proteins amino acids

Amino acids are the monomer units that polymerise to form proteins through condensation reactions. Each amino acid contains a central carbon atom (α-carbon) bonded to four groups: a carboxyl group (-COOH), an amine/amino group (-NH₂), a hydrogen atom (-H), and a distinctive R group (side chain) that varies between the 20 naturally occurring amino acids.

General Structure Formula: NH₂-CHR-COOH

Amino acids are classified by their R group properties:

• Acidic: R groups containing extra -COOH (e.g., aspartic acid, glutamic acid)
• Basic: R groups with extra -NH₂ (e.g., lysine, arginine)
• Polar: R groups with -OH or other hydrophilic groups (e.g., serine, threonine)
• Non-polar/Hydrophobic: R groups containing hydrocarbon chains (e.g., alanine, valine, leucine)

Peptide bonds form through condensation reactions between the carboxyl group of one amino acid and the amino group of another, releasing one water molecule (H₂O). The resulting C-N bond is called a peptide bond, creating a dipeptide. Multiple amino acids joined create polypeptides.

Condensation Equation: Amino acid₁ + Amino acid₂ → Dipeptide + H₂O

The reverse process, hydrolysis, breaks peptide bonds by adding water, splitting polypeptides back into amino acids. This occurs during digestion via enzyme action.

Key Cambridge Definition: A peptide bond is a covalent bond formed between the carbon atom of the carboxyl group of one amino acid and the nitrogen atom of the amino group of another amino acid, with the elimination of water.

Zwitterions form when amino acids exist in solution at physiological pH—the amino group accepts a proton (NH₃⁺) while the carboxyl group loses one (COO⁻), creating an internal salt structure.

Think of amino acids as LEGO bricks with different attachments—the basic brick structure is identical (the α-carbon with its carboxyl, amino, and hydrogen groups), but each has a unique attachment piece (the R group) that determines its properties and function.

Real-World Application 1: Sickle Cell Disease A single amino acid substitution demonstrates peptide bond importance. In sickle cell anaemia, glutamic acid (hydrophilic/polar) is replaced by valine (hydrophobic) at position 6 of the β-globin chain. This single change, linked by the same peptide bonds, alters haemoglobin's structure, causing red blood cells to sickle under low oxygen conditions. This shows how the sequence matters as much as the bonds.

Real-World Application 2: Food Digestion When you eat a chicken breast, proteolytic enzymes (pepsin, trypsin) catalyse hydrolysis reactions, systematically breaking peptide bonds. This is why cooking denatures proteins first—heat unfolds the structure, exposing peptide bonds to enzymatic attack more easily.

Analogy for Condensation/Hydrolysis Imagine two people shaking hands (peptide bond formation) and each person removes their glove—the two gloves combine to form a pair (water molecule released). Breaking the handshake requires pulling the hands apart and giving each person a glove back (adding water in hydrolysis).

Industrial Application Insulin production for diabetes treatment involves creating specific peptide bond sequences through recombinant DNA technology. Bacteria synthesise human insulin by correctly joining 51 amino acids (two polypeptide chains) with precise peptide bonds—an error in just one bond position renders the insulin non-functional.

Memory Aid - CORN: Around the α-carbon, remember COOH, Oxygen (in COOH), R group, NH₂ arranged tetrahedrally.