Amino Acids: Structure, Classification & General Properties of α-Amino Acids
Learning Objectives
- Describe the general structure of an α-amino acid and explain the zwitterionic form.
- Classify α-amino acids based on R-group chemistry and nutritional requirement.
- Explain the general physicochemical properties of amino acids.
- Discuss the physiological importance of essential and non-essential amino acids in animal systems.
1. Introduction
Amino acids are the structural units (monomers) of proteins, joined together by peptide bonds. Of the more than 300 amino acids found in nature, only 20 (the "standard" or "proteinogenic" amino acids) are genetically coded and incorporated into proteins during translation. With the exception of proline (an imino acid) and glycine (achiral), all standard amino acids share a common structural framework built around a central, or alpha (α), carbon atom — hence the term α-amino acids.
2. General Structure of an α-Amino Acid
Click on any part of the diagram to learn its identity and function.
Tap a group above ⬆
- All four groups — –NH₂, –COOH, H and R — are attached to the same α-carbon.
- The α-carbon is a chiral centre in all amino acids except glycine (R = H).
- Naturally occurring amino acids belong to the L-series (based on L-glyceraldehyde configuration).
- The nature of the R group differentiates the 20 standard amino acids from one another.
Interactive: The Zwitterion — Effect of pH on Ionic Form
Amino acids are amphoteric — they carry both positive and negative charges depending on the surrounding pH. Drag the slider to see how the ionic form changes.
3. Classification of α-Amino Acids
Amino acids can be classified on several bases. Click each tab below to explore.
(a) Non-polar / Hydrophobic (aliphatic & aromatic)
(b) Polar Uncharged
(c) Acidic (negatively charged R-group)
(d) Basic (positively charged R-group)
4. General Properties of α-Amino Acids
Amphoteric Nature
Amino acids act as both acid and base due to the presence of –COOH and –NH₂ groups, allowing them to act as buffers.Isoelectric Point (pI)
The pH at which an amino acid carries no net charge and exists predominantly as a zwitterion; it does not migrate in an electric field at this pH.Optical Activity
All amino acids except glycine possess a chiral α-carbon and are optically active, rotating plane-polarised light.Solubility
Generally soluble in water and insoluble in non-polar organic solvents, owing to their ionic (zwitterionic) nature.High Melting Point
Amino acids have relatively high melting points (usually >200°C) due to strong electrostatic (ionic) interactions in the crystal lattice.UV Absorption
Aromatic amino acids (Trp, Tyr, Phe) absorb UV light around 280 nm — used to estimate protein concentration.Ninhydrin Reaction
Amino acids react with ninhydrin to produce a purple (Ruhemann's purple) colour, used for their qualitative and quantitative detection (proline gives yellow).Peptide Bond Formation
The α-carboxyl group of one amino acid condenses with the α-amino group of another, releasing water and forming a peptide bond.5. Physiological Importance of Essential and Non-Essential α-Amino Acids
Essential Amino Acids
Essential amino acids cannot be synthesised by animal tissues (owing to the absence of the requisite enzymatic pathways) and must be supplied through diet. Their physiological roles include:
| Amino Acid | Physiological Importance |
|---|---|
| Leucine, Isoleucine, Valine (BCAAs) | Muscle protein synthesis; energy source during exercise; regulate blood glucose |
| Lysine | Collagen formation, calcium absorption, carnitine synthesis; growth in young animals |
| Methionine | Methyl group donor (via SAM); synthesis of cysteine, creatine, and choline; antioxidant defence |
| Threonine | Component of mucin/collagen; supports gut and immune function |
| Phenylalanine | Precursor of tyrosine, catecholamines (dopamine, adrenaline) and thyroid hormones |
| Tryptophan | Precursor of serotonin, melatonin and niacin (vitamin B3) |
| Histidine (semi-essential) | Precursor of histamine; important in growth and tissue repair |
| Arginine (semi-essential) | Nitric oxide synthesis, urea cycle, wound healing, immune function |
Non-Essential Amino Acids
These are synthesised endogenously from metabolic intermediates (glycolysis, TCA cycle) or from essential amino acids, yet remain physiologically vital:
| Amino Acid | Physiological Importance |
|---|---|
| Glycine | Component of collagen, glutathione, haem and purine synthesis; inhibitory neurotransmitter |
| Alanine | Glucose-alanine cycle for inter-organ nitrogen and energy transport |
| Glutamic acid / Glutamine | Excitatory neurotransmitter (Glu); ammonia detoxification and nitrogen transport (Gln); fuel for rapidly dividing cells |
| Aspartic acid | Urea cycle intermediate; involved in purine and pyrimidine biosynthesis |
| Serine | Phospholipid synthesis; precursor of glycine and cysteine |
| Cysteine | Disulphide bond formation in proteins; glutathione and taurine synthesis |
| Tyrosine | Precursor of melanin, thyroid hormones and catecholamines |
| Proline | Structural rigidity in collagen triple helix |
Clinical/comparative note: Deficiency of essential amino acids in diet leads to conditions such as kwashiorkor (protein-energy malnutrition), while inherited defects in amino acid metabolism (e.g. phenylketonuria, due to inability to convert phenylalanine to tyrosine) illustrate their physiological significance.
Interactive Learning Activities
This quadrant reinforces Quadrant I concepts through interactive exploration. Use the structure diagram and pH slider in Quadrant I actively, and try the activities below.
Activity 1 — Match the Amino Acid to its Class
Click an amino acid, then click the class you think it belongs to.
Activity 2 — Video Lecture Placeholder
Embed the recorded video lecture link here (e.g., institutional SWAYAM/YouTube unlisted link) when available.
Activity 3 — Quick Recall Flashcards
Click a card to flip and reveal the answer.
Self-Assessment Quiz
Answer all questions and click "Submit Quiz" to see your score and correct answers.
Glossary
- Zwitterion
- A dipolar ion carrying both a positive and a negative charge simultaneously, with net charge zero.
- Isoelectric point (pI)
- pH at which an amino acid has no net electric charge.
- Chiral centre
- A carbon atom attached to four different substituents, giving rise to optical isomerism.
- Essential amino acid
- An amino acid that cannot be synthesised by the organism and must be obtained from the diet.
- Semi-essential amino acid
- Normally non-essential but required from diet during growth, stress, or illness.
- Ninhydrin reaction
- Colorimetric reaction used to detect and quantify free amino acids.
Suggested Readings & References
- Lehninger, A. L., Nelson, D. L., & Cox, M. M. Principles of Biochemistry. W.H. Freeman & Co. (Latest edition) — Chapter on Amino Acids, Peptides and Proteins.
- Voet, D., Voet, J. G., & Pratt, C. W. Fundamentals of Biochemistry. Wiley.
- Satyanarayana, U., & Chakrapani, U. Biochemistry. Elsevier India.
- Berg, J. M., Tymoczko, J. L., & Stryer, L. Biochemistry. W.H. Freeman & Co.
- UGC-SWAYAM/e-PG Pathshala modules on Amino Acid Biochemistry, Biochemistry Paper.
- NCBI Bookshelf — Biochemistry, LibreTexts Biological Chemistry (for supplementary reading).
Points to Ponder / Discussion Questions
- Why is glycine the only achiral standard amino acid?
- Why does an amino acid exhibit minimum solubility at its isoelectric point?
- Compare essential amino acid requirements across herbivores, carnivores, and omnivores.
- Discuss the biochemical basis of phenylketonuria (PKU) in relation to phenylalanine metabolism.
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