Protein Structure, Amino Acids, and Nucleic Acids: Key Concepts in Cell Biology

Protein Structure and Amino Acids

Categories of Amino Acids

Amino acids, the building blocks of proteins, are classified based on the properties of their side chains (R groups). The 20 essential amino acids fall into three main categories:

• Nonpolar (Hydrophobic) Amino Acids: Side chains are mostly hydrocarbons; these amino acids tend to be found in the interior of proteins.

• Polar (Uncharged) Amino Acids: Side chains contain groups that can form hydrogen bonds (e.g., -OH, -NH2), but are not charged at physiological pH.

• Charged (Acidic and Basic) Amino Acids: Side chains are either positively (basic) or negatively (acidic) charged at physiological pH.

Example: Leucine is nonpolar, serine is polar uncharged, and aspartic acid is acidic (negatively charged).

Protein Directionality: N and C Termini

Proteins have directionality, meaning they have two distinct ends:

• N-terminus (Amino terminus): The end of the protein with a free amino group (-NH2).

• C-terminus (Carboxyl terminus): The end with a free carboxyl group (-COOH).

During protein synthesis, amino acids are added to the C-terminus.

Peptide Bonds and Protein Synthesis

• Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water.

• Dehydration Synthesis: The process of forming a peptide bond by removing water.

• Hydrolysis: The reverse process, breaking a peptide bond by adding water.

Reaction Type: Peptide bond formation is a condensation (dehydration) reaction.

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

Cardinal Rules for Proteins

• Proteins are made of L-amino acids (except glycine, which is achiral).

• Proteins are synthesized from N-terminus to C-terminus.

• Primary structure determines higher-level structures and function.

Forces Stabilizing Protein 3D Structure

• Hydrogen Bonds: Between backbone atoms or side chains.

• Disulfide Bonds: Covalent bonds between cysteine residues.

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

• Ionic Bonds (Salt Bridges): Between oppositely charged side chains.

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

Covalent Modifications of Proteins

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

• Glycosylation: Addition of carbohydrate groups; affects folding, stability, and cell signaling.

• Acetylation, Methylation, Ubiquitination: Other modifications that regulate protein function and fate.

Residues Modified: Specific amino acids (e.g., serine for phosphorylation) are targeted.

Chaperones

Chaperones are proteins that assist in the proper folding of other proteins, preventing misfolding and aggregation. They do not form part of the final structure.

Levels of Protein Organization

Proteins have four structural levels:

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding patterns (e.g., alpha helices, beta sheets) stabilized by hydrogen bonds.

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

• Quaternary Structure: Association of multiple polypeptide chains (subunits).

Alpha helices and beta sheets are common secondary structures.

Motifs vs. Domains

• Motif: A short, recurring structural element (e.g., helix-turn-helix) found in different proteins, often associated with a specific function.

• Domain: A larger, independently folding unit within a protein, often associated with a particular function and can exist in different proteins.

Example: The SH2 domain binds phosphotyrosine residues in signaling proteins.

Nucleic Acids: DNA and Nucleotides

Basic Structure of DNA

• DNA is a double helix composed of two antiparallel strands.

• Each strand is a polymer of nucleotides linked by phosphodiester bonds.

• Base pairing: Adenine (A) pairs with Thymine (T), Guanine (G) pairs with Cytosine (C).

Basic Structure of a Nucleotide

• Nitrogenous Base: Purine or pyrimidine.

• Pentose Sugar: Deoxyribose in DNA, ribose in RNA.

• Phosphate Group: Attached to the 5' carbon of the sugar.

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

Structural Difference: Purines are larger (two rings), while pyrimidines are smaller (one ring).

Additional info: The above notes expand on the brief questions in the original material, providing definitions, examples, and context for each concept relevant to cell biology.