Proteins and Enzymes: Structure, Function, and Catalysis

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Amino acids are the fundamental building blocks of proteins, each with distinct chemical properties that influence protein structure and function.

General Structure: Each amino acid contains a central carbon (α-carbon), an amino group, a carboxyl group, and a variable side chain (R group).
Chirality: Most amino acids exist as chiral molecules, typically in the L-form in biological systems.
Ionization: Amino acids can exist in different ionization states depending on pH; they are zwitterions at physiological pH (~7).
Side Chain Classification: Side chains are classified as polar, nonpolar, acidic, or basic, affecting solubility and reactivity.
UV Absorption: Aromatic amino acids (e.g., tryptophan, tyrosine) absorb UV light.
Post-translational Modifications: Amino acids can be modified after protein synthesis (e.g., phosphorylation, hydroxylation, methylation).
Genetic Code: Amino acids are encoded by nucleotide triplets (codons) in DNA/RNA.

Example: Serine can be phosphorylated to regulate enzyme activity.

Proteins are synthesized from amino acids via peptide bonds, guided by the genetic code.

Peptide Bond Formation: Peptide bonds are formed by condensation reactions, releasing H2O and requiring ATP.
Polypeptide Chains: Proteins are polymers of amino acids; dipeptides (2), oligopeptides (3–14), polypeptides (>15).
Polarity: Peptide chains have directionality: N-terminus (amino end) to C-terminus (carboxyl end).
Genetic Code: Each amino acid is specified by a codon (three-nucleotide sequence) in mRNA.
Post-translational Processing: Proteins may undergo modifications (e.g., glycosylation, proteolytic cleavage) for proper function.

Example: Insulin is produced as a precursor and processed to its active form.

Proteins have hierarchical structures that determine their function and stability.

Primary Structure: Linear sequence of amino acids.
Secondary Structure: Local folding into α-helices and β-sheets stabilized by hydrogen bonds.
Tertiary Structure: Overall 3D folding driven by hydrophobic, ionic, and hydrogen bonding interactions.
Quaternary Structure: Assembly of multiple polypeptide chains (subunits) into a functional protein.
Stabilizing Forces: Disulfide bonds, salt bridges, and hydrophobic interactions maintain structure.

Example: Hemoglobin is a tetramer with quaternary structure.

Protein folding is a complex process influenced by sequence, environment, and chaperones.

Ramachandran Plot: Shows allowed angles of polypeptide backbone rotation.
Secondary Structure Variations: α-helix, β-sheet, collagen triple helix.
Collagen: Unique triple helix stabilized by post-translational hydroxylation.
Globular Proteins: Compact, soluble, diverse functions (e.g., enzymes, antibodies).
Protein Folding: Assisted by molecular chaperones; misfolding can cause disease.

Example: Prion diseases result from protein misfolding.

Proteins play key roles in immunity, with antibodies recognizing and neutralizing foreign molecules.

Antibody Structure: Y-shaped proteins with variable (V) and constant (C) regions; heavy and light chains.
Antigen Recognition: Specificity determined by variable regions.
Immunoglobulin Classes: IgG, IgM, IgA, IgD, IgE.
Immunological Memory: Rapid response upon re-exposure to antigen (basis of vaccination).
Therapeutic Antibodies: Engineered for targeted cancer therapy and autoimmune disease treatment.

Example: Monoclonal antibodies are used in immunotherapy.

Myoglobin and hemoglobin are oxygen-binding proteins with distinct structures and functions.

Myoglobin: Monomeric, stores O2 in muscle.
Hemoglobin: Tetrameric, transports O2; exhibits cooperative binding via T (tense) and R (relaxed) states.
Heme Group: Contains Fe2+; binds O2 reversibly.
Allosteric Effectors: CO2, H+, 2,3-BPG modulate O2 affinity.
Bohr Effect: Lower pH decreases O2 affinity, facilitating release in tissues.

Example: Hemoglobin adapts to high altitude by increasing 2,3-BPG levels.

Enzymes are proteins that accelerate biochemical reactions by lowering activation energy.

Enzyme Function: Catalyze reactions without altering equilibrium; highly specific for substrates.
Reaction Types: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Active Site: Region where substrate binds and reaction occurs; specificity determined by shape and chemical environment.
Catalytic Mechanisms: Acid-base catalysis, covalent catalysis, metal ion catalysis.
Enzyme Kinetics: Rate of reaction depends on substrate concentration and enzyme properties.
Regulation: Allosteric control, covalent modification, feedback inhibition.
Ribozymes: RNA molecules with catalytic activity.

Example: Hexokinase catalyzes phosphorylation of glucose in glycolysis.

Level	Description	Stabilizing Forces
Primary	Sequence of amino acids	Peptide bonds
Secondary	α-helix, β-sheet	Hydrogen bonds
Tertiary	3D folding of polypeptide	Hydrophobic interactions, ionic bonds, disulfide bridges
Quaternary	Assembly of multiple subunits	Non-covalent interactions, sometimes disulfide bonds

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