Amino Acids, Peptides, and Proteins – Structure, Properties, and Stereochemistry

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Amino Acids, Peptides, and Proteins – Part 1

Introduction and Learning Objectives

This section introduces the foundational concepts of amino acids, peptides, and proteins, focusing on their structure, properties, and the relationship between sequence and function. Understanding these biomolecules is essential for grasping protein chemistry and its implications in health and disease.

Relationship between amino acids and proteins: Amino acids are the building blocks of proteins. Proteins are formed by joining amino acids via peptide bonds, which then fold into functional three-dimensional structures.
General structure of an amino acid: All amino acids share a common backbone but differ in their side chains (R groups).
Recognizing and drawing R groups: The 20 protein-coding amino acids each have a unique R group that determines their properties.
Three-letter and one-letter codes: Each amino acid is represented by standard codes for ease of communication.
Side chain labeling: Side chains are attached to the α-carbon of the amino acid backbone.
Bonding interactions: Side chains participate in various interactions (ionic, hydrogen bonding, disulfide bonds) that stabilize protein structure.
Charge states and pKa: Amino acids can be charged or uncharged at physiological pH, depending on their side chains and pKa values.
Stereochemistry: Amino acids are chiral and exist as L- or D- isomers; only L-amino acids are used in proteins.
Peptide bond formation: Amino acids are linked by peptide bonds through condensation reactions.

From Amino Acids to Proteins

Levels of Protein Structure

Proteins are hierarchical structures, with each level of organization contributing to their final function.

Primary structure: The linear sequence of amino acids in a polypeptide chain.
Secondary structure: Local folding patterns such as α-helices and β-sheets, stabilized by hydrogen bonds.
Tertiary structure: The overall three-dimensional shape of a single polypeptide, determined by interactions among side chains.
Quaternary structure: The assembly of multiple polypeptide chains into a functional protein complex.

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Importance of Amino Acid Sequence

The Amino Acid 'Code' and Protein Function

The specific sequence of amino acids determines the three-dimensional structure and function of a protein. Even a single amino acid mutation can disrupt protein folding and function, leading to disease.

Dynamic movement, catalysis, and binding sites: The 3D shape enables proteins to perform diverse roles such as catalysis (enzymes), molecular recognition (antibodies, receptors), and structural support.
Clinical relevance: Many diseases result from mutations that alter amino acid sequence, affecting protein function (e.g., sickle cell anemia, cystic fibrosis).

General Structure of Amino Acids

Alpha (α) Amino Acids

All proteinogenic amino acids share a common structure:

Amino group (–NH2): Basic group attached to the α-carbon.
Carboxylic acid group (–COOH): Acidic group attached to the α-carbon.
α-Carbon: Central carbon atom bonded to the amino group, carboxyl group, hydrogen atom, and R group.
R group (side chain): Variable group that defines the identity and properties of each amino acid.

Note: Only α-amino acids (with the amino and carboxyl groups attached to the same carbon) are used in protein synthesis.

Zwitterions and Charge States

Ionization of Amino Acids

At physiological pH (~7), amino acids exist as zwitterions, carrying both positive and negative charges but being overall neutral.

Zwitterion: A molecule with both a positively charged (protonated) amino group () and a negatively charged (deprotonated) carboxyl group ().
pH dependence: The charge state of amino acids depends on the pH and the pKa values of their ionizable groups.

The 20 Protein-Coding Amino Acids

Classification by R Group Properties

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

Hydrophobic (nonpolar): Side chains are typically buried within protein structures (e.g., alanine, valine, leucine, isoleucine, phenylalanine, methionine, tryptophan, proline).
Hydrophilic (polar): Side chains are often found on the protein surface and can form hydrogen bonds (e.g., serine, threonine, asparagine, glutamine, tyrosine, cysteine).
Charged (acidic or basic): Side chains carry a charge at physiological pH (acidic: aspartate, glutamate; basic: lysine, arginine, histidine).

Group	Examples	Properties
Nonpolar (Hydrophobic)	Ala, Val, Leu, Ile, Met, Phe, Trp, Pro	Buried in protein core
Polar (Uncharged)	Ser, Thr, Asn, Gln, Tyr, Cys	Surface, H-bonding
Acidic (Negatively Charged)	Asp, Glu	Surface, ionic bonds
Basic (Positively Charged)	Lys, Arg, His	Surface, ionic bonds

Representative Amino Acids: Structure and Properties

Alanine (Ala, A)

Structure: Small, nonpolar, aliphatic side chain (–CH3).
Hydrophobicity: Hydrophobic, often found in protein interiors.
Role: Non-essential, synthesized in the body.

