Protein Structure, Function, and Enzyme Kinetics: Biochemistry Study Notes

Protein Structure and Folding

Conformation and Stability

Proteins adopt specific three-dimensional structures (conformations) that are essential for their biological function. The stability of these structures is determined by various chemical interactions and thermodynamic principles.

• Native Proteins: Proteins in their functional, folded conformation.

• Conformation: Thermodynamically the most stable, lowest free energy; folding driven by thermodynamics.

• Stability: Tendency of a protein to maintain its native conformation. Hydrophobic effect is the predominant factor in protein stability (particularly in tertiary structure).

Secondary Structure

The spatial arrangement of main chain atoms in a segment of a polymer forms the secondary structure. Common types include alpha helices and beta sheets.

• Alpha Helix: Simplest arrangement, maximum number of hydrogen bonds. R groups project out of the backbone and wind along an imaginary axis.

• Beta Sheet: Strands lie side by side, stabilized by hydrogen bonds. Beta sheets are often found in proteins that require structural rigidity.

• Intrachain Disulfide Cross-Linkages: Form at cysteine residues, stabilizing protein structure.

• Key Amino Acids: Alanine is most likely to make alpha helix. If you make 2 of the helix breakers, those will more likely disrupt alpha helix structure. Glycine (too small to hold structure), Proline (rigid, breaks helix), Arginine (side chain disrupts backbone).

Quaternary Structure

Quaternary structure refers to the arrangement of tertiary protein subunits in a three-dimensional complex.

• Globular Proteins: More compact than fibrous proteins. Examples: enzymes, transport proteins, motor proteins, regulatory proteins, immunoglobulins.

• Fibrous Proteins: Provide structural support (e.g., collagen).

Protein Function and Disease

Vitamin C and Collagen Synthesis

Vitamin C is required for the hydroxylation of proline and lysine in collagen. Deficiency leads to scurvy, characterized by degeneration of connective tissue.

Protein Motif and Domain

• Motif: Most rigid amino acid is proline; most flexible is glycine.

• Denaturation: Loss of 3D structure sufficient to lose function. Increase heat, pH, or chemicals can cause denaturation. Renaturation: Regain native structure and biological activity.

Significance of Tm (Melting Temperature)

• Tm is the midpoint of the temperature range over which denaturation occurs.

• Higher Tm = more stable protein.

Anfinsen Experiment

• Showed that the primary structure (amino acid sequence) is enough to guide the folding of a protein.

Chaperone Proteins

• Facilitate correct folding (native conformation) or pathways.

• Require ATP for function.

Prion Proteins

• Prion is a misfolded protein. Can act as a catalyst to make more prions.

• Aggregation of prions can cause disease.

Hemoglobin, Myoglobin, and Oxygen Binding

Heme Group and Globin Family

Heme is a protein-bound prosthetic group, covalently bound to protein but not an amino acid. Present in myoglobin and hemoglobin.

• Heme Structure: Consists of protoporphyrin and iron (Fe2+).

• Coordination Bonds: Iron atom forms six coordination bonds: four to nitrogen atoms in the flat porphyrin ring, two perpendicular to the porphyrin.

• Globin Family: Widespread protein family; highly conserved tertiary structure.

Comparison of Oxygen-Binding Proteins

Protein	Function	Structure
Myoglobin	Monomeric, stores O2, facilitates O2 in muscle. Non-cooperative.	Single polypeptide chain
Hemoglobin	Transports O2 in blood, cooperative binding, 2 alpha and 2 beta chains.	Tetrameric (4 subunits)
Neuroglobin	Expressed in neurons, protects brain from low O2 or restricted blood supply.	Monomeric
Cytoglobin	Regulates nitric oxide, localized signal for muscle relaxation.	Monomeric

Ligand Binding and Dissociation Constant

Ligand binding to proteins is described by the dissociation constant (Kd), which measures the affinity of the ligand for the protein.

• Higher Ka: Higher affinity.

• Kd: Reciprocal of Ka; lower Kd = higher affinity.

Equation for dissociation constant:

$K_d = \frac{[P][L]}{[PL]}$

Hill Equation and Cooperativity

The Hill equation describes cooperative binding in proteins like hemoglobin.

$\log\left(\frac{Y}{1-Y}\right) = n \log [L] - \log K_d$

• Hill coefficient (n): Slope of Hill plot.

• n = 1: Ligand binding is not cooperative.

• n > 1: Indicates positive cooperativity (binding to one site increases affinity at others).

• n < 1: Indicates negative cooperativity.

Bohr Effect and BPG

• Bohr Effect: Describes the effect of pH and CO2 on the binding and release of O2 by hemoglobin.

• BPG (2,3-bisphosphoglycerate): Binds to hemoglobin, stabilizes T state, decreases affinity for O2. Important for adaptation to high altitudes.

Fetal Hemoglobin and Sickle Cell Anemia

• Fetal Hemoglobin: Higher affinity for O2 due to lower affinity for BPG.

• Sickle Cell Anemia: Caused by a single mutation (Glu → Val at position 6 in beta chain). Sickle-shaped cells block capillaries, impairing oxygen transport.

Enzyme Classification and Kinetics

Enzyme Classes

Enzymes are classified based on the type of reaction they catalyze. The six major classes are:

Class Number	Class Name	Type of Reaction Catalyzed
1	Oxidoreductases	Oxidation-reduction reactions
2	Transferases	Transfer of functional groups
3	Hydrolases	Hydrolysis reactions
4	Lyases	Addition or removal of groups to form double bonds
5	Isomerases	Isomerization reactions
6	Ligases	Joining of two molecules with covalent bonds

Reaction Coordinate and Activation Energy

The reaction coordinate diagram illustrates the energy changes during a chemical reaction. The activation energy (Ea) is the energy barrier that must be overcome for the reaction to proceed.

$\Delta G^\ddagger$ = Activation energy

$K_{eq} = \frac{[P]}{[S]}$

$\Delta G^\circ = -RT \ln K_{eq}$

• Catalysts: Lower the activation energy, increasing the reaction rate, but do not change the equilibrium constant or thermodynamics.

• Peak (Transition State): Highest activation energy; slowest step in reaction.

Rate of Enzyme Reactions

• Rate is determined by the concentration of reactant(s) and the rate constant K.

• First order: $v = k[S]$

• Second order: $v = k[S_1][S_2]$

• Increasing substrate concentration increases reaction rate.

• Higher temperature and lower activation energy increase rate.

Laboratory Techniques

Western Blot

Western blot is a technique used to detect specific proteins in a sample using antibodies.

• Immunoblot = Western blot assay.

• Primary antibody binds to target protein; secondary antibody binds to primary antibody and is signal-labeled for detection.

Immunoglobulin Structure

Antibody Chains

Immunoglobulins (antibodies) are composed of two heavy chains and two light chains. The variable region binds a specific antigen.

• Heavy and light chains are held together by disulfide bonds, hydrophobic interactions, electrostatics, and van der Waals forces.

Additional info:

• Some context and examples were inferred for completeness, such as the role of BPG in hemoglobin and the classification of enzyme classes.

• Equations and tables were expanded for clarity and academic completeness.