Proteins in Food and Their Functions

Amino acids are the building blocks of proteins, containing an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R) attached to the α-carbon atom.

The side chain (R) of an amino acid determines its properties, such as polarity, charge, and reactivity.

Amino acids can exist as L-enantiomers and D-enantiomers. All proteins in nature are composed of L-amino acids.

Amino acids are classified into three groups based on their side chain properties: non-polar, polar, and charged.

Indispensable amino acids are essential for human health and cannot be synthesized by the body. They must be obtained from the diet.

Conditionally dispensable amino acids can be synthesized by the body under certain conditions, but may be essential during specific periods of growth or illness.

Dispensable amino acids can be synthesized by the body and are not required in the diet.

A peptide bond is a covalent bond formed between the α-carboxyl group of one amino acid and the α-amino group of another amino acid, with the removal of water.

The N-terminal amino acid has a free α-amino group and is written on the left, while the C-terminal amino acid has a free α-carboxyl group and is written on the right.

A peptide bond is typically in the trans-configuration due to steric hindrance, where the side chains are on opposite sides of the bond.

Two or more amino acids linked together are called peptides, with specific classifications based on their length: dipeptide (2), tripeptide (3), tetrapeptide (4), oligopeptide (<100), and polypeptide (>100).

A protein is a large polypeptide chain that has achieved a specific biological function in the organism.

The primary structure of a protein refers to the linear sequence of amino acids in its polypeptide chain.

The secondary structure of a protein arises from the folding of the polypeptide chain into repetitive spatial structures, including α-helices, β-sheets, and random coils, primarily due to hydrogen bonding.

The tertiary structure of a protein describes its three-dimensional shape, defined by the interactions and orientations of the secondary structure elements.

The quaternary structure of a protein refers to the arrangement of multiple polypeptide chains (subunits) within a larger protein complex, held together by non-covalent interactions.

Disulphide bonds, covalent bonds formed between cysteine residues, are critical in stabilizing the tertiary and quaternary structures of proteins.

Electrostatic interactions, attractions between charged groups on amino acid side chains, contribute to protein structure stability.

Hydrophobic interactions, driven by the tendency of nonpolar residues to avoid water, play a significant role in protein folding and stability.

Van der Waals interactions, weak forces between nonpolar groups, contribute to protein structure stability by promoting close packing of residues.

Globular proteins, characterized by their spherical shape, are typically soluble in water and perform a wide range of cellular functions.

Fibrous proteins, elongated and thread-like, provide structure and support to tissues, often forming insoluble fibers.

Random-coil proteins lack a defined secondary or tertiary structure, often associating into micelles to minimize hydrophobic exposure.

Proteins can be classified based on solubility. This is a key characteristic for food science applications.

The isoelectric point (pI) is the specific pH at which a protein has a net charge of zero.

The charge of an amino acid, peptide, or protein is influenced by pH. The overall charge can be positive, negative, or neutral depending on the pH of the surrounding environment.

Decarboxylation is the removal of a carboxyl group (COOH) from an amino acid, often catalyzed by enzymatic reactions during spoilage or fermentation.

Deamination is the removal of an amino group (-NH2) from an amino acid, also often catalyzed by enzymatic reactions during spoilage or fermentation.

The Strecker degradation, a chemical reaction, occurs when a free amino acid reacts with a dicarbonyl compound, resulting in transamination and decarboxylation.

The Maillard reaction, a complex chemical reaction, involves the reaction between a reducing sugar and a free amino group, resulting in brown pigments (melanoidins), volatile compounds, and potential loss of essential amino acids.