Protein Structure and Function

Protein Structure

• Proteins are complex macromolecules composed of amino acids linked together by peptide bonds.
• The sequence of amino acids dictates the protein's unique three-dimensional structure, which is critical for its function.
• Four levels of protein structure are recognized: primary, secondary, tertiary, and quaternary.
• The primary structure is the linear sequence of amino acids.
• Secondary structure involves local folding patterns like alpha-helices and beta-sheets stabilized by hydrogen bonds.
• Tertiary structure refers to the overall three-dimensional shape of a polypeptide chain, often stabilized by interactions between amino acid side chains (e.g., disulfide bonds, hydrophobic interactions).
• Quaternary structure arises when multiple polypeptide chains associate to form a functional protein complex.
• Protein structure is very sensitive to changes in pH, temperature, and the presence of denaturing agents like urea and strong acids or bases. These changes disrupt the weak interactions that maintain the protein's structure, leading to denaturation.

Protein Classification

• Proteins can be classified based on their functions and properties.
• Some common classifications include globular proteins (e.g., enzymes, antibodies) and fibrous proteins (e.g., collagen, keratin).
• Globular proteins are typically soluble in water and have a compact, roughly spherical shape, while fibrous proteins are often insoluble and have elongated shapes.
• Other classifications include structural proteins involved in supporting tissues such as hair or bone, as well as transport proteins needed to carry molecules within an organism.

Protein Functionality in Food Systems

• Proteins play crucial roles in various food systems, influencing texture, color, and stability of food products and the sensory experience of a particular food.
• Proteins can contribute to emulsification, gelation, foaming, and viscosity.
• Emulsification is the process of stabilizing two immiscible liquids (e.g., oil and water) in a mixture by the presence of proteins.
• Gelation is the formation of a gel from a solution, where proteins can aggregate to form a network trapping water molecules.
• Foaming, another property of proteins in food, results from protein's ability to stabilize air bubbles in a liquid system, contributing to the texture of foods like whipped cream and meringues.
• Protein viscosity plays a vital role in the mouthfeel of foods, whether liquids or solids, and affects how they are experienced by the consumer.
• The denaturation of proteins during thermal processing affects their functionality in various food applications, and can affect texture, color, taste, and shelf-life.

Protein Degradation and Nutritional Value

• Protein degradation is a complex process, often involving enzymes, impacting the nutritional quality of food products.
• Protein degradation through enzymatic activity can yield smaller, more readily digestible peptides, or it can result in complete protein hydrolysis.
• The quality of protein in food sources is often measured by factors like digestibility, amino acid profile, and biological value.
• Optimal digestibility is important for the body to absorb the protein and use it for different cellular functions.
• Good amino acid profiles provide the essential amino acids necessary for human growth and metabolism.
• Biological value measures how effectively the body utilizes the ingested protein.

Protein Interactions with Other Food Components

• Protein interactions with other food components influence the overall properties of food systems.
• These interactions include interactions with lipids, carbohydrates, acids, and other proteins.
• Protein-lipid interactions play major roles in food stability and texture, particularly concerning the emulsion properties of oil and water in foods.
• Protein-carbohydrate interactions often involve cross-linking, which is important in the development of gels and structural components of food, e.g., the formation of gluten in dough or the texture of grains.
• Protein-acid interactions can cause changes in protein structure, leading to alteration in texture and even color.

Factors Affecting Protein Functionality

• Various factors impact protein functionality in food products.
• pH and temperature are crucial factors. Changes in pH affect the ionization state of amino acid side chains, altering protein structure and consequently, functionality.
• Also, heat treatments such as cooking or pasteurization can cause denaturation and subsequent changes in protein structure and functionality.
• Other factors include water activity, concentration, and the presence of other food components.
• The nature of the protein itself such as its conformation or source, will greatly influence its interaction with other food components and thus its overall functionality.