Biochemistry and Properties of Proteins

A comprehensive vocabulary review of protein biochemistry covering amino acid classification, structural hierarchy, biological functions, physical properties, denaturation, and digestion.

Amino Acid

The basic monomer unit of proteins, consisting of a central carbon atom bonded to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom, and a variable side chain ($R\text{-group}$).

Peptide Bond

The covalent chemical bond formed between the carboxyl group ($-COOH$) of one amino acid and the amino group ($-NH_2$) of another through a condensation reaction that releases a molecule of water ($H_2O$).

Essential Amino Acids

Amino acids that cannot be synthesized by the human body at a rate adequate to meet metabolic needs and therefore must be supplied directly through the diet.

Non-Essential Amino Acids

Amino acids that the body can synthesize endogenously from other compounds and metabolic intermediates, making dietary intake non-mandatory.

Primary Structure of Protein

The unique linear sequence of amino acids linked together by covalent peptide bonds along a polypeptide chain.

Secondary Structure of Protein

The localized folding or coiling of a polypeptide chain into repetitive geometric conformations, such as $\alpha$-helices and $\beta$-pleated sheets, stabilized by hydrogen bonding between backbone atoms.

$\alpha$-Helix

A common secondary protein structure where the polypeptide chain winds into a right-handed spiral coil stabilized by intramolecular hydrogen bonds parallel to the central axis.

$\beta$-Pleated Sheet

A secondary protein structure formed by hydrogen bonds between adjacent, parallel or antiparallel polypeptide strands, producing a folded or pleated sheet arrangement.

Tertiary Structure of Protein

The overall three-dimensional conformation of a single polypeptide chain formed by interactions among side-chain $R\text{-groups}$, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

Quaternary Structure of Protein

The higher-order structural organization resulting from the assembly and non-covalent association of two or more distinct polypeptide subunits into a single functional protein complex.

Disulfide Bridge

A strong covalent bond formed by the oxidation and linkage of sulfhydryl ($-SH$) groups between two cysteine residues, contributing to tertiary and quaternary protein stability.

Denaturation

The unfolding or disruption of a protein's secondary, tertiary, and quaternary structures due to factors like heat, extreme pH, or chemical denaturants, leading to loss of biological function while the primary structure remains intact.

Simple Protein

A protein composed exclusively of amino acid residues that yields only amino acids and no other chemical groups upon complete hydrolysis.

Conjugated Protein

A protein that contains a non-protein prosthetic group permanently associated with or bound to its polypeptide chain, such as a glycoprotein, lipoprotein, or chromoprotein.

Prosthetic Group

The non-amino acid chemical component permanently bound to a conjugated protein that is required for the protein to carry out its biological activity (e.g., the iron-containing heme group in hemoglobin).

Globular Protein

A compact, spherical, generally water-soluble protein that carries out dynamic metabolic functions such as enzymatic catalysis, molecular transport, and immune defense.

Fibrous Protein

An elongated, insoluble protein characterized by repetitive secondary structures that provides tensile strength, structural support, and protective frameworks in tissues (e.g., collagen, keratin).

High Biological Value (HBV) Protein

A complete dietary protein typically derived from animal sources (such as eggs, meat, and milk) that supplies all essential amino acids in optimal proportions for human physiological needs.

Low Biological Value (LBV) Protein

An incomplete dietary protein typically obtained from plant sources (such as cereals, grains, and pulses) that is deficient in one or more essential amino acids.

Sickle Cell Anemia

A genetic hemoglobin disorder resulting from a point mutation where hydrophilic glutamic acid is replaced by hydrophobic valine at position 6 of the $\beta$-globin chain, causing hemoglobin polymerization and red blood cell sickling under low oxygen.

Zwitterion

A dipolar molecule containing both positively charged ($-NH_3^+$) and negatively charged ($-COO^-$) functional groups, yielding an overall net neutral electrical charge.

Isoelectric Point ($\text{pI}$)

The specific pH level at which a protein or amino acid possesses zero net electrical charge, exhibiting minimal aqueous solubility and no mobility in an electric field.

Pepsin

A gastric endopeptidase enzyme secreted by the stomach lining that cleaves dietary proteins into smaller polypeptides and peptones in an acidic environment.

Trypsin

A pancreatic protease secreted into the small intestine that hydrolyzes peptide bonds in polypeptides to produce shorter peptides and amino acids during digestion.