Proteins and amino acids- FOOD ANALYST EXAMINATION SEREIES

 Proteins are macromolecules composed of amino acids that play a vital role in various biological processes such as metabolism, signaling, and transportation. The amino acids that make up proteins are classified based on their chemical properties and structure. In this article, we will discuss the classification of amino acids based on structure, globular and fibrous proteins, hierarchy in structure, isoelectric point, protein denaturation, and digestibility.

Classification of amino acids based on structure:

Amino acids can be classified based on their chemical properties and structure. There are 20 amino acids that make up proteins. They can be divided into several categories based on their chemical properties. The categories include aromatic, aliphatic, acidic, basic, sulfur-containing, branched-chain, essential, and non-essential amino acids.

·         Aromatic amino acids: These amino acids have an aromatic ring in their side chain. The three aromatic amino acids are phenylalanine, tyrosine, and tryptophan.

·         Aliphatic amino acids: These amino acids have a straight-chain structure in their side chain. The aliphatic amino acids include glycine, alanine, valine, leucine, isoleucine, and proline.

·         Acidic amino acids: These amino acids have a carboxylic acid group in their side chain. The two acidic amino acids are aspartic acid and glutamic acid.

·         Basic amino acids: These amino acids have an amino group in their side chain. The basic amino acids include lysine, arginine, and histidine.

·         Sulfur-containing amino acids: These amino acids contain a sulfur atom in their side chain. The two sulfur-containing amino acids are cysteine and methionine.

·         Branched-chain amino acids: These amino acids have a branched side chain. The three branched-chain amino acids are valine, leucine, and isoleucine.

·         Essential amino acids: These amino acids cannot be synthesized by the body and must be obtained from the diet. There are nine essential amino acids, including phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.

Non-essential amino acids: These amino acids can be synthesized by the body. The non-essential amino acids include alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.

Globular and fibrous proteins:

Proteins can be classified based on their shape and structure. There are two main types of proteins: globular and fibrous.

Globular proteins: These proteins are water-soluble and have a rounded, compact shape. They are usually enzymes, hormones, or transport proteins. Examples of globular proteins include insulin, hemoglobin, and antibodies.

Fibrous proteins: These proteins are insoluble in water and have a long, thin shape. They are usually structural proteins and provide support and shape to cells and tissues. Examples of fibrous proteins include collagen, keratin, and elastin.

Hierarchy in structure:

Proteins are complex biomolecules that play essential roles in various physiological processes. Their structure is hierarchically organized into four levels, each with unique features and functions. The primary structure of a protein refers to its linear sequence of amino acids, which are covalently linked through peptide bonds. The sequence determines the protein's unique identity and function.

The secondary structure describes the local folding patterns of the polypeptide chain. This level of structure is stabilized by hydrogen bonds between the backbone atoms of the amino acids. Two common secondary structures are the alpha helix and the beta sheet. In the alpha helix, the polypeptide chain is coiled into a spiral shape, while in the beta sheet, the chain forms a series of zigzag folds.

The tertiary structure is the overall 3D arrangement of the protein molecule, resulting from the folding of the secondary structure. The tertiary structure is stabilized by various interactions between the amino acid side chains, such as hydrogen bonds, disulfide bonds, ionic bonds, and hydrophobic interactions. The tertiary structure determines the protein's specific function and interaction with other molecules.

The quaternary structure refers to the arrangement of multiple protein subunits to form a functional complex. The subunits can be identical or different, and the interactions between them can be noncovalent, such as hydrogen bonds, or covalent, such as disulfide bonds.

Proteins are classified based on their overall shape and solubility. Globular proteins are compact, water-soluble proteins with a roughly spherical shape, and are involved in functions such as enzyme catalysis, transport, and signaling. In contrast, fibrous proteins are elongated, insoluble proteins that form structural components, such as collagen in connective tissues and keratin in hair and nails.

Amino acids, the building blocks of proteins, can be classified based on their chemical properties. Aromatic amino acids, such as phenylalanine and tyrosine, contain a benzene ring and are involved in diverse functions, including hormone synthesis and signal transduction. Aliphatic amino acids, such as alanine and valine, contain a straight or branched chain and are involved in hydrophobic interactions and structural stability. Acidic amino acids, such as aspartic acid and glutamic acid, are negatively charged at physiological pH and are involved in enzymatic activity and signal transduction. Basic amino acids, such as lysine and arginine, are positively charged at physiological pH and are involved in binding negatively charged molecules and DNA. Sulfur-containing amino acids, such as cysteine and methionine, contain a sulfur atom and are involved in disulfide bond formation and antioxidant activity. Branched-chain amino acids, such as leucine and isoleucine, have a branched side chain and are involved in energy metabolism and muscle protein synthesis. Essential amino acids cannot be synthesized by the human body and must be obtained through the diet, while non-essential amino acids can be synthesized by the body.

