Definition and Classification of Amino Acids
What are Amino Acids?
Amino acids are the basic building blocks of proteins. Each amino acid consists of a central carbon atom (called the alpha carbon) bonded to four groups:
- An amino group (-NH2)
- A carboxyl group (-COOH)
- A hydrogen atom (H)
- A distinctive side chain or R group
The R group varies among different amino acids and determines their chemical nature and function in proteins.
Classification of Amino Acids
Amino acids can be classified in multiple ways. One common method is based on the nature of the side chain (R group). This affects the amino acid’s solubility, polarity, and reactivity.
1. Based on Polarity and Charge
a) Non-Polar (Hydrophobic) Amino Acids:
These have side chains that do not interact well with water. Examples include:
- Glycine (Gly)
- Alanine (Ala)
- Valine (Val)
- Leucine (Leu)
- Isoleucine (Ile)
- Phenylalanine (Phe)
- Proline (Pro)
b) Polar Uncharged Amino Acids:
These can form hydrogen bonds with water but are not charged.
- Serine (Ser)
- Threonine (Thr)
- Asparagine (Asn)
- Glutamine (Gln)
- Tyrosine (Tyr)
c) Polar Charged Amino Acids:
These have side chains that carry a charge at physiological pH.
- Acidic (Negatively charged): Aspartic acid (Asp), Glutamic acid (Glu)
- Basic (Positively charged): Lysine (Lys), Arginine (Arg), Histidine (His)
2. Based on Nutritional Requirement
- Essential Amino Acids: Cannot be synthesized by the body and must be obtained from diet (e.g., leucine, lysine, methionine).
- Non-Essential Amino Acids: Can be synthesized by the human body (e.g., alanine, aspartic acid).
3. Based on Metabolic Fate
- Glucogenic Amino Acids: Converted into glucose (e.g., alanine)
- Ketogenic Amino Acids: Converted into ketone bodies (e.g., leucine)
- Both: Isoleucine, phenylalanine, and tyrosine
Conclusion
Amino acids are vital for protein structure and function. Understanding their classification helps in grasping how proteins behave in different environments and perform various biological functions.