Valine (Val, V)

Structure: Branched, nonpolar side chain (–CH(CH3)2).
Hydrophobicity: Hydrophobic, essential amino acid (must be obtained from diet).

Glutamate/Glutamic Acid (Glu, E)

Structure: Acidic side chain (–CH2CH2COOH).
Charge at pH 7: –1 (negatively charged).
Role: Forms ionic bonds with basic amino acids; non-essential.

Arginine (Arg, R)

Structure: Basic side chain with guanidinium group.
Charge at pH 7: +1 (positively charged).
Role: Forms ionic bonds with acidic amino acids; non-essential.

Serine (Ser, S)

Structure: Polar side chain (–CH2OH).
Charge at pH 7: Uncharged.
Role: Forms hydrogen bonds; can participate in catalysis.

Cysteine (Cys, C)

Structure: Contains thiol group (–CH2SH).
Charge at pH 7: Uncharged.
Role: Can form disulfide bonds, stabilizing protein structure.

Histidine (His, H)

Structure: Imidazole side chain.
Charge at pH 7: Can be both charged (+1) and uncharged (0); pKa ≈ 6.0.
Role: Involved in catalysis and metal binding.

Bonding Interactions in Proteins

Ionic Bonds

Formed between: Oppositely charged side chains (e.g., Glutamate (–1) and Arginine (+1)).
Role: Stabilize protein folds and contribute to protein function.

Hydrogen Bonds

Formed between: Polar side chains (e.g., Serine, Threonine, Histidine) or between side chains and the peptide backbone.
Role: Stabilize secondary and tertiary structures.

Disulfide Bonds

Formed between: Two cysteine residues (–SH + –SH → –S–S–).
Role: Covalent bonds that stabilize protein tertiary and quaternary structures.

pKa and Charge States of Amino Acids

Definition of pKa

pKa: The negative logarithm of the acid dissociation constant ().
Interpretation: The pH at which 50% of the group is dissociated (HA ⇌ H+ + A–).

Typical pKa Values

α-Amino group: pKa ≈ 8.8–9.7
α-Carboxyl group: pKa ≈ 1.7–2.6
Side chains: Vary by amino acid (e.g., Histidine side chain pKa ≈ 6.0)

Physiological Relevance

If the pKa of a group is significantly above or below pH 7, it is not physiologically relevant (i.e., the group will be fully protonated or deprotonated in cells).
Only groups with pKa values near physiological pH (~7) can change their charge state in biological conditions.

Stereochemistry of Amino Acids

Chirality and Isomerism

Tetrahedral α-carbon: Most amino acids (except glycine) have four different groups attached, making them chiral.
L- and D- isomers: Only L-amino acids are incorporated into proteins.
CORN rule: Used to assign L- or D- configuration by arranging the groups around the α-carbon and reading the order of CO (carboxyl), R (side chain), N (amino group).
Cahn-Ingold-Prelog (CIP) rules: Assign R/S configuration based on atomic number priorities.

Peptide Bond Formation

Condensation Reaction

Peptide bond: Formed by a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing water.
Polypeptide: A chain of amino acids linked by peptide bonds, with an N-terminus (amino end) and a C-terminus (carboxyl end).

Equation:

Summary Table: Key Properties of Representative Amino Acids

Amino Acid	Three-Letter Code	One-Letter Code	Side Chain Type	Charge at pH 7
Alanine	Ala	A	Nonpolar	0
Valine	Val	V	Nonpolar	0
Glutamate	Glu	E	Acidic	–1
Arginine	Arg	R	Basic	+1
Serine	Ser	S	Polar	0
Cysteine	Cys	C	Polar (thiol)	0
Histidine	His	H	Basic (imidazole)	0 or +1

Conclusion

Understanding the structure, properties, and stereochemistry of amino acids is fundamental to the study of proteins and their functions in biological systems. Mastery of these concepts is essential for further exploration of protein folding, enzymatic catalysis, and the molecular basis of disease.