Chapter 10 & 11

Chapter 10: Proteins – Workers of the Cell

General Properties of Amino Acids (AAs)

Proteins are polymers made from amino acids, which are the building blocks of life. Understanding their properties is essential for grasping protein structure and function.

• Nonessential vs. Essential Amino Acids: Nonessential amino acids can be synthesized by the body, while essential amino acids must be obtained from the diet.

• General Structure of Amino Acids: Each amino acid contains a central carbon (alpha carbon), an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group).

• Acid-Base Nature of Amino Acids: Amino acids can act as both acids and bases due to their amino and carboxyl groups. They exist as zwitterions at physiological pH.

• Abbreviations: Amino acids are commonly represented by three-letter and one-letter abbreviations (e.g., Gly for Glycine, G for Glycine).

Example: Glycine (Gly, G) is the simplest amino acid, with a hydrogen as its side chain.

Protein Structure and Organization

Proteins have complex structures that determine their function in biological systems.

• Levels of Protein Structure:

  • Primary Structure: Sequence of amino acids in a polypeptide chain.

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

  • Tertiary Structure: Overall three-dimensional shape of a single polypeptide, determined by interactions among side chains.

  • Quaternary Structure: Association of multiple polypeptide chains into a functional protein complex.

• Denaturation and Renaturation: Denaturation is the loss of protein structure due to external stress (heat, pH, chemicals), while renaturation is the restoration of structure under suitable conditions.

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

Protein Functions

Proteins perform a wide variety of functions in the body, including:

• Enzymes: Catalyze biochemical reactions.

• Transporters: Move molecules across membranes (e.g., oxygen transport by hemoglobin).

• Antibodies: Defend against pathogens.

• Structural Proteins: Provide support and shape to cells and tissues (e.g., collagen).

Enzyme Structure and Function

Enzymes are specialized proteins that accelerate chemical reactions.

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate during the reaction.

• Specificity: Enzymes are highly specific for their substrates due to the unique shape of their active sites.

• Types of Enzyme Inhibition:

  • Irreversible Inhibition: Inhibitor permanently binds to the enzyme, inactivating it.

  • Reversible Inhibition: Inhibitor binds non-covalently and can be removed, restoring enzyme activity.

Example: Lactase is an enzyme that catalyzes the breakdown of lactose into glucose and galactose.

Chapter 11: Nucleic Acids – Big Molecules with a Big Role

Structure and Function of Nucleic Acids

Nucleic acids are polymers that store and transmit genetic information. The two main types are DNA and RNA.

• Nucleotides: The building blocks of nucleic acids, each consisting of a nitrogenous base, a five-carbon sugar, and a phosphate group.

• DNA (Deoxyribonucleic Acid): Stores genetic information; composed of deoxyribose sugar.

• RNA (Ribonucleic Acid): Involved in protein synthesis; composed of ribose sugar.

DNA Structure and Replication

DNA is a double helix formed by two complementary strands held together by base pairing.

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

• Double Helix Model: Proposed by Watson and Crick, with supporting evidence from Rosalind Franklin's X-ray diffraction studies.

• Replication: DNA copies itself before cell division, ensuring genetic continuity.

Example: During replication, the enzyme DNA polymerase synthesizes a new strand complementary to the template strand.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a protein at the ribosome, using transfer RNA (tRNA) and ribosomal RNA (rRNA).

• Codons: Three-nucleotide sequences in mRNA that specify amino acids.

Example: The codon AUG codes for the amino acid methionine and serves as the start signal for protein synthesis.

Table: Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double-stranded	Single-stranded
Bases	A, T, G, C	A, U, G, C
Function	Genetic storage	Protein synthesis, regulation

Key Equations and Concepts

• Peptide Bond Formation: Amino acids are linked by peptide bonds during protein synthesis. $\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Base Pairing Rule: $\text{A} = \text{T}$, $\text{G} = \text{C}$

• Central Dogma: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: The study guide references the importance of knowing amino acid abbreviations and the structure of nucleic acids, which are foundational for understanding protein synthesis and genetic information flow in biochemistry.