 2009;Ma & Harwalkar, 1984).…”

“…Liu et al (2009) had previously shown that oat protein isolate prepared by alkaline extraction was mainly (~80%) comprised of globulins. The band around 15 kDa may belong to oat albumins and/or oat prolamins as both proteins were shown to have a fraction around 15 kDa together with fractions of higher molecular weights (Liu et al, 2009;Ma & Harwalkar, 1984). In non-reducing conditions, the single band around 50 kDa belongs to a subunit of globulins which comprises of a-and b-polypeptides linked with one disulphide bond (~54 kDa).…”

Oat protein solubility and emulsion properties improved by enzymatic deamidation

“…The combined effects of the increase in the protein structural flexibility and the increase in the protein net charge might be assumed as the mechanism for the improvement in oat protein water solubility by PG deamidation. According to Liu et al (2009), the self-assembly of oat proteins was responsible for causing the association of neighboring proteins and, hence, resulted in forming large protein aggregates in an aqueous solution at neutral pH and having low salt concentration. The PG deamidation increased the electronic repulsion forces among the oat protein molecules by introducing negatively charged carbonyl side chains to the oat proteins.…”