Amino Acids and Proteins: Structure, Properties, and Biological Significance

Proteins and Amino Acids

Introduction

Proteins are essential macromolecules in all living organisms, composed of linear polymers of amino acids. The structure and function of proteins are determined by the sequence and properties of their constituent amino acids. Understanding amino acids is fundamental to biochemistry, as they are the building blocks of proteins and play diverse roles in cellular processes.

Protein-Related Diseases

Sickle Cell Anemia: Amino Acid Substitution

• Sickle cell anemia is caused by a single amino acid substitution in the hemoglobin protein: glutamic acid (Glu) is replaced by valine (Val).

• This substitution leads to the formation of rigid, sickle-shaped red blood cells, impairing their ability to transport oxygen efficiently.

• Example: Normal hemoglobin: Glu at position 6; Sickle cell hemoglobin: Val at position 6.

Alzheimer’s Disease: Protein Misfolding

• In Alzheimer’s disease, abnormal deposits of tau protein accumulate in the brain.

• These deposits disrupt normal cellular function and are associated with neurodegeneration.

• Example: Brain scans show tau protein deposits in healthy individuals (top) and those with Alzheimer’s (bottom).

Amino Acids

General Structure of Amino Acids

Amino acids share a common structure, consisting of a central (alpha, α) carbon atom bonded to four different groups:

• An amino group (–NH2)

• A carboxyl group (–COOH)

• A hydrogen atom

• A distinctive side chain (R group) that determines the identity and properties of the amino acid

At physiological pH (~7), amino acids exist as zwitterions, with a positively charged amino group (–NH3+) and a negatively charged carboxylate group (–COO–).

Stereochemistry of Amino Acids

• With the exception of glycine (where R = H), all α-amino acids have a chiral (asymmetric) α-carbon atom.

• This chirality gives rise to two enantiomers: L- and D- forms. Proteins are composed exclusively of L-amino acids.

• Fischer projection and ball-and-stick models are used to represent the three-dimensional arrangement of atoms.

• Example: Alanine and β-alanine are mirror images (enantiomers).

Classification of Amino Acids

The 20 common amino acids are classified based on the chemical nature of their side chains (R groups):

• Nonpolar (hydrophobic): e.g., alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline, glycine

• Polar, uncharged: e.g., serine, threonine, asparagine, glutamine, cysteine, tyrosine

• Acidic (negatively charged): aspartic acid, glutamic acid

• Basic (positively charged): lysine, arginine, histidine

• Aromatic: phenylalanine, tyrosine, tryptophan

General Properties of Amino Acids

• Abbreviations: Each amino acid has a three-letter and a one-letter code (e.g., Alanine: Ala, A).

• Molecular weight: Varies depending on the side chain.

• pKa values: Amino acids have characteristic pKa values for their α-carboxyl, α-amino, and (if present) side chain groups.

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

Table: Typical pKa Ranges for Ionizable Groups

Group	Typical pKa Range
α-Carboxyl	2.0–2.5
α-Amino	9.0–10.5
Aspartic acid, Glutamic acid (side chain)	3.5–4.8
Histidine (side chain)	6.5–7.4
Cysteine (side chain)	8.5–9.0
Tyrosine (side chain)	8.5–10.5
Lysine (side chain)	10.0–10.4
Arginine (side chain)	12.0

Properties of Aromatic Amino Acids

• Aromatic amino acids (phenylalanine, tyrosine, tryptophan) absorb ultraviolet (UV) light, with a peak absorbance at 280 nm.

• This property is used to estimate protein concentration in solution.

• Comparison: Nucleic acids absorb most strongly at 260 nm.

Properties of Polar and Reactive Side Chains

• Serine, threonine, asparagine, and glutamine have polar side chains that can form hydrogen bonds.

• The –OH group of serine and the –SH group of cysteine are nucleophilic and often participate in enzyme catalysis.

• Cysteine can form disulfide bonds through oxidation, stabilizing protein structure:

$2 \text{Cys-SH} \rightarrow \text{Cys-S-S-Cys} + 2H^+ + 2e^-$

Properties of Basic Side Chains

• Lysine, arginine, and histidine have basic side chains that are positively charged at physiological pH.

• Histidine’s charge state can vary with pH, making it important in enzyme active sites.

• Titration curve: The pI (isoelectric point) is the pH at which the net charge is zero.

Post-Translational Modifications of Amino Acids

• Amino acids in proteins can be chemically modified after translation, affecting protein function and regulation.

• Examples:

  • Phosphorylation (e.g., phosphoserine) – important in cell signaling

  • Carboxylation (e.g., γ-carboxyglutamate) – introduces binding sites in blood-clotting proteins

  • Hydroxylation (e.g., hydroxyproline in collagen) – stabilizes connective tissue

Overview of Protein Synthesis in Eukaryotic Cells

• DNA is transcribed to form messenger RNA (mRNA) in the nucleus.

• mRNA is exported to the cytoplasm.

• mRNA is translated into a linear sequence of amino acids, which folds into a three-dimensional protein structure.

Summary

• Proteins are polymers of α-amino acids, joined by peptide bonds.

• Twenty common amino acids (and two rare ones) are incorporated into proteins, each distinguished by its side chain (R group).

• Chirality is a key feature of amino acids (except glycine), with proteins containing only L-amino acids.

• Amino acid properties influence protein structure, stability, and function.

• Post-translational modifications expand the functional diversity of proteins.