Bioscience Evidence 2025, Vol.15, No.5, 228-236 http://bioscipublisher.com/index.php/be 229 2 Gluten Proteins: Structure and Function 2.1 Overview of wheat seed storage proteins (prolamins) The main storage protein in wheat grains is prolamins, collectively known as gluten, which accounts for approximately 80% of the total protein in wheat. Gluten proteins are divided into two types: monomer gliadins and polymerized glutenins. These two types of proteins jointly determine the special processing properties of wheat dough (Biesiekierski, 2017; Wang et al., 2020; Wieser et al., 2022). 2.2 Molecular organization of gliadins and glutenins Alcohol-soluble proteins can be classified into α-, β-, γ- and ω- types based on molecular weight and electrophoretic characteristics, with molecular weights ranging from 30 000 to 80 000 Da. They contain a lot of glutamine and proline, mainly composed of repetitive sequences, lacking cysteine, and generally exist as monomers (Wang et al., 2020; Wieser et al., 2022). Gluten is classified into two types: high molecular weight (HMW-GS, 80 000-160 000 Da) and low molecular weight (LMW-GS, 30 000-51 000 Da). They are rich in cysteine and can form macromolecular polymers through disulfide bonds, forming the gluten network. Both types of proteins have central repetitive domains and non-repetitive N/ C-terminal domains. The gluten subunits also maintain the three-dimensional structural skeleton through disulfide bonds and hydrogen bonds, etc. (Shewry et al., 2002). 2.3 Their role in dough elasticity and baking performance The special structure of gluten protein endows wheat dough with excellent processing characteristics. Glutenin mainly provides viscosity and extensibility, while gluten determines the elasticity and strength of the dough. High-molecular-weight gluten forms a three-dimensional network through disulfide bonds, which is the key to elasticity. Low-molecular-weight gluten and gliadin regulate plasticity and adaptability (Shewry et al., 2002; Biesiekierski, 2017; Wang et al., 2020). The proportion and polymerization state of gluten protein will directly affect the volume, structure and taste of bread and noodles. 2.4 Immunogenic peptides responsible for triggering celiac disease (e.g., 33-mer in α-gliadin) Some gluten proteins, especially α -alcohol-soluble proteins, contain peptides with strong immunogenicity. One of the most typical is 33 - peptide (33 - killing, LQLQPFPQPQLPYPQPQLPYPQPQLPYPQPQPF). It is not easily broken down by digestive enzymes and can induce celiac disease in genetically susceptible populations (Biesiekierski, 2017; Schalk et al., 2017; Shewry, 2019; Wang et al., 2020). These peptides are rich in glutamine and proline and are easily modified by tissue transglutaminase, thereby better binding to HLA-DQ2/8 molecules, activating T cells and triggering immune responses, which is the molecular basis for the occurrence of celiac disease. 3 Conventional Breeding Approaches 3.1 Historical breeding attempts to reduce immunogenic proteins Traditional breeding methods, such as variety selection, hybridization, backcrossing and mutagenesis, have always been mainly used to increase the yield and processing quality of wheat. However, there is not much breeding work specifically aimed at reducing immunogenic proteins such as gliadin and glutenin. Research has found that there are significant differences among different wheat varieties and related species in terms of gluten protein content and the number of immunogenic epitopes. Hard-grain wheat and some wild species usually have lower immunogenicity (Ozuna and Barro, 2018; Pilolli et al., 2019; Marin-Sanz et al., 2023). In addition, mutagenesis breeding methods (such as gamma-ray and EMS mutagenesis) have also been used to obtain mutants lacking certain gluten genes. These mutants sometimes exhibit a decrease in immunogenic proteins (Jouanin et al., 2019; 2020). 3.2 Trade-offs between low-gluten content and technological properties Reducing gluten protein often causes problems. Because gluten is the key to forming the dough structure, reducing it will affect elasticity, extensibility and baking performance (Jouanin et al., 2019; Pilolli et al., 2019; Call et al., 2020; Jouanin et al., 2020). For instance, those mutants lacking alcohol-soluble protein or low-molecular-weight gluten, although having lower immunogenicity, often have poorer dough quality, and the
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