Amino Acids and Protein Structure

Amino Acids

• Proteins are large molecules made of 20 amino acids linked together in long chains called polypeptides.
• Amino acids are the building blocks for proteins found in all living cells.
• About 300 amino acids occur in nature.
• Only 20 enter protein synthesis.
• Proteins found in cells and in proteins are in the L-configuration.

Amino Acid Structure

• Amino acids consist of a carboxyl group, an amine group, and a side chain (R group) bonded to a central carbon atom.
• The R group varies between amino acids, determining their specific properties.

Amino Acids in Proteins

• All naturally occurring amino acids in proteins are α-amino acids.
• Amino acids can occur in one of two forms: L-form (left-handed) or D-form (right-handed).
• L-amino acids are used in protein synthesis.
• α-carbon has 4 different groups attached to it, except glycine (which has two identical groups).
• This makes them chiral.

Amino Acid Classification

• Classification by Chemical Structure (R group): Aliphatic (non-ring), Aromatic (benzene ring), and Heterocyclic (ring containing heteroatoms: S, N, O).
• Classification by Polarity of Side Chains (R): Polar (forms hydrogen bonds because they contain polar hydrophilic groups) or Nonpolar (hydrophobic, can't form hydrogen bonds.)
• Nonpolar, Aliphatic: glycine, alanine, valine, leucine, isoleucine, phenylalanine, tryptophan, proline, methionine
• Polar, uncharged: serine, threonine, cysteine, asparagine, glutamine, tyrosine, histidine
• Acidic: aspartic acid, glutamic acid
• Basic: lysine, arginine
• Classification by Nutritional Requirements: Essential (must be obtained from food), Nonessential (synthesized by the body), Semi-essential (usually not made in sufficient quantities, especially in children)
• Classification by Metabolic Properties: Purely ketogenic (give ketone bodies), Purely glucogenic (give glucose), Ketogenic and Glucogenic (give both)

Amino Acid Properties

• Physical: Colourless, crystalline, soluble in polar solvents, insoluble in non-polar solvents. Melting point generally above 200°C. Glycine is an exception as it is more stable and hindered.
• Acid-base: Amino acids are amphoteric, meaning they can act as both acids and bases. They're zwitterions in water. The molecule contains both positive and negative charges.
• Isoelectric point (pI): The pH at which the amino acid carries no net electrical charge. In this form, the amino acid is more stable.
• Chemical: Various reactions like the Ninhydrin test (detection of amino acids which form deep violet product), Decarboxylation (removal of carboxyl group to form amines), reaction with Alkalis (to form carboxylate ions), reaction with alcohols (form esters)

Function of Amino Acids

• Participate in cellular functions including nerve transmission, neurotransmitter synthesis, hormone synthesis, etc. Amino acids are critical in the formation of proteins, and therefore a key component to health and wellbeing.