The isoelectric point (pI) of a protein is the pH at which the protein has no net charge and does not move in an electric field. At pH values below the pI, the protein is positively charged and migrates towards the negatively charged electrode, while at pH values above the pI, the protein is negatively charged and migrates towards the positively charged electrode. The pI depends on the amino acid sequence and the charge of the side chains, and can be calculated using various methods.

Color reactions of proteins and amino acid

Color reactions of proteins and amino acids are commonly used to identify and quantify proteins and amino acids in different samples. Some of the commonly used color reactions are as follows:

Biuret test: This test is used to identify the presence of peptide bonds in proteins. When copper (II) ions react with peptide bonds, they form a violet-colored complex. For example, when the biuret reagent (copper sulfate and sodium hydroxide) is added to egg white or milk, a violet color appears, indicating the presence of proteins.

Ninhydrin test: This test is used to detect the presence of free amino groups in amino acids. When ninhydrin reacts with amino acids, it forms a blue or purple-colored compound. For example, when ninhydrin is added to a solution containing amino acids, a blue color appears.

Xanthoproteic test: This test is used to identify the presence of aromatic amino acids, such as phenylalanine and tyrosine. When these amino acids react with nitric acid, they form a yellow-colored compound. For example, when nitric acid is added to a solution containing phenylalanine or tyrosine, a yellow color appears.

Millon's test: This test is used to detect the presence of tyrosine in proteins. When proteins are treated with Millon's reagent (a solution of mercury (II) nitrate and nitric acid), a red-colored compound is formed. For example, when Millon's reagent is added to egg white, a red color appears, indicating the presence of tyrosine.

Sakaguchi test: This test is used to identify the presence of arginine in proteins. When arginine reacts with ninhydrin and sodium hypochlorite, it forms a red-colored compound. For example, when ninhydrin and sodium hypochlorite are added to a solution containing arginine, a red color appears.

Color reactions are widely used to identify and quantify proteins and amino acids in different samples. The choice of the color reaction depends on the amino acid or protein being analyzed and the purpose of the analysis.

Amino acid analysis

Amino acid analysis is a method used to determine the composition of amino acids in a protein or peptide. This analysis is important for the characterization and quantification of proteins, as well as for the identification of unknown proteins. The following are some notes on amino acid analysis:

Principle: The principle of amino acid analysis involves the hydrolysis of the protein or peptide, followed by the separation and quantification of the resulting amino acids.

Hydrolysis: The protein or peptide is hydrolyzed using acid or base to break the peptide bonds and release the constituent amino acids.

Derivatization: The amino acids are often derivatized prior to analysis, which involves the addition of a functional group to make them more amenable to analysis. Common derivatization methods include ion exchange chromatography and HPLC.

Separation: The amino acids are separated by chromatography, usually by reverse-phase HPLC. Each amino acid has a unique retention time, which allows for their identification and quantification.

Detection: The amino acids are detected by a variety of methods, including UV, fluorescence, and mass spectrometry. The choice of detection method depends on the specific analytical requirements.

A common application of amino acid analysis is in the determination of protein quality. For example, the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is a method used to assess the quality of dietary protein. The PDCAAS is based on the amino acid profile of the protein and its digestibility. Amino acid analysis is used to determine the amino acid composition of the protein, which is used to calculate the PDCAAS.

Rheological properties of protein- solubility, viscosity, gelling, surfactants/

Rheological properties of proteins are important characteristics that determine their functionality in various food applications. These properties include solubility, viscosity, gelling, and surfactant properties.

·         Solubility is the ability of a protein to dissolve in a given solvent. The solubility of a protein depends on various factors such as pH, temperature, and the type of solvent used. For example, casein is more soluble in alkaline solutions than acidic solutions.

·         Viscosity is the resistance of a fluid to flow. Proteins can increase the viscosity of a fluid due to their large molecular size and ability to form complexes with water molecules. For example, egg white proteins can increase the viscosity of a batter, giving it a thick and sticky consistency.

·         Gelling is the ability of a protein to form a three-dimensional network that traps water molecules. Proteins that have the ability to form gels are commonly used in food applications such as desserts and jellies. For example, gelatin, which is derived from collagen, can form a gel when mixed with water and heated.

·         Surfactants are molecules that have both hydrophilic (water-loving) and hydrophobic (water-fearing) properties. Proteins can act as surfactants due to their ability to interact with both water and oil. For example, soy protein isolate can be used as an emulsifier in salad dressings to prevent oil and water from separating.

The rheological properties of proteins play a crucial role in determining their functionality in various food applications. Understanding these properties can help food scientists optimize the use of proteins in food formulations.

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