Explore the intricate world of rice proteins—albumins, globulins, glutelins, and more. Learn their structures, types, and transformative effects. Dive in now!

Rice proteins exhibit a diverse array, primarily classified based on their solubility properties. The proteins extracted from rice or rice syrup using water are referred to as albumins, while those obtained from the residue using a dilute salt solution are termed globulins. Proteins extracted with 75% ethanol are known as alcohol-soluble proteins, and those remaining in the residue, soluble only in acid or alkali, are collectively referred to as glutelins.

Glutelins and alcohol-soluble proteins, also known as storage proteins, constitute the major protein components in rice. Glutelins constitute over 80% of total proteins, while alcohol-soluble proteins make up around 10%. In contrast, albumins and globulins are present in minimal amounts, serving as physiologically active proteins during the early stages of rice germination.

Distinct amino acid compositions characterize different proteins. Albumins contain a higher proportion of non-polar hydrophobic amino acids without a charge, and lower levels of acidic amino acids. Globulins, on the other hand, exhibit an elevated content of basic amino acids, exceeding 15%, while alcohol-soluble proteins have only about half the alkaline amino acid content of globulins. However, alcohol-soluble proteins have significantly higher hydrophobic amino acid content than other protein types.

The solubility of proteins is not only influenced by their amino acid composition but also by their structural form. Research indicates the presence of two protein aggregates in the endosperm, namely PB-I and PB-II. PB-I appears as a layered structure with dense particles of 0.5–2 μm, containing alcohol-soluble proteins. In contrast, PB-II is spherical, uniform in texture, with a diameter of approximately 4 μm, encompassing glutelins and globulins. Both aggregates often coexist.

During rice germination, these protein aggregates disintegrate, yet their digestibility differs significantly. PB-I, lacking a dense core, is more susceptible to hydrolysis, while PB-I maintains its layered structure even after 9 days of germination. SDS-PAGE analysis reveals continuous appearance of new protein bands in PB-II, signifying ongoing protein synthesis, whereas PB-I components remain stable, indicating metabolic differences in their protein molecules.

Rice proteins exhibit a high cysteine content with numerous intra- and intermolecular disulfide bonds. These bonds contribute to the formation of dense protein molecules and may play a crucial role in protein polymerization. Polyacrylamide gel electrophoresis (PAGE) analysis shows protein components in PB-I, ranging from 64 to over 2000 kDa. Molecular biology studies suggest that the initial synthesis of storage proteins in rice yields a 57 kDa protein, which subsequently cleaves into 22 kDa and 37 kDa subunits. Glutenin, comprising proteins of varying sizes, is formed by the assembly of these subunits via disulfide bonds.

While albumins also contain high molecular weight protein components, the low cysteine content in albumins prevents the formation of disulfide bonds, making them more soluble in water. This underscores the crucial role of disulfide bonds in stabilizing protein aggregates.

Analysis of the amino acid composition of rice proteins reveals that certain proteins are not simple amino acid-based proteins but rather contain sugar (raffinose) or lipid components. These non-amino acid constituents not only influence the properties of proteins but also confer unique physiological functions.

Furthermore, extensive research indicates that the types of proteins in rice are not fixed. During the aging process of rice, although the total protein content remains constant, structural changes occur, affecting the rheological properties of rice. Notable changes include an increase in disulfide bond quantity, larger protein molecular weights, denser protein aggregates, and a more compact network structure between proteins and starch. Consequently, water absorption, swelling, and tenderness of starch granules decrease, leading to a reduction in rice stickiness and an increase in hardness. Introducing a suitable reducing agent to disrupt disulfide bonds during this stage enhances rice stickiness.

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It is evident that the development and utilization of rice proteins necessitate careful consideration of the impact of rice aging, heating, and oxidation and reduction of disulfide bonds on protein properties.

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