Protein Structure, Amino Acids, and Nucleic Acids: Core Concepts for Cell Biology

Protein Structure and Amino Acids

General Structure of an Amino Acid

Amino acids are the building blocks of proteins, each sharing a common structure but differing in their side chains.

• Central Carbon (α-carbon): The core atom to which all groups are attached.

• Amino Group (-NH2): Acts as a base, accepting protons.

• Carboxyl Group (-COOH): Acts as an acid, donating protons.

• Hydrogen Atom: Attached to the α-carbon.

• R Group (Side Chain): Variable group that determines the properties and identity of the amino acid.

Example: Glycine has a hydrogen as its R group, making it the simplest amino acid.

Categories of Amino Acids

The 20 essential amino acids are classified based on the chemical nature of their side chains:

• Nonpolar (Hydrophobic): Side chains are mostly hydrocarbons (e.g., leucine, valine).

• Polar (Uncharged): Side chains contain groups that can form hydrogen bonds (e.g., serine, threonine).

• Charged: Side chains are either acidic (negatively charged, e.g., aspartic acid) or basic (positively charged, e.g., lysine).

Additional info: These categories influence protein folding and function.

Protein Directionality: N and C Termini

Proteins are linear polymers with directionality:

• N-terminus: The end with a free amino group.

• C-terminus: The end with a free carboxyl group.

Directionality is crucial for protein synthesis and function.

Peptide Bonds and Chemical Reactions

A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another.

• Dehydration Synthesis: Formation of a peptide bond releases a molecule of water.

• Hydrolysis: Breaking a peptide bond consumes a molecule of water.

Equation for peptide bond formation:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + H_2O$

Additional info: Peptide bond formation is an example of a condensation reaction.

Cardinal Rules for Being a Protein

• Must be composed of amino acids linked by peptide bonds.

• Must fold into a specific three-dimensional structure.

• Must have directionality (N-terminus to C-terminus).

Forces Stabilizing Protein 3D Structure

Proteins fold into complex shapes stabilized by several forces:

• Hydrogen Bonds: Between backbone atoms and side chains.

• Hydrophobic Interactions: Nonpolar side chains cluster away from water.

• Van der Waals Forces: Weak attractions between atoms in close proximity.

• Ionic Bonds: Between oppositely charged side chains.

• Disulfide Bridges: Covalent bonds between cysteine residues.

Covalent Modifications of Proteins

Covalent modifications regulate protein function and localization:

• Phosphorylation: Addition of phosphate groups (usually to serine, threonine, or tyrosine).

• Acetylation: Addition of acetyl groups (often to lysine).

• Methylation: Addition of methyl groups (commonly to lysine or arginine).

• Glycosylation: Addition of carbohydrate groups (to asparagine, serine, or threonine).

Additional info: These modifications can alter activity, stability, or cellular location.

Chaperones

Chaperones are specialized proteins that assist in the proper folding of other proteins, preventing misfolding and aggregation.

• Help newly synthesized proteins achieve correct conformation.

• Can refold misfolded proteins or target them for degradation.

Four Levels of Protein Organization

Proteins have hierarchical structural organization:

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding patterns, such as alpha helices and beta sheets.

• Tertiary Structure: Overall 3D shape of a single polypeptide.

• Quaternary Structure: Assembly of multiple polypeptide chains.

Alpha helices: Right-handed coils stabilized by hydrogen bonds. Beta sheets: Sheet-like structures formed by hydrogen bonding between backbone atoms in different strands.

Motif vs. Domain

Motif: A short, recurring sequence or structural element found in proteins, often associated with a specific function.

Domain: A larger, independently folding region of a protein that can have a distinct function.

• Motifs are usually smaller and may not fold independently.

• Domains are larger and can often function independently.

Nucleic Acids: DNA and Nucleotides

Basic Structure of DNA

DNA (deoxyribonucleic acid) is a double-helical molecule that stores genetic information.

• Composed of two antiparallel strands.

• Strands are held together by hydrogen bonds between complementary bases.

Basic Structure of a Nucleotide

Nucleotides are the monomers of nucleic acids, each consisting of:

• Phosphate Group

• Pentose Sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogenous Base: Purine or pyrimidine

Purines vs. Pyrimidines

Type	Bases	Structure
Purines	Adenine (A), Guanine (G)	Double-ring structure
Pyrimidines	Cytosine (C), Thymine (T), Uracil (U in RNA)	Single-ring structure

Additional info: Purines are larger than pyrimidines due to their two-ring structure.

Structural Differences

• Purines: Fused imidazole and pyrimidine rings.

• Pyrimidines: Single six-membered ring.

Example: In DNA, adenine pairs with thymine, and guanine pairs with cytosine.