Structure and Properties of Amino Acids: Foundations for Biochemistry

Structure and Properties of Amino Acids

Overview of Amino Acids

Amino acids are the monomeric units of proteins, one of the four major classes of biological macromolecules. Their unique structures and chemical properties underpin the diversity and function of proteins in living systems.

• Definition: Amino acids are organic molecules containing both an amino group (–NH2) and a carboxyl group (–COOH), attached to a central (α) carbon.

• General Structure: The α-carbon is bonded to a hydrogen atom, an amino group, a carboxyl group, and a variable side chain (R group).

• Side Chain (R group): Determines the identity and properties of each amino acid.

• Physiological pH: Amino acids exist predominantly as zwitterions, carrying both positive and negative charges but with a net zero charge.

Example: At pH 7, glycine exists as a zwitterion: $\ce{H3N^+-CH2-COO^-}$

The Four Classes of Biological Macromolecules

Biological macromolecules are polymers composed of repeating monomeric units. Their three-dimensional structures are essential for their biological functions.

• Proteins: Monomer = amino acids

• Nucleic acids: Monomer = nucleotides

• Carbohydrates: Monomer = sugars

• Lipids: Monomer = acetate (not a true polymer)

Common Features:

• Three-dimensional structures

• Structure is critical to function

• Flexibility and dynamic nature

• Responsive to environment, modifications, and ligand interactions

Proteins: Amino Acids and Chirality

Functions of Proteins

Proteins perform a wide variety of functions in cells, determined by their amino acid composition and structure.

• Enzymes: Catalyze biochemical reactions

• Storage and Transport: Carry and store molecules

• Membrane Channels: Facilitate transport across membranes

• Structural Components: Provide support to cells and tissues

• Mechanical Motors: Enable movement

• Regulators: Control gene expression and replication

• Receptors: Mediate cell signaling

• Specialized Functions: Antibodies, hormones, etc.

General Structure of Amino Acids

The structure of amino acids is central to their function and reactivity.

• α-Carbon: The central carbon atom, attached to four different groups, making it chiral (except for glycine).

• Amino Group: Weak base, pKa ≈ 9–10

• Carboxyl Group: Weak acid, pKa ≈ 2–3

• Zwitterion: At physiological pH, amino acids have both a positively charged amino group and a negatively charged carboxylate group.

Equations:

• Acidic dissociation: $\ce{COOH <=> COO^- + H^+}$ (pKa ≈ 2–3)

• Basic dissociation: $\ce{R-NH3^+ <=> R-NH2 + H^+}$ (pKa ≈ 9–10)

Amino Acid Stereochemistry

The α-carbon of amino acids (except glycine) is chiral, leading to two possible stereoisomers: L- and D-forms.

• Chirality: Four different substituents attached to the α-carbon.

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

• L- and D- Enantiomers: Most naturally occurring amino acids in proteins are L-enantiomers.

• Optical Activity: L-amino acids rotate plane-polarized light, as first observed by Emil Fischer.

Example: L-Alanine and D-Alanine are mirror images and non-superimposable.

Structural Representations and Mnemonics

Amino acids can be represented using Fischer projections and mnemonic aids to help identify their configuration.

• Fischer Projection: Horizontal lines represent bonds coming out of the plane; vertical lines go behind.

• CORN Rule: When viewing from the hydrogen, the groups COOH, R, NH2 spell 'CORN' in a clockwise direction for L-amino acids.

• R,S System: Assigns priority based on atomic number; used for systematic naming of chiral centers.

Classification of Amino Acids

Categories Based on Side Chain Properties

The 20 naturally occurring amino acids are classified according to the chemical nature of their side chains.

• Hydrophobic (Non-polar): Gly, Ala, Val, Leu, Ile, Met, Pro

• Polar (Uncharged): Ser, Thr, Cys, Gln, Asn

• Charged: Asp, Glu (negative); Lys, Arg, His (positive)

• Aromatic: Phe, Tyr, Trp

Side Chain Properties: Influence hydrophobicity, hydrophilicity, reactivity, and protein structure.

Structural Features of Selected Amino Acids

• Proline: Only cyclic amino acid; often found in cis peptide bonds.

• Aromatic Amino Acids: Phenylalanine (nonreactive), Tyrosine (hydroxyl group, pKa ≈ 10), Tryptophan (indole ring).

• Spectroscopic Properties: Aromatic amino acids absorb at 280 nm; used to quantify protein concentration via Beer-Lambert Law: $A = \varepsilon c l$

Acidic and Basic Amino Acids

• Basic: Lys (pKa ≈ 10.5), Arg (pKa ≈ 12.5), His (pKa ≈ 6.5)

• Acidic: Asp (pKa ≈ 4), Glu (pKa ≈ 4)

• Nucleophilic Side Chains: Lys, Arg, His, Asp, Glu, Cys, Ser, Thr

Special Properties

• Cysteine: Can form disulfide bonds, important for protein stability.

• Glycine: No side chain; not chiral; highly flexible.

• Methionine: Contains sulfur; can act as a nucleophile.

Post-Translational Modifications

Importance of Amino Acid Modifications

Amino acids in proteins can be chemically modified after translation, expanding the diversity of protein function.

• Phosphorylation: Addition of phosphate group (e.g., phosphoserine)

• Hydroxylation: Addition of hydroxyl group (e.g., 4-hydroxyproline)

• Acetylation: Addition of acetyl group (e.g., N-ε-acetyllysine)

• Carboxylation: Addition of carboxyl group (e.g., γ-carboxyglutamate)

Acid/Base Properties and Titration Curves

Ionization States and pI Calculation

The ionization state of amino acids depends on the pH of the environment, affecting their charge and reactivity.

• Titration Curve: Shows the sequential ionization of amino acid groups as pH increases.

• pKa Values: Each titratable group has a characteristic pKa.

• Isoelectric Point (pI): The pH at which the amino acid has no net charge.

Equation for pI (for amino acids with two ionizable groups):

• $pI = \frac{pK_{a1} + pK_{a2}}{2}$

Example: For glycine, $pK_{a1} = 2.4$, $pK_{a2} = 9.8$, so $pI = 6.1$

Summary Table: Amino Acid Classification

Category	Amino Acids	Key Properties
Hydrophobic (Non-polar)	Gly, Ala, Val, Leu, Ile, Met, Pro	Non-polar side chains, often found in protein interiors
Polar (Uncharged)	Ser, Thr, Cys, Gln, Asn	Hydrogen bonding, reactive side chains
Charged (Acidic/Basic)	Asp, Glu (acidic); Lys, Arg, His (basic)	Ionizable side chains, involved in catalysis and binding
Aromatic	Phe, Tyr, Trp	Absorb UV light, participate in stacking interactions

Additional info: The study notes have expanded on the brief points in the original slides, providing definitions, examples, and equations for clarity and completeness. The table summarizes the main categories of amino acids for quick reference.