Protein Structure and Bonds

Peptide Bonds

• A peptide bond forms between the carboxylic acid of one amino acid and the amino group of another, releasing water (H2O).

Structure of Proteins

• Primary structure: Sequence of amino acids linked by peptide bonds.
• Secondary structure: Includes alpha helices (coiled peptide chains) and beta-pleated sheets (formed between protein strands).
• Tertiary structure: 3D shape due to interactions between side groups (ionic bonds, disulfide bonds, hydrophilic interactions).
• Quaternary structure: Combination of two or more tertiary structures or polypeptide chains.

Definition of Proteins

• Composed of amino acids; a molecule is categorized as a protein if it contains more than 50 amino acids with biological activity.

Functions of Proteins

• Essential for the structure of skin, muscle, cartilage, hair, fingernails, enzymes, and hormones.

Protein Bonding

• Primary: Linking through peptide bonds, forming dipeptides, tripeptides, etc.
• Secondary: Stabilized by hydrogen bonding; alpha helix resembles a spiral staircase, while beta pleated sheets resemble folding or pleating.

Denaturation of Proteins

• Temperature: Breaks hydrogen bonds and disrupts hydrophobic interactions.
• pH: Affects hydrogen bonds and salt bridges.
• Heavy metals: Disrupt disulfide bonds through ionic bond formation.
• Organic molecules: Disrupt hydrophobic interactions.

Biuret Test

• Positive test indicates presence of peptide bonds; the solution changes from blue (Cu²⁺) to violet for proteins with two or more peptide bonds.
• Negative reaction remains blue, showing only individual amino acids or dipeptides.

Ninhydrin Test

• Used to detect amino acids in proteins; most amino acids yield a blue-violet color, while proline and hydroxyproline result in a yellow color.

Xanthoproteic Test

• Detects amino acids with aromatic rings; concentrated nitric acid produces yellow-colored nitro-substituted products from tyrosine and tryptophan.

Disinfection Methods

• Heat and alcohol disinfect by denaturing proteins, destabilizing secondary, tertiary, and quaternary structures.

Milk and Heavy Metal Ingestion

• Milk acts as a buffer by reacting with heavy metal ions like silver or mercury, protecting the body from toxicity.

pH Changes and Protein Structure

• Alterations in pH disrupt bonds maintaining tertiary structures.

Xanthoproteic Test Reaction

• The yellow spot on skin after nitric acid exposure indicates oxidization leading to the formation of a yellow product.

Negative Biuret Test

• Glycine and tyrosine yield negative results due to lacking multiple peptide bonds; biuret test requires the presence of those bonds for a positive outcome.

Functional Group in Xanthoproteic Test

• The presence of an aromatic ring confirms a positive reaction in the xanthoproteic test.

Differentiating Amino Acids and Proteins

• The biuret test can distinguish between amino acids and proteins in an unlabeled test tube.

This quiz covers the fundamental concepts of protein structure, including peptide bonds and the four levels of protein organization: primary, secondary, tertiary, and quaternary. Additionally, it highlights the biological significance of proteins and their various functions in the body.