Amino Acids, Peptides, and Proteins: Structure, Properties, and Biological Relevance

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 chemical structure, properties, and biological significance. Understanding these molecules is essential for grasping protein function and the molecular basis of 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 and folding into specific three-dimensional structures.

• General structure of an amino acid: Each amino acid contains an amino group, a carboxylic acid group, a central (alpha) carbon, and a unique side chain (R group).

• Recognizing and drawing R groups: The 20 protein-coding amino acids have distinct R groups that determine their properties.

• Three-letter and single-letter codes: Each amino acid is assigned a standard code for identification.

• Bonding interactions: Side chains form various interactions (ionic, hydrogen, disulfide) that stabilize protein structure.

• Charge states and pKa values: Amino acids can be charged or neutral at physiological pH, depending on their side chains and pKa values.

• Stereochemistry: Amino acids are chiral and only one stereoisomer (L-form) is used in proteins.

• Peptide bond formation: Amino acids join via condensation reactions to form peptides and proteins.

From Amino Acids to Proteins

Hierarchical Structure of Proteins

Proteins are formed through a series of structural levels, each contributing to their final function.

• Primary structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as alpha-helices and beta-sheets.

• Tertiary structure: The overall three-dimensional shape of a single polypeptide.

• Quaternary structure: Complexes formed by multiple polypeptide chains.

Example: The sequence MALKRHKELND... forms a polypeptide that folds into secondary, tertiary, and possibly quaternary structures.

Importance of Amino Acids in Biology and Disease

Biological Relevance

Learning amino acids is crucial because protein function depends on the correct sequence and structure. Mutations or misfolding can lead to diseases such as cancer, neurodegenerative disorders, and immune conditions.

• Drug design: Targeted therapies (e.g., Vemurafenib) exploit specific amino acid changes.

• Gene therapy: Understanding protein structure enables advances in treating genetic diseases.

General Structure of Amino Acids

Alpha Amino Acid Structure

All proteinogenic amino acids share a common backbone:

• Amino group: $\mathrm{NH_2}$

• Carboxylic acid group: $\mathrm{COOH}$

• Alpha carbon: Central carbon atom

• R group (side chain): Unique to each amino acid

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

Zwitterionic Nature of Amino Acids

Charge Properties

At physiological pH, amino acids exist as zwitterions, carrying both positive and negative charges:

• Amino group: Protonated ($\mathrm{NH_3^+}$)

• Carboxyl group: Deprotonated ($\mathrm{COO^-}$)

• Net charge: Overall neutral, but side chains may contribute additional charges

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): Typically buried within protein structures (e.g., Alanine, Valine)

• Hydrophilic (polar): Often found on protein surfaces (e.g., Serine, Cysteine)

• Charged: Acidic (e.g., Glutamate) or basic (e.g., Arginine)

• Special roles: Some side chains form stabilizing bonds (e.g., disulfide bonds in Cysteine) or participate in catalysis (e.g., Histidine)

Representative Amino Acids: Structure and Properties

Alanine

• Three-letter code: Ala

• One-letter code: A

• Hydrophobic, nonpolar

• Smallest amino acid with a stereocentre

Valine

• Three-letter code: Val

• One-letter code: V

• Hydrophobic, nonpolar

• Essential amino acid

Glutamate (Glutamic Acid)

• Three-letter code: Glu

• One-letter code: E

• Acidic, negatively charged at neutral pH

• Forms ionic bonds

Arginine

• Three-letter code: Arg

• One-letter code: R

• Basic, positively charged at neutral pH

• Forms ionic bonds

Serine

• Three-letter code: Ser

• One-letter code: S

• Polar, uncharged at neutral pH

• Forms hydrogen bonds

Cysteine

• Three-letter code: Cys

• One-letter code: C

• Hydrophobic, uncharged at neutral pH

• Forms disulfide bonds

Histidine

• Three-letter code: His

• One-letter code: H

• Basic, can be charged (+1) or uncharged (0) at neutral pH

• Forms hydrogen and ionic bonds

• Involved in catalysis

Bonding Interactions in Proteins

Ionic Bonds

Ionic bonds form between oppositely charged side chains, such as glutamate (-1) and arginine (+1), stabilizing protein folds.

Hydrogen Bonds

Polar side chains (e.g., serine, threonine, histidine) can form hydrogen bonds, contributing to protein stability and function.

Disulfide Bonds

Cysteine residues can form covalent disulfide bonds ($\mathrm{S-S}$), further stabilizing protein structure.

Naming and Stereochemistry of Amino Acids

Chirality and Stereoisomers

Amino acids (except glycine) are chiral, with four unique substituents around the alpha carbon. Only the L-stereoisomer is used in proteins.

• CORN rule: Assigns L- or D- configuration based on the arrangement of groups around the alpha carbon.

• Cahn-Ingold-Prelog (CIP) rules: Assigns R or S configuration based on atomic number priorities.

pKa and Charge States of Amino Acids

Definition and Relevance

The pKa is the negative logarithm of the dissociation constant of an acid. It determines the charge state of amino acid groups at different pH values.

• General equation: $\mathrm{HA \rightleftharpoons H^+ + A^-}$

• pKa of alpha amino group: $6.8 - 9.7$

• pKa of carboxyl group: $1.7 - 2.6$

• Side chain pKa: Varies by amino acid (e.g., histidine side chain pKa = 6.0)

Physiological relevance: Only charge states and pKa values near pH 7 are relevant for cellular function.

Summary Table: Properties of Representative Amino Acids

Amino Acid	Three-Letter Code	One-Letter Code	Side Chain Type	Charge at pH 7	Key Interactions
Alanine	Ala	A	Hydrophobic	0	Hydrophobic
Valine	Val	V	Hydrophobic	0	Hydrophobic
Glutamate	Glu	E	Acidic	-1	Ionic
Arginine	Arg	R	Basic	+1	Ionic
Serine	Ser	S	Polar	0	Hydrogen bonds
Cysteine	Cys	C	Hydrophobic	0	Disulfide bonds
Histidine	His	H	Basic	0 or +1	Hydrogen/Ionic bonds

Peptide Bond Formation

Condensation Reaction

Amino acids are joined by peptide bonds through condensation reactions, forming polypeptides and ultimately functional proteins.

• Reaction: The carboxyl group of one amino acid reacts with the amino group of another, releasing water and forming a peptide bond.

• Equation: $\mathrm{Amino\ Acid_1 + Amino\ Acid_2 \rightarrow Dipeptide + H_2O}$

Conclusion

Understanding the structure, properties, and interactions of amino acids is fundamental to organic chemistry and molecular biology. These concepts underpin protein function, disease mechanisms, and modern advances in biotechnology and medicine.

Additional info: The notes also reference the role of AI (AlphaFold) in protein structure prediction, highlighting the interdisciplinary nature of modern protein science.