Amino Acids: Structure, Properties, and Classification

Chapter 3 – Amino Acids

Introduction to Amino Acids

Amino acids are the fundamental building blocks of proteins and play diverse roles in biochemistry. There are 20 genetically encoded α-amino acids that are incorporated into proteins, while other amino acids exist as metabolic intermediates or have specialized functions.

• Definition: An amino acid is an organic molecule containing both an amino group (–NH2) and a carboxyl group (–COOH) attached to a central (α) carbon atom.

• General Structure: The α-carbon is bonded to four different groups:

  • α-carboxylate group (–COOH)

  • α-amino group (–NH2)

  • α-hydrogen (–H)

  • R-group or "side chain" (unique for each amino acid)

• Side Chains: The chemical nature of the R-group determines the properties and classification of each amino acid (e.g., hydrophobicity, hydrophilicity, reactivity).

Chirality and Isomerism

The α-carbon of amino acids (except glycine) is chiral, meaning it is attached to four different substituents, resulting in two possible stereoisomers.

• Chiral Center: All amino acids except glycine (where R = H) are chiral.

• Stereoisomers: Molecules with the same molecular formula but different spatial arrangements.

• Enantiomers: Nonsuperimposable mirror images, designated as L (levo) and D (dextro) isomers.

• Biological Relevance: Only L-amino acids are found in proteins.

Ionization and Zwitterions

Amino acids contain at least two ionizable groups (the amino and carboxyl groups), and some have ionizable side chains. The ionization state depends on the pH of the environment.

• Zwitterion: At physiological pH (~7.4), amino acids exist as zwitterions, carrying both a positive and a negative charge but having no net charge.

• Isoelectric Point (pI): The pH at which the amino acid has no net charge. At this point, the molecule is electrically neutral.

Equation for Isoelectric Point (pI):

$pI = \frac{pK_a^{(1)} + pK_a^{(2)}}{2}$

where $pK_a^{(1)}$ and $pK_a^{(2)}$ are the dissociation constants of the ionizable groups.

Amino Acid Codes and Structures

Each amino acid has a unique name, one-letter code, and three-letter code. Students should be able to recognize and draw the structures of all 20 standard amino acids.

• Example: Glycine (Gly, G), Alanine (Ala, A), Valine (Val, V), etc.

• Application: Knowledge of codes is essential for interpreting protein sequences and biochemical literature.

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains:

• Nonpolar (Hydrophobic) Amino Acids: Side chains are aliphatic or aromatic, contributing to hydrophobic interactions in proteins.

• Polar (Hydrophilic) Amino Acids: Side chains can form hydrogen bonds and interact with water.

• Charged Amino Acids: Side chains are either positively or negatively charged at physiological pH.

Table: Amino Acid Classification

Group	Examples	Properties
Nonpolar (Hydrophobic)	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan	Aliphatic or aromatic side chains; found in protein interiors
Polar (Uncharged)	Serine, Threonine, Tyrosine, Asparagine, Glutamine, Cysteine	Can form hydrogen bonds; often found on protein surfaces
Positively Charged (Basic)	Lysine, Arginine, Histidine	Side chains carry positive charge at physiological pH
Negatively Charged (Acidic)	Aspartate, Glutamate	Side chains carry negative charge at physiological pH

Essential and Non-Essential Amino Acids

Some amino acids must be obtained from the diet (essential), while others can be synthesized by the human body (non-essential).

• Essential Amino Acids: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

• Non-Essential Amino Acids: Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamine, Glutamate, Glycine, Proline, Serine, Tyrosine

• Note: Arginine is considered semi-essential; required in the diet for children.

Modified and Non-Proteinogenic Amino Acids

Some amino acids are modified after protein synthesis or occur in specialized proteins.

• Post-Translational Modifications: Hydroxyproline, Selenocysteine, etc.

• Non-Proteinogenic Amino Acids: Serve as precursors for biologically active molecules (e.g., histamine, GABA, serotonin, epinephrine).

Summary Table: Ionizable Groups and pKa Values

Amino Acid	pKa (α-COOH)	pKa (α-NH3+)	pKa (R-group)
Alanine	2.3	9.69	—
Arginine	2.3	9.69	12.48
Aspartic acid	2.09	9.82	3.65
Glutamic acid	2.19	9.67	4.25
Histidine	1.82	9.17	6.00
Lysine	2.18	8.95	10.53
Cysteine	1.96	10.28	8.33

Key Takeaways

• Understand the structure and classification of amino acids.

• Recognize the importance of chirality and isomerism in biochemistry.

• Know the ionization properties and the concept of zwitterions and isoelectric point.

• Be familiar with amino acid codes, essentiality, and modifications.

Additional info: Some table entries and explanations were inferred and expanded for completeness and clarity.